Merge pull request #2141 from aspnet/dev

Merging changes from dev

.appveyor.yml

@@ -1,17 +0,0 @@
-init:
-- git config --global core.autocrlf true
-branches:
-  only:
-  - dev
-  - /^release\/.*$/
-  - /^(.*\/)?ci-.*$/
-build_script:
-- ps: .\run.ps1 default-build
-clone_depth: 1
-environment:
-  global:
-    DOTNET_SKIP_FIRST_TIME_EXPERIENCE: true
-    DOTNET_CLI_TELEMETRY_OPTOUT: 1
-test: 'off'
-deploy: 'off'
-os: Visual Studio 2017 Preview

README.md

@@ -1,8 +1,6 @@
 Razor
 =====
 
-AppVeyor: [![AppVeyor](https://ci.appveyor.com/api/projects/status/olbc8ur2jna0v27j/branch/dev?svg=true)](https://ci.appveyor.com/project/aspnetci/razor/branch/dev)
-
 Travis:   [![Travis](https://travis-ci.org/aspnet/Razor.svg?branch=dev)](https://travis-ci.org/aspnet/Razor)
 
 The Razor syntax provides a fast, terse, clean and lightweight way to combine server code with HTML to create dynamic web content. This repo contains the parser and the C# code generator for the Razor syntax.

Razor.sln

@@ -1,6 +1,7 @@
-Microsoft Visual Studio Solution File, Format Version 12.00
+
+Microsoft Visual Studio Solution File, Format Version 12.00
 # Visual Studio 15
-VisualStudioVersion = 15.0.27107.3000
+VisualStudioVersion = 15.0.27130.2036
 MinimumVisualStudioVersion = 15.0.26730.03
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "src", "src", "{3C0D6505-79B3-49D0-B4C3-176F0F1836ED}"
 	ProjectSection(SolutionItems) = preProject
@@ -92,9 +93,11 @@ Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Microsoft.CodeAnalysis.Razo
 EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Microsoft.AspNetCore.Razor.Tools", "src\Microsoft.AspNetCore.Razor.Tools\Microsoft.AspNetCore.Razor.Tools.csproj", "{3E7F2D49-3B45-45A8-9893-F73EC1EEBAAB}"
 EndProject
-Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Microsoft.NET.Sdk.Razor", "src\Microsoft.NET.Sdk.Razor\Microsoft.NET.Sdk.Razor.csproj", "{7D9ECCEE-71D1-4A42-ABEE-876AFA1B4FC9}"
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Microsoft.NET.Sdk.Razor", "src\Microsoft.NET.Sdk.Razor\Microsoft.NET.Sdk.Razor.csproj", "{7D9ECCEE-71D1-4A42-ABEE-876AFA1B4FC9}"
 EndProject
-Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Microsoft.AspNetCore.Razor.Tools.Test", "test\Microsoft.AspNetCore.Razor.Tools.Test\Microsoft.AspNetCore.Razor.Tools.Test.csproj", "{6EA56B2B-89EC-4C38-A384-97D203375B06}"
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Microsoft.AspNetCore.Razor.Tools.Test", "test\Microsoft.AspNetCore.Razor.Tools.Test\Microsoft.AspNetCore.Razor.Tools.Test.csproj", "{6EA56B2B-89EC-4C38-A384-97D203375B06}"
+EndProject
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Microsoft.AspNetCore.Razor.Test.MvcShim.ClassLib", "test\Microsoft.AspNetCore.Razor.Test.MvcShim.ClassLib\Microsoft.AspNetCore.Razor.Test.MvcShim.ClassLib.csproj", "{72E89155-86C7-454E-BDD9-39F497F2F61B}"
 EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
@@ -398,6 +401,14 @@ Global
 		{6EA56B2B-89EC-4C38-A384-97D203375B06}.Release|Any CPU.Build.0 = Release|Any CPU
 		{6EA56B2B-89EC-4C38-A384-97D203375B06}.ReleaseNoVSIX|Any CPU.ActiveCfg = Release|Any CPU
 		{6EA56B2B-89EC-4C38-A384-97D203375B06}.ReleaseNoVSIX|Any CPU.Build.0 = Release|Any CPU
+		{72E89155-86C7-454E-BDD9-39F497F2F61B}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{72E89155-86C7-454E-BDD9-39F497F2F61B}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{72E89155-86C7-454E-BDD9-39F497F2F61B}.DebugNoVSIX|Any CPU.ActiveCfg = Debug|Any CPU
+		{72E89155-86C7-454E-BDD9-39F497F2F61B}.DebugNoVSIX|Any CPU.Build.0 = Debug|Any CPU
+		{72E89155-86C7-454E-BDD9-39F497F2F61B}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{72E89155-86C7-454E-BDD9-39F497F2F61B}.Release|Any CPU.Build.0 = Release|Any CPU
+		{72E89155-86C7-454E-BDD9-39F497F2F61B}.ReleaseNoVSIX|Any CPU.ActiveCfg = Release|Any CPU
+		{72E89155-86C7-454E-BDD9-39F497F2F61B}.ReleaseNoVSIX|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE
@@ -440,6 +451,7 @@ Global
 		{3E7F2D49-3B45-45A8-9893-F73EC1EEBAAB} = {3C0D6505-79B3-49D0-B4C3-176F0F1836ED}
 		{7D9ECCEE-71D1-4A42-ABEE-876AFA1B4FC9} = {3C0D6505-79B3-49D0-B4C3-176F0F1836ED}
 		{6EA56B2B-89EC-4C38-A384-97D203375B06} = {92463391-81BE-462B-AC3C-78C6C760741F}
+		{72E89155-86C7-454E-BDD9-39F497F2F61B} = {92463391-81BE-462B-AC3C-78C6C760741F}
 	EndGlobalSection
 	GlobalSection(ExtensibilityGlobals) = postSolution
 		SolutionGuid = {0035341D-175A-4D05-95E6-F1C2785A1E26}

benchmarks/Microsoft.AspNetCore.Razor.Performance/CodeGenerationBenchmark.cs

@@ -2,7 +2,6 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
-using System.Collections.Generic;
 using System.IO;
 using BenchmarkDotNet.Attributes;
 using Microsoft.AspNetCore.Mvc.Razor.Extensions;
@@ -21,33 +20,22 @@ namespace Microsoft.AspNetCore.Razor.Performance
             }
 
             var root = current;
-
-            var engine = RazorEngine.Create(b => { RazorExtensions.Register(b); });
-
             var fileSystem = RazorProjectFileSystem.Create(root.FullName);
+            
+            ProjectEngine = RazorProjectEngine.Create(RazorConfiguration.Default, fileSystem, b => RazorExtensions.Register(b)); ;
 
-            DesignTimeTemplateEngine = new MvcRazorTemplateEngine(RazorEngine.CreateDesignTime(b => { RazorExtensions.Register(b); }), fileSystem);
+            MSN = fileSystem.GetItem(Path.Combine(root.FullName, "MSN.cshtml"));
-            RuntimeTemplateEngine = new MvcRazorTemplateEngine(RazorEngine.Create(b => { RazorExtensions.Register(b); }), fileSystem);
-
-            var codeDocument = RuntimeTemplateEngine.CreateCodeDocument(Path.Combine(root.FullName, "MSN.cshtml"));
-
-            Imports = codeDocument.Imports;
-            MSN = codeDocument.Source;
         }
 
-        public RazorTemplateEngine DesignTimeTemplateEngine { get; }
+        public RazorProjectEngine ProjectEngine { get; }
 
-        public RazorTemplateEngine RuntimeTemplateEngine { get; }
+        public RazorProjectItem MSN { get; }
-
-        public IReadOnlyList<RazorSourceDocument> Imports { get; }
-
-        public RazorSourceDocument MSN { get; }
 
         [Benchmark(Description = "Razor Design Time Code Generation of MSN.com")]
         public void CodeGeneration_DesignTime_LargeStaticFile()
         {
-            var codeDocument = RazorCodeDocument.Create(MSN, Imports);
+            var codeDocument = ProjectEngine.ProcessDesignTime(MSN);
-            var generated = DesignTimeTemplateEngine.GenerateCode(codeDocument);
+            var generated = codeDocument.GetCSharpDocument();
 
             if (generated.Diagnostics.Count != 0)
             {
@@ -58,8 +46,8 @@ namespace Microsoft.AspNetCore.Razor.Performance
         [Benchmark(Description = "Razor Runtime Code Generation of MSN.com")]
         public void CodeGeneration_Runtime_LargeStaticFile()
         {
-            var codeDocument = RazorCodeDocument.Create(MSN, Imports);
+            var codeDocument = ProjectEngine.Process(MSN);
-            var generated = RuntimeTemplateEngine.GenerateCode(codeDocument);
+            var generated = codeDocument.GetCSharpDocument();
 
             if (generated.Diagnostics.Count != 0)
             {

build/dependencies.props

@@ -4,23 +4,23 @@
   </PropertyGroup>
   <PropertyGroup Label="Package Versions">
     <BenchmarkDotNetPackageVersion>0.10.11</BenchmarkDotNetPackageVersion>
-    <InternalAspNetCoreSdkPackageVersion>2.1.0-preview2-15698</InternalAspNetCoreSdkPackageVersion>
+    <InternalAspNetCoreSdkPackageVersion>2.1.0-preview2-15726</InternalAspNetCoreSdkPackageVersion>
-    <MicrosoftAspNetCoreBenchmarkRunnerSourcesPackageVersion>2.1.0-preview2-30077</MicrosoftAspNetCoreBenchmarkRunnerSourcesPackageVersion>
+    <MicrosoftAspNetCoreBenchmarkRunnerSourcesPackageVersion>2.1.0-preview2-30230</MicrosoftAspNetCoreBenchmarkRunnerSourcesPackageVersion>
-    <MicrosoftAspNetCoreHtmlAbstractionsPackageVersion>2.1.0-preview2-30077</MicrosoftAspNetCoreHtmlAbstractionsPackageVersion>
+    <MicrosoftAspNetCoreHtmlAbstractionsPackageVersion>2.1.0-preview2-30230</MicrosoftAspNetCoreHtmlAbstractionsPackageVersion>
-    <MicrosoftAspNetCoreTestingPackageVersion>2.1.0-preview2-30077</MicrosoftAspNetCoreTestingPackageVersion>
+    <MicrosoftAspNetCoreTestingPackageVersion>2.1.0-preview2-30230</MicrosoftAspNetCoreTestingPackageVersion>
-    <MicrosoftBuildFrameworkPackageVersion>15.3.409</MicrosoftBuildFrameworkPackageVersion>
+    <MicrosoftBuildFrameworkPackageVersion>15.6.82</MicrosoftBuildFrameworkPackageVersion>
-    <MicrosoftBuildPackageVersion>15.3.409</MicrosoftBuildPackageVersion>
+    <MicrosoftBuildPackageVersion>15.6.82</MicrosoftBuildPackageVersion>
-    <MicrosoftBuildUtilitiesCorePackageVersion>15.3.409</MicrosoftBuildUtilitiesCorePackageVersion>
+    <MicrosoftBuildUtilitiesCorePackageVersion>15.6.82</MicrosoftBuildUtilitiesCorePackageVersion>
     <MicrosoftCodeAnalysisCommonPackageVersion>2.6.1</MicrosoftCodeAnalysisCommonPackageVersion>
     <MicrosoftCodeAnalysisCSharpPackageVersion>2.6.1</MicrosoftCodeAnalysisCSharpPackageVersion>
-    <MicrosoftExtensionsCommandLineUtilsSourcesPackageVersion>2.1.0-preview2-30077</MicrosoftExtensionsCommandLineUtilsSourcesPackageVersion>
+    <MicrosoftExtensionsCommandLineUtilsSourcesPackageVersion>2.1.0-preview2-30230</MicrosoftExtensionsCommandLineUtilsSourcesPackageVersion>
-    <MicrosoftExtensionsCopyOnWriteDictionarySourcesPackageVersion>2.1.0-preview2-30077</MicrosoftExtensionsCopyOnWriteDictionarySourcesPackageVersion>
+    <MicrosoftExtensionsCopyOnWriteDictionarySourcesPackageVersion>2.1.0-preview2-30230</MicrosoftExtensionsCopyOnWriteDictionarySourcesPackageVersion>
     <MicrosoftExtensionsDependencyModelPackageVersion>2.1.0-preview2-25711-01</MicrosoftExtensionsDependencyModelPackageVersion>
-    <MicrosoftExtensionsHashCodeCombinerSourcesPackageVersion>2.1.0-preview2-30077</MicrosoftExtensionsHashCodeCombinerSourcesPackageVersion>
+    <MicrosoftExtensionsHashCodeCombinerSourcesPackageVersion>2.1.0-preview2-30230</MicrosoftExtensionsHashCodeCombinerSourcesPackageVersion>
-    <MicrosoftExtensionsWebEncodersPackageVersion>2.1.0-preview2-30077</MicrosoftExtensionsWebEncodersPackageVersion>
+    <MicrosoftExtensionsWebEncodersPackageVersion>2.1.0-preview2-30230</MicrosoftExtensionsWebEncodersPackageVersion>
     <MicrosoftNETCoreApp20PackageVersion>2.0.0</MicrosoftNETCoreApp20PackageVersion>
-    <MicrosoftNETCoreApp21PackageVersion>2.1.0-preview2-26130-04</MicrosoftNETCoreApp21PackageVersion>
+    <MicrosoftNETCoreApp21PackageVersion>2.1.0-preview2-26225-03</MicrosoftNETCoreApp21PackageVersion>
-    <MicrosoftNETTestSdkPackageVersion>15.3.0</MicrosoftNETTestSdkPackageVersion>
+    <MicrosoftNETTestSdkPackageVersion>15.6.0</MicrosoftNETTestSdkPackageVersion>
     <MicrosoftVisualStudioComponentModelHostPackageVersion>15.0.26606</MicrosoftVisualStudioComponentModelHostPackageVersion>
     <MicrosoftVisualStudioEditorPackageVersion>15.6.161-preview</MicrosoftVisualStudioEditorPackageVersion>
     <MicrosoftVisualStudioLanguageIntellisensePackageVersion>15.6.161-preview</MicrosoftVisualStudioLanguageIntellisensePackageVersion>
@@ -42,8 +42,8 @@
     <NETStandardLibrary20PackageVersion>2.0.1</NETStandardLibrary20PackageVersion>
     <NewtonsoftJsonPackageVersion>10.0.1</NewtonsoftJsonPackageVersion>
     <StreamJsonRpcPackageVersion>1.1.92</StreamJsonRpcPackageVersion>
-    <SystemDiagnosticsDiagnosticSourcePackageVersion>4.5.0-preview2-26130-01</SystemDiagnosticsDiagnosticSourcePackageVersion>
+    <SystemDiagnosticsDiagnosticSourcePackageVersion>4.5.0-preview2-26224-02</SystemDiagnosticsDiagnosticSourcePackageVersion>
-    <SystemValueTuplePackageVersion>4.5.0-preview2-26130-01</SystemValueTuplePackageVersion>
+    <SystemValueTuplePackageVersion>4.5.0-preview2-26224-02</SystemValueTuplePackageVersion>
     <VisualStudio_NewtonsoftJsonPackageVersion>9.0.1</VisualStudio_NewtonsoftJsonPackageVersion>
     <VSIX_MicrosoftCodeAnalysisCommonPackageVersion>2.7.0-beta3-62512-06</VSIX_MicrosoftCodeAnalysisCommonPackageVersion>
     <VSIX_MicrosoftCodeAnalysisCSharpFeaturesPackageVersion>2.7.0-beta3-62512-06</VSIX_MicrosoftCodeAnalysisCSharpFeaturesPackageVersion>

build/sources.props

@@ -1,10 +1,11 @@
-<Project>
+<Project>
   <Import Project="$(DotNetRestoreSourcePropsPath)" Condition="'$(DotNetRestoreSourcePropsPath)' != ''"/>
 
   <PropertyGroup Label="RestoreSources">
     <RestoreSources>$(DotNetRestoreSources)</RestoreSources>
     <RestoreSources Condition="'$(DotNetBuildOffline)' != 'true' AND '$(AspNetUniverseBuildOffline)' != 'true' ">
       $(RestoreSources);
+      https://dotnet.myget.org/F/dotnet-core/api/v3/index.json;
       https://dotnet.myget.org/F/aspnetcore-dev/api/v3/index.json;
       https://dotnet.myget.org/F/aspnetcore-tools/api/v3/index.json;
       https://dotnet.myget.org/F/msbuild/api/v3/index.json;

korebuild-lock.txt

@@ -1,2 +1,2 @@
-version:2.1.0-preview2-15698
+version:2.1.0-preview2-15726
-commithash:7216e5068cb1957e09d45fcbe58a744dd5c2de73
+commithash:599e691c41f502ed9e062b1822ce13b673fc916e

src/Microsoft.AspNetCore.Mvc.Razor.Extensions.Version1_X/DefaultMvcImportFeature.cs

@@ -1,84 +0,0 @@
-// Copyright (c) .NET Foundation. All rights reserved.
-// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
-
-using System;
-using System.Collections.Generic;
-using System.IO;
-using System.Linq;
-using System.Text;
-using Microsoft.AspNetCore.Razor.Language;
-
-namespace Microsoft.AspNetCore.Mvc.Razor.Extensions.Version1_X
-{
-    internal class DefaultMvcImportFeature : RazorProjectEngineFeatureBase, IRazorImportFeature
-    {
-        private const string ImportsFileName = "_ViewImports.cshtml";
-
-        public IReadOnlyList<RazorSourceDocument> GetImports(RazorProjectItem projectItem)
-        {
-            if (projectItem == null)
-            {
-                throw new ArgumentNullException(nameof(projectItem));
-            }
-
-            var imports = new List<RazorSourceDocument>();
-            AddDefaultDirectivesImport(imports);
-
-            // We add hierarchical imports second so any default directive imports can be overridden.
-            AddHierarchicalImports(projectItem, imports);
-
-            return imports;
-        }
-
-        // Internal for testing
-        internal static void AddDefaultDirectivesImport(List<RazorSourceDocument> imports)
-        {
-            using (var stream = new MemoryStream())
-            using (var writer = new StreamWriter(stream, Encoding.UTF8))
-            {
-                writer.WriteLine("@using System");
-                writer.WriteLine("@using System.Collections.Generic");
-                writer.WriteLine("@using System.Linq");
-                writer.WriteLine("@using System.Threading.Tasks");
-                writer.WriteLine("@using Microsoft.AspNetCore.Mvc");
-                writer.WriteLine("@using Microsoft.AspNetCore.Mvc.Rendering");
-                writer.WriteLine("@using Microsoft.AspNetCore.Mvc.ViewFeatures");
-                writer.WriteLine("@inject global::Microsoft.AspNetCore.Mvc.Rendering.IHtmlHelper<TModel> Html");
-                writer.WriteLine("@inject global::Microsoft.AspNetCore.Mvc.Rendering.IJsonHelper Json");
-                writer.WriteLine("@inject global::Microsoft.AspNetCore.Mvc.IViewComponentHelper Component");
-                writer.WriteLine("@inject global::Microsoft.AspNetCore.Mvc.IUrlHelper Url");
-                writer.WriteLine("@inject global::Microsoft.AspNetCore.Mvc.ViewFeatures.IModelExpressionProvider ModelExpressionProvider");
-                writer.WriteLine("@addTagHelper Microsoft.AspNetCore.Mvc.Razor.TagHelpers.UrlResolutionTagHelper, Microsoft.AspNetCore.Mvc.Razor");
-                writer.Flush();
-
-                stream.Position = 0;
-                var defaultMvcImports = RazorSourceDocument.ReadFrom(stream, fileName: null, encoding: Encoding.UTF8);
-                imports.Add(defaultMvcImports);
-            }
-        }
-
-        // Internal for testing
-        internal void AddHierarchicalImports(RazorProjectItem projectItem, List<RazorSourceDocument> imports)
-        {
-            // We want items in descending order. FindHierarchicalItems returns items in ascending order.
-            var importProjectItems = ProjectEngine.FileSystem.FindHierarchicalItems(projectItem.FilePath, ImportsFileName).Reverse();
-            foreach (var importProjectItem in importProjectItems)
-            {
-                RazorSourceDocument importSourceDocument;
-
-                if (importProjectItem.Exists)
-                {
-                    importSourceDocument = RazorSourceDocument.ReadFrom(importProjectItem);
-                }
-                else
-                {
-                    // File doesn't exist on disk so just add a marker source document as an identifier for "there could be something here".
-                    var sourceDocumentProperties = new RazorSourceDocumentProperties(importProjectItem.FilePath, importProjectItem.RelativePhysicalPath);
-                    importSourceDocument = RazorSourceDocument.Create(string.Empty, sourceDocumentProperties);
-                }
-
-                imports.Add(importSourceDocument);
-            }
-        }
-    }
-}

src/Microsoft.AspNetCore.Mvc.Razor.Extensions.Version1_X/ModelDirective.cs

@@ -29,7 +29,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions.Version1_X
             }
 
             builder.AddDirective(Directive);
-            builder.Features.Add(new Pass(builder.Configuration.DesignTime));
+            builder.Features.Add(new Pass());
             return builder;
         }
 
@@ -64,13 +64,6 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions.Version1_X
 
         internal class Pass : IntermediateNodePassBase, IRazorDirectiveClassifierPass
         {
-            private readonly bool _designTime;
-
-            public Pass(bool designTime)
-            {
-                _designTime = designTime;
-            }
-
             // Runs after the @inherits directive
             public override int Order => 5;
 
@@ -79,7 +72,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions.Version1_X
                 var visitor = new Visitor();
                 var modelType = GetModelType(documentNode, visitor);
 
-                if (_designTime)
+                if (documentNode.Options.DesignTime)
                 {
                     // Alias the TModel token to a known type.
                     // This allows design time compilation to succeed for Razor files where the token isn't replaced.
@@ -143,7 +136,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions.Version1_X
             }
 
             builder.AddDirective(Directive);
-            builder.Features.Add(new Pass(builder.DesignTime));
+            builder.Features.Add(new Pass());
             return builder;
         }
         #endregion

src/Microsoft.AspNetCore.Mvc.Razor.Extensions.Version1_X/MvcImportProjectFeature.cs

@@ -0,0 +1,89 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+using Microsoft.AspNetCore.Razor.Language;
+
+namespace Microsoft.AspNetCore.Mvc.Razor.Extensions.Version1_X
+{
+    internal class MvcImportProjectFeature : RazorProjectEngineFeatureBase, IImportProjectFeature
+    {
+        private const string ImportsFileName = "_ViewImports.cshtml";
+
+        public IReadOnlyList<RazorProjectItem> GetImports(RazorProjectItem projectItem)
+        {
+            if (projectItem == null)
+            {
+                throw new ArgumentNullException(nameof(projectItem));
+            }
+
+            var imports = new List<RazorProjectItem>();
+            AddDefaultDirectivesImport(imports);
+
+            // We add hierarchical imports second so any default directive imports can be overridden.
+            AddHierarchicalImports(projectItem, imports);
+
+            return imports;
+        }
+
+        // Internal for testing
+        internal static void AddDefaultDirectivesImport(List<RazorProjectItem> imports)
+        {
+            imports.Add(DefaultDirectivesProjectItem.Instance);
+        }
+
+        // Internal for testing
+        internal void AddHierarchicalImports(RazorProjectItem projectItem, List<RazorProjectItem> imports)
+        {
+            // We want items in descending order. FindHierarchicalItems returns items in ascending order.
+            var importProjectItems = ProjectEngine.FileSystem.FindHierarchicalItems(projectItem.FilePath, ImportsFileName).Reverse();
+            imports.AddRange(importProjectItems);
+        }
+
+        private class DefaultDirectivesProjectItem : RazorProjectItem
+        {
+            private readonly byte[] _defaultImportBytes;
+
+            private DefaultDirectivesProjectItem()
+            {
+                var preamble = Encoding.UTF8.GetPreamble();
+                var content = @"
+@using System
+@using System.Collections.Generic
+@using System.Linq
+@using System.Threading.Tasks
+@using Microsoft.AspNetCore.Mvc
+@using Microsoft.AspNetCore.Mvc.Rendering
+@using Microsoft.AspNetCore.Mvc.ViewFeatures
+@inject global::Microsoft.AspNetCore.Mvc.Rendering.IHtmlHelper<TModel> Html
+@inject global::Microsoft.AspNetCore.Mvc.Rendering.IJsonHelper Json
+@inject global::Microsoft.AspNetCore.Mvc.IViewComponentHelper Component
+@inject global::Microsoft.AspNetCore.Mvc.IUrlHelper Url
+@inject global::Microsoft.AspNetCore.Mvc.ViewFeatures.IModelExpressionProvider ModelExpressionProvider
+@addTagHelper Microsoft.AspNetCore.Mvc.Razor.TagHelpers.UrlResolutionTagHelper, Microsoft.AspNetCore.Mvc.Razor
+";
+                var contentBytes = Encoding.UTF8.GetBytes(content);
+
+                _defaultImportBytes = new byte[preamble.Length + contentBytes.Length];
+                preamble.CopyTo(_defaultImportBytes, 0);
+                contentBytes.CopyTo(_defaultImportBytes, preamble.Length);
+            }
+
+            public override string BasePath => null;
+
+            public override string FilePath => null;
+
+            public override string PhysicalPath => null;
+
+            public override bool Exists => true;
+
+            public static DefaultDirectivesProjectItem Instance { get; } = new DefaultDirectivesProjectItem();
+
+            public override Stream Read() => new MemoryStream(_defaultImportBytes);
+        }
+    }
+}

src/Microsoft.AspNetCore.Mvc.Razor.Extensions.Version1_X/RazorExtensions.cs

@@ -16,8 +16,6 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions.Version1_X
                 throw new ArgumentNullException(nameof(builder));
             }
 
-            EnsureDesignTime(builder);
-
             InjectDirective.Register(builder);
             ModelDirective.Register(builder);
 
@@ -37,7 +35,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions.Version1_X
             builder.Features.Add(new ModelExpressionPass());
             builder.Features.Add(new MvcViewDocumentClassifierPass());
 
-            builder.SetImportFeature(new DefaultMvcImportFeature());
+            builder.SetImportFeature(new MvcImportProjectFeature());
         }
 
         public static void RegisterViewComponentTagHelpers(RazorProjectEngineBuilder builder)
@@ -47,22 +45,10 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions.Version1_X
                 throw new ArgumentNullException(nameof(builder));
             }
 
-            EnsureDesignTime(builder);
-
             builder.Features.Add(new ViewComponentTagHelperPass());
             builder.AddTargetExtension(new ViewComponentTagHelperTargetExtension());
         }
 
-        private static void EnsureDesignTime(RazorProjectEngineBuilder builder)
-        {
-            if (builder.Configuration.DesignTime)
-            {
-                return;
-            }
-
-            throw new NotSupportedException(Resources.RuntimeCodeGenerationNotSupported);
-        }
-
         #region Obsolete
         public static void Register(IRazorEngineBuilder builder)
         {

src/Microsoft.AspNetCore.Mvc.Razor.Extensions/AssemblyAttributeInjectionPass.cs

@@ -14,6 +14,11 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
 
         protected override void ExecuteCore(RazorCodeDocument codeDocument, DocumentIntermediateNode documentNode)
         {
+            if (documentNode.Options.DesignTime)
+            {
+                return;
+            }
+
             var @namespace = documentNode.FindPrimaryNamespace();
             if (@namespace == null || string.IsNullOrEmpty(@namespace.Content))
             {

src/Microsoft.AspNetCore.Mvc.Razor.Extensions/DefaultMvcImportFeature.cs

@@ -1,86 +0,0 @@
-// Copyright (c) .NET Foundation. All rights reserved.
-// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
-
-using System;
-using System.Collections.Generic;
-using System.IO;
-using System.Linq;
-using System.Text;
-using Microsoft.AspNetCore.Razor.Language;
-
-namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
-{
-    internal class DefaultMvcImportFeature : RazorProjectEngineFeatureBase, IRazorImportFeature
-    {
-        private const string ImportsFileName = "_ViewImports.cshtml";
-
-        public IReadOnlyList<RazorSourceDocument> GetImports(RazorProjectItem projectItem)
-        {
-            if (projectItem == null)
-            {
-                throw new ArgumentNullException(nameof(projectItem));
-            }
-
-            var imports = new List<RazorSourceDocument>();
-            AddDefaultDirectivesImport(imports);
-
-            // We add hierarchical imports second so any default directive imports can be overridden.
-            AddHierarchicalImports(projectItem, imports);
-
-            return imports;
-        }
-
-        // Internal for testing
-        internal static void AddDefaultDirectivesImport(List<RazorSourceDocument> imports)
-        {
-            using (var stream = new MemoryStream())
-            using (var writer = new StreamWriter(stream, Encoding.UTF8))
-            {
-                writer.WriteLine("@using System");
-                writer.WriteLine("@using System.Collections.Generic");
-                writer.WriteLine("@using System.Linq");
-                writer.WriteLine("@using System.Threading.Tasks");
-                writer.WriteLine("@using Microsoft.AspNetCore.Mvc");
-                writer.WriteLine("@using Microsoft.AspNetCore.Mvc.Rendering");
-                writer.WriteLine("@using Microsoft.AspNetCore.Mvc.ViewFeatures");
-                writer.WriteLine("@inject global::Microsoft.AspNetCore.Mvc.Rendering.IHtmlHelper<TModel> Html");
-                writer.WriteLine("@inject global::Microsoft.AspNetCore.Mvc.Rendering.IJsonHelper Json");
-                writer.WriteLine("@inject global::Microsoft.AspNetCore.Mvc.IViewComponentHelper Component");
-                writer.WriteLine("@inject global::Microsoft.AspNetCore.Mvc.IUrlHelper Url");
-                writer.WriteLine("@inject global::Microsoft.AspNetCore.Mvc.ViewFeatures.IModelExpressionProvider ModelExpressionProvider");
-                writer.WriteLine("@addTagHelper Microsoft.AspNetCore.Mvc.Razor.TagHelpers.UrlResolutionTagHelper, Microsoft.AspNetCore.Mvc.Razor");
-                writer.WriteLine("@addTagHelper Microsoft.AspNetCore.Mvc.Razor.TagHelpers.HeadTagHelper, Microsoft.AspNetCore.Mvc.Razor");
-                writer.WriteLine("@addTagHelper Microsoft.AspNetCore.Mvc.Razor.TagHelpers.BodyTagHelper, Microsoft.AspNetCore.Mvc.Razor");
-                writer.Flush();
-
-                stream.Position = 0;
-                var defaultMvcImports = RazorSourceDocument.ReadFrom(stream, fileName: null, encoding: Encoding.UTF8);
-                imports.Add(defaultMvcImports);
-            }
-        }
-
-        // Internal for testing
-        internal void AddHierarchicalImports(RazorProjectItem projectItem, List<RazorSourceDocument> imports)
-        {
-            // We want items in descending order. FindHierarchicalItems returns items in ascending order.
-            var importProjectItems = ProjectEngine.FileSystem.FindHierarchicalItems(projectItem.FilePath, ImportsFileName).Reverse();
-            foreach (var importProjectItem in importProjectItems)
-            {
-                RazorSourceDocument importSourceDocument;
-
-                if (importProjectItem.Exists)
-                {
-                    importSourceDocument = RazorSourceDocument.ReadFrom(importProjectItem);
-                }
-                else
-                {
-                    // File doesn't exist on disk so just add a marker source document as an identifier for "there could be something here".
-                    var sourceDocumentProperties = new RazorSourceDocumentProperties(importProjectItem.FilePath, importProjectItem.RelativePhysicalPath);
-                    importSourceDocument = RazorSourceDocument.Create(string.Empty, sourceDocumentProperties);
-                }
-
-                imports.Add(importSourceDocument);
-            }
-        }
-    }
-}

src/Microsoft.AspNetCore.Mvc.Razor.Extensions/ExtensionInitializer.cs

@@ -0,0 +1,15 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using Microsoft.AspNetCore.Razor.Language;
+
+namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
+{
+    internal class ExtensionInitializer : RazorExtensionInitializer
+    {
+        public override void Initialize(RazorProjectEngineBuilder builder)
+        {
+            RazorExtensions.Register(builder);
+        }
+    }
+}

src/Microsoft.AspNetCore.Mvc.Razor.Extensions/InstrumentationPass.cs

@@ -15,6 +15,11 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
 
         protected override void ExecuteCore(RazorCodeDocument codeDocument, DocumentIntermediateNode documentNode)
         {
+            if (documentNode.Options.DesignTime)
+            {
+                return;
+            }
+
             var walker = new Visitor();
             walker.VisitDocument(documentNode);
 

src/Microsoft.AspNetCore.Mvc.Razor.Extensions/ModelDirective.cs

@@ -29,7 +29,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
             }
 
             builder.AddDirective(Directive);
-            builder.Features.Add(new Pass(builder.Configuration.DesignTime));
+            builder.Features.Add(new Pass());
             return builder;
         }
 
@@ -71,13 +71,6 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
 
         internal class Pass : IntermediateNodePassBase, IRazorDirectiveClassifierPass
         {
-            private readonly bool _designTime;
-
-            public Pass(bool designTime)
-            {
-                _designTime = designTime;
-            }
-
             // Runs after the @inherits directive
             public override int Order => 5;
 
@@ -86,7 +79,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
                 var visitor = new Visitor();
                 var modelType = GetModelType(documentNode, visitor);
 
-                if (_designTime)
+                if (documentNode.Options.DesignTime)
                 {
                     // Alias the TModel token to a known type.
                     // This allows design time compilation to succeed for Razor files where the token isn't replaced.
@@ -150,7 +143,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
             }
 
             builder.AddDirective(Directive);
-            builder.Features.Add(new Pass(builder.DesignTime));
+            builder.Features.Add(new Pass());
             return builder;
         }
         #endregion

src/Microsoft.AspNetCore.Mvc.Razor.Extensions/MvcImportProjectFeature.cs

@@ -0,0 +1,91 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+using Microsoft.AspNetCore.Razor.Language;
+
+namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
+{
+    internal class MvcImportProjectFeature : RazorProjectEngineFeatureBase, IImportProjectFeature
+    {
+        private const string ImportsFileName = "_ViewImports.cshtml";
+
+        public IReadOnlyList<RazorProjectItem> GetImports(RazorProjectItem projectItem)
+        {
+            if (projectItem == null)
+            {
+                throw new ArgumentNullException(nameof(projectItem));
+            }
+
+            var imports = new List<RazorProjectItem>();
+            AddDefaultDirectivesImport(imports);
+
+            // We add hierarchical imports second so any default directive imports can be overridden.
+            AddHierarchicalImports(projectItem, imports);
+
+            return imports;
+        }
+
+        // Internal for testing
+        internal static void AddDefaultDirectivesImport(List<RazorProjectItem> imports)
+        {
+            imports.Add(DefaultDirectivesProjectItem.Instance);
+        }
+
+        // Internal for testing
+        internal void AddHierarchicalImports(RazorProjectItem projectItem, List<RazorProjectItem> imports)
+        {
+            // We want items in descending order. FindHierarchicalItems returns items in ascending order.
+            var importProjectItems = ProjectEngine.FileSystem.FindHierarchicalItems(projectItem.FilePath, ImportsFileName).Reverse();
+            imports.AddRange(importProjectItems);
+        }
+
+        private class DefaultDirectivesProjectItem : RazorProjectItem
+        {
+            private readonly byte[] _defaultImportBytes;
+
+            private DefaultDirectivesProjectItem()
+            {
+                var preamble = Encoding.UTF8.GetPreamble();
+                var content = @"
+@using System
+@using System.Collections.Generic
+@using System.Linq
+@using System.Threading.Tasks
+@using Microsoft.AspNetCore.Mvc
+@using Microsoft.AspNetCore.Mvc.Rendering
+@using Microsoft.AspNetCore.Mvc.ViewFeatures
+@inject global::Microsoft.AspNetCore.Mvc.Rendering.IHtmlHelper<TModel> Html
+@inject global::Microsoft.AspNetCore.Mvc.Rendering.IJsonHelper Json
+@inject global::Microsoft.AspNetCore.Mvc.IViewComponentHelper Component
+@inject global::Microsoft.AspNetCore.Mvc.IUrlHelper Url
+@inject global::Microsoft.AspNetCore.Mvc.ViewFeatures.IModelExpressionProvider ModelExpressionProvider
+@addTagHelper Microsoft.AspNetCore.Mvc.Razor.TagHelpers.UrlResolutionTagHelper, Microsoft.AspNetCore.Mvc.Razor
+@addTagHelper Microsoft.AspNetCore.Mvc.Razor.TagHelpers.HeadTagHelper, Microsoft.AspNetCore.Mvc.Razor
+@addTagHelper Microsoft.AspNetCore.Mvc.Razor.TagHelpers.BodyTagHelper, Microsoft.AspNetCore.Mvc.Razor
+";
+                var contentBytes = Encoding.UTF8.GetBytes(content);
+
+                _defaultImportBytes = new byte[preamble.Length + contentBytes.Length];
+                preamble.CopyTo(_defaultImportBytes, 0);
+                contentBytes.CopyTo(_defaultImportBytes, preamble.Length);
+            }
+
+            public override string BasePath => null;
+
+            public override string FilePath => null;
+
+            public override string PhysicalPath => null;
+
+            public override bool Exists => true;
+
+            public static DefaultDirectivesProjectItem Instance { get; } = new DefaultDirectivesProjectItem();
+
+            public override Stream Read() => new MemoryStream(_defaultImportBytes);
+        }
+    }
+}

src/Microsoft.AspNetCore.Mvc.Razor.Extensions/Properties/AssemblyInfo.cs

@@ -2,6 +2,11 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System.Runtime.CompilerServices;
+using Microsoft.AspNetCore.Mvc.Razor.Extensions;
+using Microsoft.AspNetCore.Razor.Language;
+
+[assembly: ProvideRazorExtensionInitializer("MVC-2.0", typeof(ExtensionInitializer))]
+[assembly: ProvideRazorExtensionInitializer("MVC-2.1", typeof(ExtensionInitializer))]
 
 [assembly: InternalsVisibleTo("Microsoft.AspNetCore.Mvc.Razor.Extensions.Test, PublicKey=0024000004800000940000000602000000240000525341310004000001000100f33a29044fa9d740c9b3213a93e57c84b472c84e0b8a0e1ae48e67a9f8f6de9d5f7f3d52ac23e48ac51801f1dc950abe901da34d2a9e3baadb141a17c77ef3c565dd5ee5054b91cf63bb3c6ab83f72ab3aafe93d0fc3c2348b764fafb0b1c0733de51459aeab46580384bf9d74c4e28164b7cde247f891ba07891c9d872ad2bb")]
 [assembly: InternalsVisibleTo("Microsoft.VisualStudio.Editor.Razor, PublicKey=0024000004800000940000000602000000240000525341310004000001000100f33a29044fa9d740c9b3213a93e57c84b472c84e0b8a0e1ae48e67a9f8f6de9d5f7f3d52ac23e48ac51801f1dc950abe901da34d2a9e3baadb141a17c77ef3c565dd5ee5054b91cf63bb3c6ab83f72ab3aafe93d0fc3c2348b764fafb0b1c0733de51459aeab46580384bf9d74c4e28164b7cde247f891ba07891c9d872ad2bb")]

src/Microsoft.AspNetCore.Mvc.Razor.Extensions/RazorExtensions.cs

@@ -25,6 +25,8 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
             InheritsDirective.Register(builder);
             SectionDirective.Register(builder);
 
+            builder.Features.Add(new ViewComponentTagHelperDescriptorProvider());
+
             builder.AddTargetExtension(new ViewComponentTagHelperTargetExtension());
             builder.AddTargetExtension(new TemplateTargetExtension()
             {
@@ -36,14 +38,10 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
             builder.Features.Add(new ViewComponentTagHelperPass());
             builder.Features.Add(new RazorPageDocumentClassifierPass());
             builder.Features.Add(new MvcViewDocumentClassifierPass());
+            builder.Features.Add(new AssemblyAttributeInjectionPass());
+            builder.Features.Add(new InstrumentationPass());
 
-            if (!builder.Configuration.DesignTime)
+            builder.SetImportFeature(new MvcImportProjectFeature());
-            {
-                builder.Features.Add(new AssemblyAttributeInjectionPass());
-                builder.Features.Add(new InstrumentationPass());
-            }
-
-            builder.SetImportFeature(new DefaultMvcImportFeature());
         }
 
         #region Obsolete

src/Microsoft.AspNetCore.Mvc.Razor.Extensions/build/netstandard2.0/Microsoft.AspNetCore.Mvc.Razor.Extensions.props

@@ -10,6 +10,9 @@
       Set the primary configuration supported by this pacakge as the default configuration for Razor. 
     -->
     <RazorDefaultConfiguration Condition="'$(RazorDefaultConfiguration)'==''">MVC-2.1</RazorDefaultConfiguration>
+
+    <!-- Override for testing. This path is only correct inside a nuget package. -->
+    <_MvcExtensionAssemblyPath Condition="'$(_MvcExtensionAssemblyPath)'==''">$(MSBuildThisFileDirectory)..\..\lib\netstandard2.0\Microsoft.AspNetCore.Mvc.Razor.Extensions.dll</_MvcExtensionAssemblyPath>
   </PropertyGroup>
 
   <ItemGroup>
@@ -26,7 +29,7 @@
   <ItemGroup>
     <RazorExtension Include="MVC-2.1">
       <AssemblyName>Microsoft.AspNetCore.Mvc.Razor.Extensions</AssemblyName>
-      <AssemblyFilePath>$(MSBuildThisFileDirectory)..\..\lib\netstandard2.0\Microsoft.AspNetCore.Mvc.Razor.Extensions.dll</AssemblyFilePath>
+      <AssemblyFilePath>$(_MvcExtensionAssemblyPath)</AssemblyFilePath>
     </RazorExtension>
   </ItemGroup>
 </Project>

src/Microsoft.AspNetCore.Mvc.Razor.Extensions/build/netstandard2.0/Microsoft.AspNetCore.Mvc.Razor.Extensions.targets

@@ -5,5 +5,10 @@
       MVC will generally want to add support for runtime compilation, but only for applications. 
     -->
     <GenerateRazorAssemblyInfo Condition="'$(GenerateRazorAssemblyInfo)'=='' and '$(OutputType)'=='Exe'">true</GenerateRazorAssemblyInfo>
+
+    <!--
+      Use the suffix .Views when producing compiled view assemblies. This matches the requirements for Mvc's ViewsFeatureProvider.
+    -->
+    <RazorTargetNameSuffix Condition="'$(RazorTargetNameSuffix)'==''">.Views</RazorTargetNameSuffix>
   </PropertyGroup>
-</Project>
+</Project>

src/Microsoft.AspNetCore.Razor.Design/build/netstandard2.0/Microsoft.AspNetCore.Razor.Design.CodeGeneration.targets

@@ -60,11 +60,11 @@
       -->
     <Touch
       Files="$(_RazorTagHelperInputCache)"
-      AlwaysCreate="true">
+      AlwaysCreate="true" />
-      <Output
+
-        TaskParameter="TouchedFiles"
+    <ItemGroup>
-        ItemName="FileWrites" />
+      <FileWrites Include="$(_RazorTagHelperInputCache)" />
-    </Touch>
+    </ItemGroup>
 
     <RazorTagHelper
       Debug="$(_RazorDebugTagHelperTask)"
@@ -73,6 +73,9 @@
       UseServer="$(UseRazorBuildServer)"
       ForceServer="$(_RazorForceBuildServer)"
       PipeName="$(_RazorBuildServerPipeName)"
+      Version="$(RazorLangVersion)"
+      Configuration="@(ResolvedRazorConfiguration)"
+      Extensions="@(ResolvedRazorExtension)"
       Assemblies="@(RazorReferencePath)"
       ProjectRoot="$(MSBuildProjectDirectory)"
       TagHelperManifest="$(_RazorTagHelperOutputCache)">
@@ -121,6 +124,9 @@
       UseServer="$(UseRazorBuildServer)"
       ForceServer="$(_RazorForceBuildServer)"
       PipeName="$(_RazorBuildServerPipeName)"
+      Version="$(RazorLangVersion)"
+      Configuration="@(ResolvedRazorConfiguration)"
+      Extensions="@(ResolvedRazorExtension)"
       Sources="@(RazorGenerateWithTargetPath)"
       ProjectRoot="$(MSBuildProjectDirectory)"
       TagHelperManifest="$(_RazorTagHelperOutputCache)" />

src/Microsoft.AspNetCore.Razor.Language/AssemblyExtension.cs

@@ -0,0 +1,55 @@
+// Copyright(c) .NET Foundation.All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Reflection;
+
+namespace Microsoft.AspNetCore.Razor.Language
+{
+    public class AssemblyExtension : RazorExtension
+    {
+        public AssemblyExtension(string extensionName, Assembly assembly)
+        {
+            if (extensionName == null)
+            {
+                throw new ArgumentNullException(nameof(extensionName));
+            }
+
+            if (assembly == null)
+            {
+                throw new ArgumentNullException(nameof(assembly));
+            }
+
+            ExtensionName = extensionName;
+            Assembly = assembly;
+        }
+
+        public override string ExtensionName { get; }
+
+        public Assembly Assembly { get; }
+
+        internal RazorExtensionInitializer CreateInitializer()
+        {
+            // It's not an error to have an assembly with no initializers. This is useful to specify a dependency
+            // that doesn't really provide any Razor configuration.
+            var attributes = Assembly.GetCustomAttributes<ProvideRazorExtensionInitializerAttribute>();
+            foreach (var attribute in attributes)
+            {
+                // Using extension names and requiring them to line up allows a single assembly to ship multiple
+                // extensions/initializers for different configurations.
+                if (!string.Equals(attribute.ExtensionName, ExtensionName, StringComparison.Ordinal))
+                {
+                    continue;
+                }
+
+                // There's no real protection/exception handling here because this set isn't really user-extensible
+                // right now. This would be a great place to add some additional diagnostics and hardening in the
+                // future.
+                var initializer = (RazorExtensionInitializer)Activator.CreateInstance(attribute.InitializerType);
+                return initializer;
+            }
+
+            return null;
+        }
+    }
+}

src/Microsoft.AspNetCore.Razor.Language/CodeGeneration/CodeWriterExtensions.cs

@@ -540,7 +540,7 @@ namespace Microsoft.AspNetCore.Razor.Language.CodeGeneration
             public CSharpCodeWritingScope(CodeWriter writer, int tabSize = 4, bool autoSpace = true)
             {
                 _writer = writer;
-                _autoSpace = true;
+                _autoSpace = autoSpace;
                 _tabSize = tabSize;
                 _startIndent = -1; // Set in WriteStartScope
 

src/Microsoft.AspNetCore.Razor.Language/CodeGeneration/DefaultDocumentWriter.cs

@@ -159,11 +159,35 @@ namespace Microsoft.AspNetCore.Razor.Language.CodeGeneration
                     Context.CodeWriter.Write(" ");
                 }
 
-                Context.CodeWriter
+                Context.CodeWriter.Write(node.ReturnType);
-                    .Write(node.ReturnType)
+                Context.CodeWriter.Write(" ");
-                    .Write(" ")
+
-                    .Write(node.MethodName)
+                Context.CodeWriter.Write(node.MethodName);
-                    .WriteLine("()");
+                Context.CodeWriter.Write("(");
+
+                for (var i = 0; i < node.Parameters.Count; i++)
+                {
+                    var parameter = node.Parameters[i];
+
+                    for (var j = 0; j < parameter.Modifiers.Count; j++)
+                    {
+                        Context.CodeWriter.Write(parameter.Modifiers[j]);
+                        Context.CodeWriter.Write(" ");
+                    }
+
+                    Context.CodeWriter.Write(parameter.TypeName);
+                    Context.CodeWriter.Write(" ");
+
+                    Context.CodeWriter.Write(parameter.ParameterName);
+
+                    if (i < node.Parameters.Count - 1)
+                    {
+                        Context.CodeWriter.Write(", ");
+                    }
+                }
+
+                Context.CodeWriter.Write(")");
+                Context.CodeWriter.WriteLine();
 
                 using (Context.CodeWriter.BuildScope())
                 {

src/Microsoft.AspNetCore.Razor.Language/CodeGeneration/DocumentWriter.cs

@@ -2,12 +2,30 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.ComponentModel;
 using Microsoft.AspNetCore.Razor.Language.Intermediate;
 
 namespace Microsoft.AspNetCore.Razor.Language.CodeGeneration
 {
     public abstract class DocumentWriter
     {
+        public static DocumentWriter CreateDefault(CodeTarget codeTarget, RazorCodeGenerationOptions options)
+        {
+            if (codeTarget == null)
+            {
+                throw new ArgumentNullException(nameof(codeTarget));
+            }
+
+            if (options == null)
+            {
+                throw new ArgumentNullException(nameof(options));
+            }
+
+            return new DefaultDocumentWriter(codeTarget, options);
+        }
+
+        [EditorBrowsable(EditorBrowsableState.Never)]
+        [Obsolete("This method was intended to be static, use CreateDefault instead.")]
         public DocumentWriter Create(CodeTarget codeTarget, RazorCodeGenerationOptions options)
         {
             if (codeTarget == null)

src/Microsoft.AspNetCore.Razor.Language/DefaultImportProjectFeature.cs

@@ -6,8 +6,8 @@ using System.Collections.Generic;
 
 namespace Microsoft.AspNetCore.Razor.Language
 {
-    internal class DefaultRazorImportFeature : RazorProjectEngineFeatureBase, IRazorImportFeature
+    internal class DefaultImportProjectFeature : RazorProjectEngineFeatureBase, IImportProjectFeature
     {
-        public IReadOnlyList<RazorSourceDocument> GetImports(RazorProjectItem projectItem) => Array.Empty<RazorSourceDocument>();
+        public IReadOnlyList<RazorProjectItem> GetImports(RazorProjectItem projectItem) => Array.Empty<RazorProjectItem>();
     }
 }

src/Microsoft.AspNetCore.Razor.Language/DefaultRazorCSharpLoweringPhase.cs

@@ -25,7 +25,7 @@ namespace Microsoft.AspNetCore.Razor.Language
                 throw new InvalidOperationException(message);
             }
 
-            var writer = new DefaultDocumentWriter(documentNode.Target, documentNode.Options);
+            var writer = DocumentWriter.CreateDefault(documentNode.Target, documentNode.Options);
             var cSharpDocument = writer.WriteDocument(codeDocument, documentNode);
             codeDocument.SetCSharpDocument(cSharpDocument);
         }

src/Microsoft.AspNetCore.Razor.Language/DefaultRazorCodeGenerationOptionsBuilder.cs

@@ -1,16 +1,30 @@
 // Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
+using System;
+
 namespace Microsoft.AspNetCore.Razor.Language
 {
     internal class DefaultRazorCodeGenerationOptionsBuilder : RazorCodeGenerationOptionsBuilder
     {
-        public DefaultRazorCodeGenerationOptionsBuilder(bool designTime)
+        private bool _designTime;
+
+        public DefaultRazorCodeGenerationOptionsBuilder(RazorConfiguration configuration)
         {
-            DesignTime = designTime;
+            if (configuration == null)
+            {
+                throw new ArgumentNullException(nameof(configuration));
+            }
         }
 
-        public override bool DesignTime { get; }
+        public DefaultRazorCodeGenerationOptionsBuilder(bool designTime)
+        {
+            _designTime = designTime;
+        }
+
+        public override RazorConfiguration Configuration { get; }
+
+        public override bool DesignTime => _designTime;
 
         public override int IndentSize { get; set; } = 4;
 
@@ -22,5 +36,10 @@ namespace Microsoft.AspNetCore.Razor.Language
         {
             return new DefaultRazorCodeGenerationOptions(IndentWithTabs, IndentSize, DesignTime, SuppressChecksum, SuppressMetadataAttributes);
         }
+
+        public override void SetDesignTime(bool designTime)
+        {
+            _designTime = designTime;
+        }
     }
 }

src/Microsoft.AspNetCore.Razor.Language/DefaultRazorCodeGenerationOptionsFactoryProjectFeature.cs

@@ -0,0 +1,32 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Linq;
+
+namespace Microsoft.AspNetCore.Razor.Language
+{
+    internal class DefaultRazorCodeGenerationOptionsFactoryProjectFeature : RazorProjectEngineFeatureBase, IRazorCodeGenerationOptionsFactoryProjectFeature
+    {
+        private IConfigureRazorCodeGenerationOptionsFeature[] _configureOptions;
+
+        protected override void OnInitialized()
+        {
+            _configureOptions = ProjectEngine.EngineFeatures.OfType<IConfigureRazorCodeGenerationOptionsFeature>().ToArray();
+        }
+
+        public RazorCodeGenerationOptions Create(Action<RazorCodeGenerationOptionsBuilder> configure)
+        {
+            var builder = new DefaultRazorCodeGenerationOptionsBuilder(ProjectEngine.Configuration);
+            configure?.Invoke(builder);
+
+            for (var i = 0; i < _configureOptions.Length; i++)
+            {
+                _configureOptions[i].Configure(builder);
+            }
+
+            var options = builder.Build();
+            return options;
+        }
+    }
+}

src/Microsoft.AspNetCore.Razor.Language/DefaultRazorCodeGenerationOptionsFeature.cs

@@ -5,7 +5,9 @@ using System.Linq;
 
 namespace Microsoft.AspNetCore.Razor.Language
 {
+#pragma warning disable CS0618 // Type or member is obsolete
     internal class DefaultRazorCodeGenerationOptionsFeature : RazorEngineFeatureBase, IRazorCodeGenerationOptionsFeature
+#pragma warning restore CS0618 // Type or member is obsolete
     {
         private readonly bool _designTime;
         private IConfigureRazorCodeGenerationOptionsFeature[] _configureOptions;

src/Microsoft.AspNetCore.Razor.Language/DefaultRazorIntermediateNodeLoweringPhase.cs

@@ -11,6 +11,7 @@ using Microsoft.AspNetCore.Razor.Language.Legacy;
 
 namespace Microsoft.AspNetCore.Razor.Language
 {
+#pragma warning disable CS0618 // Type or member is obsolete
     internal class DefaultRazorIntermediateNodeLoweringPhase : RazorEnginePhaseBase, IRazorIntermediateNodeLoweringPhase
     {
         private IRazorCodeGenerationOptionsFeature _optionsFeature;
@@ -31,7 +32,7 @@ namespace Microsoft.AspNetCore.Razor.Language
             var document = new DocumentIntermediateNode();
             var builder = IntermediateNodeBuilder.Create(document);
 
-            document.Options = _optionsFeature.GetOptions();
+            document.Options = codeDocument.GetCodeGenerationOptions() ?? _optionsFeature.GetOptions();
 
             var namespaces = new Dictionary<string, SourceSpan?>(StringComparer.Ordinal);
 
@@ -785,4 +786,5 @@ namespace Microsoft.AspNetCore.Razor.Language
         private static bool IsMalformed(List<RazorDiagnostic> diagnostics)
             => diagnostics.Count > 0 && diagnostics.Any(diagnostic => diagnostic.Severity == RazorDiagnosticSeverity.Error);
     }
+#pragma warning restore CS0618 // Type or member is obsolete
 }

src/Microsoft.AspNetCore.Razor.Language/DefaultRazorParserOptionsBuilder.cs

@@ -9,23 +9,43 @@ namespace Microsoft.AspNetCore.Razor.Language
 {
     internal class DefaultRazorParserOptionsBuilder : RazorParserOptionsBuilder
     {
-        public DefaultRazorParserOptionsBuilder(bool designTime, RazorLanguageVersion version)
+        private bool _designTime;
+
+        public DefaultRazorParserOptionsBuilder(RazorConfiguration configuration)
         {
-            DesignTime = designTime;
+            if (configuration == null)
-            Version = version;
+            {
+                throw new ArgumentNullException(nameof(configuration));
+            }
+
+            Configuration = configuration;
+            LanguageVersion = configuration.LanguageVersion;
         }
 
-        public override bool DesignTime { get; }
+        public DefaultRazorParserOptionsBuilder(bool designTime, RazorLanguageVersion version)
+        {
+            _designTime = designTime;
+            LanguageVersion = version;
+        }
+
+        public override RazorConfiguration Configuration { get; }
+
+        public override bool DesignTime => _designTime;
 
         public override ICollection<DirectiveDescriptor> Directives { get; } = new List<DirectiveDescriptor>();
 
         public override bool ParseLeadingDirectives { get; set; }
 
-        public override RazorLanguageVersion Version { get; }
+        public override RazorLanguageVersion LanguageVersion { get; }
 
         public override RazorParserOptions Build()
         {
-            return new DefaultRazorParserOptions(Directives.ToArray(), DesignTime, ParseLeadingDirectives, Version);
+            return new DefaultRazorParserOptions(Directives.ToArray(), DesignTime, ParseLeadingDirectives, LanguageVersion);
+        }
+
+        public override void SetDesignTime(bool designTime)
+        {
+            _designTime = designTime;
         }
     }
 }

src/Microsoft.AspNetCore.Razor.Language/DefaultRazorParserOptionsFactoryProjectFeature.cs

@@ -0,0 +1,32 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Linq;
+
+namespace Microsoft.AspNetCore.Razor.Language
+{
+    internal class DefaultRazorParserOptionsFactoryProjectFeature : RazorProjectEngineFeatureBase, IRazorParserOptionsFactoryProjectFeature
+    {
+        private IConfigureRazorParserOptionsFeature[] _configureOptions;
+
+        protected override void OnInitialized()
+        {
+            _configureOptions = ProjectEngine.EngineFeatures.OfType<IConfigureRazorParserOptionsFeature>().ToArray();
+        }
+
+        public RazorParserOptions Create(Action<RazorParserOptionsBuilder> configure)
+        {
+            var builder = new DefaultRazorParserOptionsBuilder(ProjectEngine.Configuration);
+            configure?.Invoke(builder);
+
+            for (var i = 0; i < _configureOptions.Length; i++)
+            {
+                _configureOptions[i].Configure(builder);
+            }
+            
+            var options = builder.Build();
+            return options;
+        }
+    }
+}

src/Microsoft.AspNetCore.Razor.Language/DefaultRazorParserOptionsFeature.cs

@@ -5,7 +5,9 @@ using System.Linq;
 
 namespace Microsoft.AspNetCore.Razor.Language
 {
+#pragma warning disable CS0618 // Type or member is obsolete
     internal class DefaultRazorParserOptionsFeature : RazorEngineFeatureBase, IRazorParserOptionsFeature
+#pragma warning restore CS0618 // Type or member is obsolete
     {
         private readonly bool _designTime;
         private readonly RazorLanguageVersion _version;

src/Microsoft.AspNetCore.Razor.Language/DefaultRazorParsingPhase.cs

@@ -3,6 +3,7 @@
 
 namespace Microsoft.AspNetCore.Razor.Language
 {
+#pragma warning disable CS0618 // Type or member is obsolete
     internal class DefaultRazorParsingPhase : RazorEnginePhaseBase, IRazorParsingPhase
     {
         private IRazorParserOptionsFeature _optionsFeature;
@@ -14,7 +15,7 @@ namespace Microsoft.AspNetCore.Razor.Language
 
         protected override void ExecuteCore(RazorCodeDocument codeDocument)
         {
-            var options = _optionsFeature.GetOptions();
+            var options = codeDocument.GetParserOptions() ??_optionsFeature.GetOptions();
             var syntaxTree = RazorSyntaxTree.Parse(codeDocument.Source, options);
             codeDocument.SetSyntaxTree(syntaxTree);
 
@@ -26,4 +27,5 @@ namespace Microsoft.AspNetCore.Razor.Language
             codeDocument.SetImportSyntaxTrees(importSyntaxTrees);
         }
     }
+#pragma warning restore CS0618 // Type or member is obsolete
 }

src/Microsoft.AspNetCore.Razor.Language/DefaultRazorProjectEngine.cs

@@ -10,10 +10,16 @@ namespace Microsoft.AspNetCore.Razor.Language
     internal class DefaultRazorProjectEngine : RazorProjectEngine
     {
         public DefaultRazorProjectEngine(
+            RazorConfiguration configuration,
             RazorEngine engine,
             RazorProjectFileSystem fileSystem,
-            IReadOnlyList<IRazorProjectEngineFeature> features)
+            IReadOnlyList<IRazorProjectEngineFeature> projectFeatures)
         {
+            if (configuration == null)
+            {
+                throw new ArgumentNullException(nameof(configuration));
+            }
+
             if (engine == null)
             {
                 throw new ArgumentNullException(nameof(engine));
@@ -24,48 +30,81 @@ namespace Microsoft.AspNetCore.Razor.Language
                 throw new ArgumentNullException(nameof(fileSystem));
             }
 
-            if (features == null)
+            if (projectFeatures == null)
             {
-                throw new ArgumentNullException(nameof(features));
+                throw new ArgumentNullException(nameof(projectFeatures));
             }
 
+            Configuration = configuration;
             Engine = engine;
             FileSystem = fileSystem;
-            Features = features;
+            ProjectFeatures = projectFeatures;
 
-            for (var i = 0; i < features.Count; i++)
+            for (var i = 0; i < projectFeatures.Count; i++)
             {
-                features[i].ProjectEngine = this;
+                projectFeatures[i].ProjectEngine = this;
             }
         }
 
+        public override RazorConfiguration Configuration { get; }
+
         public override RazorProjectFileSystem FileSystem { get; }
 
         public override RazorEngine Engine { get; }
 
-        public override IReadOnlyList<IRazorProjectEngineFeature> Features { get; }
+        public override IReadOnlyList<IRazorProjectEngineFeature> ProjectFeatures { get; }
 
-        public override RazorCodeDocument Process(RazorProjectItem projectItem)
+        protected override RazorCodeDocument CreateCodeDocumentCore(RazorProjectItem projectItem)
         {
             if (projectItem == null)
             {
                 throw new ArgumentNullException(nameof(projectItem));
             }
 
-            var importFeature = GetRequiredFeature<IRazorImportFeature>();
-            var imports = importFeature.GetImports(projectItem);
             var sourceDocument = RazorSourceDocument.ReadFrom(projectItem);
 
-            var codeDocument = RazorCodeDocument.Create(sourceDocument, imports);
+            var importFeature = GetRequiredFeature<IImportProjectFeature>();
+            var importItems = importFeature.GetImports(projectItem);
+            var importSourceDocuments = GetImportSourceDocuments(importItems);
+
+            var parserOptions = GetRequiredFeature<IRazorParserOptionsFactoryProjectFeature>().Create(ConfigureParserOptions);
+            var codeGenerationOptions = GetRequiredFeature<IRazorCodeGenerationOptionsFactoryProjectFeature>().Create(ConfigureCodeGenerationOptions);
+
+            return RazorCodeDocument.Create(sourceDocument, importSourceDocuments, parserOptions, codeGenerationOptions);
+        }
+
+        protected override RazorCodeDocument CreateCodeDocumentDesignTimeCore(RazorProjectItem projectItem)
+        {
+            if (projectItem == null)
+            {
+                throw new ArgumentNullException(nameof(projectItem));
+            }
+
+            var sourceDocument = RazorSourceDocument.ReadFrom(projectItem);
+
+            var importFeature = GetRequiredFeature<IImportProjectFeature>();
+            var importItems = importFeature.GetImports(projectItem);
+            var importSourceDocuments = GetImportSourceDocuments(importItems);
+
+            var parserOptions = GetRequiredFeature<IRazorParserOptionsFactoryProjectFeature>().Create(ConfigureDesignTimeParserOptions);
+            var codeGenerationOptions = GetRequiredFeature<IRazorCodeGenerationOptionsFactoryProjectFeature>().Create(ConfigureDesignTimeCodeGenerationOptions);
+
+            return RazorCodeDocument.Create(sourceDocument, importSourceDocuments, parserOptions, codeGenerationOptions);
+        }
+
+        protected override void ProcessCore(RazorCodeDocument codeDocument)
+        {
+            if (codeDocument == null)
+            {
+                throw new ArgumentNullException(nameof(codeDocument));
+            }
 
             Engine.Process(codeDocument);
-
-            return codeDocument;
         }
 
         private TFeature GetRequiredFeature<TFeature>() where TFeature : IRazorProjectEngineFeature
         {
-            var feature = Features.OfType<TFeature>().FirstOrDefault();
+            var feature = ProjectFeatures.OfType<TFeature>().FirstOrDefault();
             if (feature == null)
             {
                 throw new InvalidOperationException(
@@ -76,5 +115,43 @@ namespace Microsoft.AspNetCore.Razor.Language
 
             return feature;
         }
+
+        private void ConfigureParserOptions(RazorParserOptionsBuilder builder)
+        {
+        }
+
+        private void ConfigureDesignTimeParserOptions(RazorParserOptionsBuilder builder)
+        {
+            builder.SetDesignTime(true);
+        }
+
+        private void ConfigureCodeGenerationOptions(RazorCodeGenerationOptionsBuilder builder)
+        {
+        }
+
+        private void ConfigureDesignTimeCodeGenerationOptions(RazorCodeGenerationOptionsBuilder builder)
+        {
+            builder.SetDesignTime(true);
+            builder.SuppressChecksum = true;
+            builder.SuppressMetadataAttributes = true;
+        }
+
+        // Internal for testing
+        internal static IReadOnlyList<RazorSourceDocument> GetImportSourceDocuments(IReadOnlyList<RazorProjectItem> importItems)
+        {
+            var imports = new List<RazorSourceDocument>();
+            for (var i = 0; i < importItems.Count; i++)
+            {
+                var importItem = importItems[i];
+
+                if (importItem.Exists)
+                {
+                    var sourceDocument = RazorSourceDocument.ReadFrom(importItem);
+                    imports.Add(sourceDocument);
+                }
+            }
+
+            return imports;
+        }
     }
 }

src/Microsoft.AspNetCore.Razor.Language/DefaultRazorProjectEngineBuilder.cs

@@ -22,29 +22,19 @@ namespace Microsoft.AspNetCore.Razor.Language
             Phases = new List<IRazorEnginePhase>();
         }
 
+        public override RazorConfiguration Configuration { get; }
+
         public override RazorProjectFileSystem FileSystem { get; }
 
         public override ICollection<IRazorFeature> Features { get; }
 
         public override IList<IRazorEnginePhase> Phases { get; }
-
+        
-        public override RazorConfiguration Configuration { get; }
-
         public override RazorProjectEngine Build()
         {
-            RazorEngine engine = null;
+            var engine = RazorEngine.CreateEmpty(ConfigureRazorEngine);
-
+            var projectFeatures = Features.OfType<IRazorProjectEngineFeature>().ToArray();
-            if (Configuration.DesignTime)
+            var projectEngine = new DefaultRazorProjectEngine(Configuration, engine, FileSystem, projectFeatures);
-            {
-                engine = RazorEngine.CreateDesignTimeEmpty(ConfigureRazorEngine);
-            }
-            else
-            {
-                engine = RazorEngine.CreateEmpty(ConfigureRazorEngine);
-            }
-
-            var projectEngineFeatures = Features.OfType<IRazorProjectEngineFeature>().ToArray();
-            var projectEngine = new DefaultRazorProjectEngine(engine, FileSystem, projectEngineFeatures);
 
             return projectEngine;
         }

src/Microsoft.AspNetCore.Razor.Language/EmptyProjectFileSystem.cs

@@ -0,0 +1,23 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Collections.Generic;
+using System.Linq;
+
+namespace Microsoft.AspNetCore.Razor.Language
+{
+    internal class EmptyProjectFileSystem : RazorProjectFileSystem
+    {
+        public override IEnumerable<RazorProjectItem> EnumerateItems(string basePath)
+        {
+            NormalizeAndEnsureValidPath(basePath);
+            return Enumerable.Empty<RazorProjectItem>();
+        }
+
+        public override RazorProjectItem GetItem(string path)
+        {
+            NormalizeAndEnsureValidPath(path);
+            return new NotFoundProjectItem(string.Empty, path);
+        }
+    }
+}

src/Microsoft.AspNetCore.Razor.Language/Extensions/DefaultTagHelperTargetExtension.cs

@@ -16,8 +16,6 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
     {
         private static readonly string[] PrivateModifiers = new string[] { "private" };
 
-        public bool DesignTime { get; set; }
-
         public string RunnerVariableName { get; set; } = "__tagHelperRunner";
 
         public string StringValueBufferVariableName { get; set; } = "__tagHelperStringValueBuffer";
@@ -82,7 +80,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
                 throw new InvalidOperationException(message);
             }
 
-            if (DesignTime)
+            if (context.Options.DesignTime)
             {
                 context.RenderChildren(node);
             }
@@ -136,7 +134,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
                 .Write(CreateTagHelperMethodName)
                 .WriteLine("<global::" + node.TypeName + ">();");
 
-            if (!DesignTime)
+            if (!context.Options.DesignTime)
             {
                 context.CodeWriter.WriteInstanceMethodInvocation(
                     ExecutionContextVariableName,
@@ -153,7 +151,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
                 throw new InvalidOperationException(message);
             }
 
-            if (!DesignTime)
+            if (!context.Options.DesignTime)
             {
                 context.CodeWriter
                     .Write("await ")
@@ -200,7 +198,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
                 throw new InvalidOperationException(message);
             }
 
-            if (DesignTime)
+            if (context.Options.DesignTime)
             {
                 context.RenderChildren(node);
             }
@@ -284,7 +282,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
                 throw new InvalidOperationException(message);
             }
 
-            if (!DesignTime)
+            if (!context.Options.DesignTime)
             {
                 // Ensure that the property we're trying to set has initialized its dictionary bound properties.
                 if (node.IsIndexerNameMatch &&
@@ -338,7 +336,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
             // If we get there, this is the first time seeing this property so we need to evaluate the expression.
             if (node.BoundAttribute.ExpectsStringValue(node.AttributeName))
             {
-                if (DesignTime)
+                if (context.Options.DesignTime)
                 {
                     context.RenderChildren(node);
 
@@ -370,7 +368,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
             }
             else
             {
-                if (DesignTime)
+                if (context.Options.DesignTime)
                 {
                     var firstMappedChild = node.Children.FirstOrDefault(child => child.Source != null) as IntermediateNode;
                     var valueStart = firstMappedChild?.Source;
@@ -456,7 +454,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
                 }
             }
 
-            if (!DesignTime)
+            if (!context.Options.DesignTime)
             {
                 // We need to inform the context of the attribute value.
                 context.CodeWriter
@@ -474,7 +472,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
 
         public void WriteTagHelperRuntime(CodeRenderingContext context, DefaultTagHelperRuntimeIntermediateNode node)
         {
-            if (!DesignTime)
+            if (!context.Options.DesignTime)
             {
                 context.CodeWriter.WriteLine("#line hidden");
 
@@ -566,7 +564,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
             }
             else if (node is IntermediateToken token)
             {
-                if (DesignTime && node.Source != null)
+                if (context.Options.DesignTime && node.Source != null)
                 {
                     context.AddSourceMappingFor(node);
                 }

src/Microsoft.AspNetCore.Razor.Language/Extensions/DesignTimeDirectivePass.cs

@@ -15,6 +15,13 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
 
         protected override void ExecuteCore(RazorCodeDocument codeDocument, DocumentIntermediateNode documentNode)
         {
+            // Only supports design time. This pass rewrites directives so they will have the right design time
+            // behavior and would break things if it ran for runtime.
+            if (!documentNode.Options.DesignTime)
+            {
+                return;
+            }
+
             var walker = new DesignTimeHelperWalker();
             walker.VisitDocument(documentNode);
         }

src/Microsoft.AspNetCore.Razor.Language/Extensions/PreallocatedTagHelperAttributeOptimizationPass.cs

@@ -15,6 +15,12 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
 
         protected override void ExecuteCore(RazorCodeDocument codeDocument, DocumentIntermediateNode documentNode)
         {
+            // There's no value in executing this pass at design time, it just prevents some allocations.
+            if (documentNode.Options.DesignTime)
+            {
+                return;
+            }
+            
             var walker = new PreallocatedTagHelperWalker();
             walker.VisitDocument(documentNode);
         }

src/Microsoft.AspNetCore.Razor.Language/IImportProjectFeature.cs

@@ -5,8 +5,8 @@ using System.Collections.Generic;
 
 namespace Microsoft.AspNetCore.Razor.Language
 {
-    public interface IRazorImportFeature : IRazorProjectEngineFeature
+    public interface IImportProjectFeature : IRazorProjectEngineFeature
     {
-        IReadOnlyList<RazorSourceDocument> GetImports(RazorProjectItem projectItem);
+        IReadOnlyList<RazorProjectItem> GetImports(RazorProjectItem projectItem);
     }
 }

src/Microsoft.AspNetCore.Razor.Language/IRazorCodeGenerationOptionsFactoryProjectFeature.cs

@@ -0,0 +1,12 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+
+namespace Microsoft.AspNetCore.Razor.Language
+{
+    internal interface IRazorCodeGenerationOptionsFactoryProjectFeature : IRazorProjectEngineFeature
+    {
+        RazorCodeGenerationOptions Create(Action<RazorCodeGenerationOptionsBuilder> configure);
+    }
+}

src/Microsoft.AspNetCore.Razor.Language/IRazorCodeGenerationOptionsFeature.cs

@@ -1,8 +1,11 @@
 // Copyright(c) .NET Foundation.All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
+using System;
+
 namespace Microsoft.AspNetCore.Razor.Language
 {
+    [Obsolete("In Razor 2.1 and newer, use RazorCodeDocument.GetCodeGenerationOptions().")]
     public interface IRazorCodeGenerationOptionsFeature : IRazorEngineFeature
     {
         RazorCodeGenerationOptions GetOptions();

src/Microsoft.AspNetCore.Razor.Language/IRazorParserOptionsFactoryProjectFeature.cs

@@ -0,0 +1,12 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+
+namespace Microsoft.AspNetCore.Razor.Language
+{
+    internal interface IRazorParserOptionsFactoryProjectFeature : IRazorProjectEngineFeature
+    {
+        RazorParserOptions Create(Action<RazorParserOptionsBuilder> configure);
+    }
+}

src/Microsoft.AspNetCore.Razor.Language/IRazorParserOptionsFeature.cs

@@ -1,8 +1,11 @@
 // Copyright(c) .NET Foundation.All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
+using System;
+
 namespace Microsoft.AspNetCore.Razor.Language
 {
+    [Obsolete("In Razor 2.1 and newer, use RazorCodeDocument.GetParserOptions().")]
     public interface IRazorParserOptionsFeature : IRazorEngineFeature
     {
         RazorParserOptions GetOptions();

src/Microsoft.AspNetCore.Razor.Language/Intermediate/MethodDeclarationIntermediateNode.cs

@@ -14,6 +14,8 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
 
         public string MethodName { get; set; }
 
+        public IList<MethodParameter> Parameters { get; } = new List<MethodParameter>();
+
         public string ReturnType { get; set; }
 
         public override void Accept(IntermediateNodeVisitor visitor)

src/Microsoft.AspNetCore.Razor.Language/Intermediate/MethodParameter.cs

@@ -0,0 +1,16 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Collections.Generic;
+
+namespace Microsoft.AspNetCore.Razor.Language.Intermediate
+{
+    public sealed class MethodParameter
+    {
+        public IList<string> Modifiers { get; } = new List<string>();
+
+        public string TypeName { get; set; }
+
+        public string ParameterName { get; set; }
+    }
+}

src/Microsoft.AspNetCore.Razor.Language/Properties/AssemblyInfo.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System.Runtime.CompilerServices;
+[assembly: InternalsVisibleTo("Microsoft.AspNetCore.Razor.Performance, PublicKey=0024000004800000940000000602000000240000525341310004000001000100f33a29044fa9d740c9b3213a93e57c84b472c84e0b8a0e1ae48e67a9f8f6de9d5f7f3d52ac23e48ac51801f1dc950abe901da34d2a9e3baadb141a17c77ef3c565dd5ee5054b91cf63bb3c6ab83f72ab3aafe93d0fc3c2348b764fafb0b1c0733de51459aeab46580384bf9d74c4e28164b7cde247f891ba07891c9d872ad2bb")]
 [assembly: InternalsVisibleTo("Microsoft.AspNetCore.Mvc.Razor.Extensions.Test, PublicKey=0024000004800000940000000602000000240000525341310004000001000100f33a29044fa9d740c9b3213a93e57c84b472c84e0b8a0e1ae48e67a9f8f6de9d5f7f3d52ac23e48ac51801f1dc950abe901da34d2a9e3baadb141a17c77ef3c565dd5ee5054b91cf63bb3c6ab83f72ab3aafe93d0fc3c2348b764fafb0b1c0733de51459aeab46580384bf9d74c4e28164b7cde247f891ba07891c9d872ad2bb")]
 [assembly: InternalsVisibleTo("Microsoft.AspNetCore.Mvc.Razor.Extensions.Version1_X.Test, PublicKey=0024000004800000940000000602000000240000525341310004000001000100f33a29044fa9d740c9b3213a93e57c84b472c84e0b8a0e1ae48e67a9f8f6de9d5f7f3d52ac23e48ac51801f1dc950abe901da34d2a9e3baadb141a17c77ef3c565dd5ee5054b91cf63bb3c6ab83f72ab3aafe93d0fc3c2348b764fafb0b1c0733de51459aeab46580384bf9d74c4e28164b7cde247f891ba07891c9d872ad2bb")]
 [assembly: InternalsVisibleTo("Microsoft.AspNetCore.Razor.GenerateTool, PublicKey=0024000004800000940000000602000000240000525341310004000001000100f33a29044fa9d740c9b3213a93e57c84b472c84e0b8a0e1ae48e67a9f8f6de9d5f7f3d52ac23e48ac51801f1dc950abe901da34d2a9e3baadb141a17c77ef3c565dd5ee5054b91cf63bb3c6ab83f72ab3aafe93d0fc3c2348b764fafb0b1c0733de51459aeab46580384bf9d74c4e28164b7cde247f891ba07891c9d872ad2bb")]

src/Microsoft.AspNetCore.Razor.Language/ProvideRazorExtensionInitializerAttribute.cs

@@ -0,0 +1,31 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+
+namespace Microsoft.AspNetCore.Razor.Language
+{
+    [AttributeUsage(AttributeTargets.Assembly, AllowMultiple = true, Inherited = false)]
+    public class ProvideRazorExtensionInitializerAttribute : Attribute
+    {
+        public ProvideRazorExtensionInitializerAttribute(string extensionName, Type initializerType)
+        {
+            if (extensionName == null)
+            {
+                throw new ArgumentNullException(nameof(extensionName));
+            }
+
+            if (initializerType == null)
+            {
+                throw new ArgumentNullException(nameof(initializerType));
+            }
+
+            ExtensionName = extensionName;
+            InitializerType = initializerType;
+        }
+
+        public string ExtensionName { get; }
+
+        public Type InitializerType { get; }
+    }
+}

src/Microsoft.AspNetCore.Razor.Language/RazorCodeDocument.cs

@@ -30,6 +30,23 @@ namespace Microsoft.AspNetCore.Razor.Language
             return new DefaultRazorCodeDocument(source, imports);
         }
 
+        public static RazorCodeDocument Create(
+            RazorSourceDocument source, 
+            IEnumerable<RazorSourceDocument> imports,
+            RazorParserOptions parserOptions,
+            RazorCodeGenerationOptions codeGenerationOptions)
+        {
+            if (source == null)
+            {
+                throw new ArgumentNullException(nameof(source));
+            }
+
+            var codeDocument = new DefaultRazorCodeDocument(source, imports);
+            codeDocument.SetParserOptions(parserOptions);
+            codeDocument.SetCodeGenerationOptions(codeGenerationOptions);
+            return codeDocument;
+        }
+
         public abstract IReadOnlyList<RazorSourceDocument> Imports { get; }
 
         public abstract ItemCollection Items { get; }

src/Microsoft.AspNetCore.Razor.Language/RazorCodeDocumentExtensions.cs

@@ -109,6 +109,46 @@ namespace Microsoft.AspNetCore.Razor.Language
             document.Items[typeof(RazorCSharpDocument)] = csharp;
         }
 
+        public static RazorParserOptions GetParserOptions(this RazorCodeDocument document)
+        {
+            if (document == null)
+            {
+                throw new ArgumentNullException(nameof(document));
+            }
+
+            return (RazorParserOptions)document.Items[typeof(RazorParserOptions)];
+        }
+
+        public static void SetParserOptions(this RazorCodeDocument document, RazorParserOptions parserOptions)
+        {
+            if (document == null)
+            {
+                throw new ArgumentNullException(nameof(document));
+            }
+
+            document.Items[typeof(RazorParserOptions)] = parserOptions;
+        }
+
+        public static RazorCodeGenerationOptions GetCodeGenerationOptions(this RazorCodeDocument document)
+        {
+            if (document == null)
+            {
+                throw new ArgumentNullException(nameof(document));
+            }
+
+            return (RazorCodeGenerationOptions)document.Items[typeof(RazorCodeGenerationOptions)];
+        }
+
+        public static void SetCodeGenerationOptions(this RazorCodeDocument document, RazorCodeGenerationOptions codeGenerationOptions)
+        {
+            if (document == null)
+            {
+                throw new ArgumentNullException(nameof(document));
+            }
+
+            document.Items[typeof(RazorCodeGenerationOptions)] = codeGenerationOptions;
+        }
+
         private class ImportSyntaxTreesHolder
         {
             public ImportSyntaxTreesHolder(IReadOnlyList<RazorSyntaxTree> syntaxTrees)

src/Microsoft.AspNetCore.Razor.Language/RazorCodeGenerationOptionsBuilder.cs

@@ -5,6 +5,8 @@ namespace Microsoft.AspNetCore.Razor.Language
 {
     public abstract class RazorCodeGenerationOptionsBuilder
     {
+        public virtual RazorConfiguration Configuration => null;
+
         public abstract bool DesignTime { get; }
 
         public abstract int IndentSize { get; set; }
@@ -41,5 +43,9 @@ namespace Microsoft.AspNetCore.Razor.Language
         public virtual bool SuppressMetadataAttributes { get; set; }
 
         public abstract RazorCodeGenerationOptions Build();
+
+        public virtual void SetDesignTime(bool designTime)
+        {
+        }
     }
 }

src/Microsoft.AspNetCore.Razor.Language/RazorConfiguration.cs

@@ -12,22 +12,12 @@ namespace Microsoft.AspNetCore.Razor.Language
         public static readonly RazorConfiguration Default = new RazorConfiguration(
             RazorLanguageVersion.Latest, 
             "unnamed",
-            Array.Empty<RazorExtension>(),
+            Array.Empty<RazorExtension>());
-            designTime: false);
-
-        // This is used only in some back-compat scenarios. We don't expose it because there's no
-        // use case for anyone else to use it.
-        internal static readonly RazorConfiguration DefaultDesignTime = new RazorConfiguration(
-            RazorLanguageVersion.Latest,
-            "unnamed",
-            Array.Empty<RazorExtension>(),
-            designTime: true);
 
         public RazorConfiguration(
             RazorLanguageVersion languageVersion, 
             string configurationName,
-            IEnumerable<RazorExtension> extensions,
+            IEnumerable<RazorExtension> extensions)
-            bool designTime)
         {
             if (languageVersion == null)
             {
@@ -47,7 +37,6 @@ namespace Microsoft.AspNetCore.Razor.Language
             LanguageVersion = languageVersion;
             ConfigurationName = configurationName;
             Extensions = extensions.ToArray();
-            DesignTime = designTime;
         }
 
         public string ConfigurationName { get; }
@@ -55,7 +44,5 @@ namespace Microsoft.AspNetCore.Razor.Language
         public IReadOnlyList<RazorExtension> Extensions { get; }
 
         public RazorLanguageVersion LanguageVersion { get; }
-
-        public bool DesignTime { get; }
     }
 }

src/Microsoft.AspNetCore.Razor.Language/RazorEngine.cs

@@ -17,27 +17,27 @@ namespace Microsoft.AspNetCore.Razor.Language
             return Create(configure: null);
         }
 
-        public static RazorEngine Create(Action<IRazorEngineBuilder> configure) => CreateCore(RazorConfiguration.Default, configure);
+        public static RazorEngine Create(Action<IRazorEngineBuilder> configure) => CreateCore(RazorConfiguration.Default, false, configure);
 
         public static RazorEngine CreateDesignTime()
         {
             return CreateDesignTime(configure: null);
         }
 
-        public static RazorEngine CreateDesignTime(Action<IRazorEngineBuilder> configure) => CreateCore(RazorConfiguration.DefaultDesignTime, configure);
+        public static RazorEngine CreateDesignTime(Action<IRazorEngineBuilder> configure) => CreateCore(RazorConfiguration.Default, true, configure);
 
         // Internal since RazorEngine APIs are going to be obsolete.
-        internal static RazorEngine CreateCore(RazorConfiguration configuration, Action<IRazorEngineBuilder> configure)
+        internal static RazorEngine CreateCore(RazorConfiguration configuration, bool designTime, Action<IRazorEngineBuilder> configure)
         {
             if (configuration == null)
             {
                 throw new ArgumentNullException(nameof(configuration));
             }
 
-            var builder = new DefaultRazorEngineBuilder(configuration.DesignTime);
+            var builder = new DefaultRazorEngineBuilder(designTime);
             AddDefaults(builder);
 
-            if (configuration.DesignTime)
+            if (designTime)
             {
                 AddDefaultDesignTimeFeatures(configuration, builder.Features);
             }
@@ -152,7 +152,7 @@ namespace Microsoft.AspNetCore.Razor.Language
             var targetExtension = features.OfType<IRazorTargetExtensionFeature>().FirstOrDefault();
             Debug.Assert(targetExtension != null);
 
-            targetExtension.TargetExtensions.Add(new DefaultTagHelperTargetExtension() { DesignTime = false });
+            targetExtension.TargetExtensions.Add(new DefaultTagHelperTargetExtension());
             targetExtension.TargetExtensions.Add(new PreallocatedAttributeTargetExtension());
         }
 
@@ -170,7 +170,7 @@ namespace Microsoft.AspNetCore.Razor.Language
             var targetExtension = features.OfType<IRazorTargetExtensionFeature>().FirstOrDefault();
             Debug.Assert(targetExtension != null);
 
-            targetExtension.TargetExtensions.Add(new DefaultTagHelperTargetExtension() { DesignTime = true });
+            targetExtension.TargetExtensions.Add(new DefaultTagHelperTargetExtension());
             targetExtension.TargetExtensions.Add(new DesignTimeDirectiveTargetExtension());
         }
 

src/Microsoft.AspNetCore.Razor.Language/RazorExtensionInitializer.cs

@@ -0,0 +1,10 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+namespace Microsoft.AspNetCore.Razor.Language
+{
+    public abstract class RazorExtensionInitializer
+    {
+        public abstract void Initialize(RazorProjectEngineBuilder builder);
+    }
+}

src/Microsoft.AspNetCore.Razor.Language/RazorParserOptionsBuilder.cs

@@ -1,21 +1,26 @@
 // Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
-using System;
 using System.Collections.Generic;
 
 namespace Microsoft.AspNetCore.Razor.Language
 {
     public abstract class RazorParserOptionsBuilder
     {
+        public virtual RazorConfiguration Configuration => null;
+
         public abstract bool DesignTime { get; }
 
         public abstract ICollection<DirectiveDescriptor> Directives { get; }
 
         public abstract bool ParseLeadingDirectives { get; set; }
 
-        public virtual RazorLanguageVersion Version { get; }
+        public virtual RazorLanguageVersion LanguageVersion { get; }
 
         public abstract RazorParserOptions Build();
+
+        public virtual void SetDesignTime(bool designTime)
+        {
+        }
     }
 }

src/Microsoft.AspNetCore.Razor.Language/RazorProjectEngine.cs

@@ -3,26 +3,61 @@
 
 using System;
 using System.Collections.Generic;
+using System.Reflection;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Razor.Language.Extensions;
 
 namespace Microsoft.AspNetCore.Razor.Language
 {
     public abstract class RazorProjectEngine
     {
+        public abstract RazorConfiguration Configuration { get; }
+
         public abstract RazorProjectFileSystem FileSystem { get; }
 
         public abstract RazorEngine Engine { get; }
 
-        public abstract IReadOnlyList<IRazorProjectEngineFeature> Features { get; }
+        public IReadOnlyList<IRazorEngineFeature> EngineFeatures => Engine.Features;
 
-        public abstract RazorCodeDocument Process(RazorProjectItem projectItem);
+        public IReadOnlyList<IRazorEnginePhase> Phases => Engine.Phases;
 
-        public static RazorProjectEngine Create(RazorProjectFileSystem fileSystem) => Create(fileSystem, configure: null);
+        public abstract IReadOnlyList<IRazorProjectEngineFeature> ProjectFeatures { get; }
 
-        public static RazorProjectEngine Create(RazorProjectFileSystem fileSystem, Action<RazorProjectEngineBuilder> configure) => Create(fileSystem, RazorConfiguration.Default, configure);
+        public virtual RazorCodeDocument Process(RazorProjectItem projectItem)
+        {
+            if (projectItem == null)
+            {
+                throw new ArgumentNullException(nameof(projectItem));
+            }
+
+            var codeDocument = CreateCodeDocumentCore(projectItem);
+            ProcessCore(codeDocument);
+            return codeDocument;
+        }
+
+        public virtual RazorCodeDocument ProcessDesignTime(RazorProjectItem projectItem)
+        {
+            if (projectItem == null)
+            {
+                throw new ArgumentNullException(nameof(projectItem));
+            }
+
+            var codeDocument = CreateCodeDocumentDesignTimeCore(projectItem);
+            ProcessCore(codeDocument);
+            return codeDocument;
+        }
+
+        protected abstract RazorCodeDocument CreateCodeDocumentCore(RazorProjectItem projectItem);
+
+        protected abstract RazorCodeDocument CreateCodeDocumentDesignTimeCore(RazorProjectItem projectItem);
+
+        protected abstract void ProcessCore(RazorCodeDocument codeDocument);
+
+        public static RazorProjectEngine Create(RazorConfiguration configuration, RazorProjectFileSystem fileSystem) => Create(configuration, fileSystem, configure: null);
 
         public static RazorProjectEngine Create(
-            RazorProjectFileSystem fileSystem,
             RazorConfiguration configuration,
+            RazorProjectFileSystem fileSystem,
             Action<RazorProjectEngineBuilder> configure)
         {
             if (fileSystem == null)
@@ -37,54 +72,96 @@ namespace Microsoft.AspNetCore.Razor.Language
 
             var builder = new DefaultRazorProjectEngineBuilder(configuration, fileSystem);
 
-            AddDefaults(builder);
+            // The intialization order is somewhat important.
+            //
+            // Defaults -> Extensions -> Additional customization
+            //
+            // This allows extensions to rely on default features, and customizations to override choices made by
+            // extensions.
+            RazorEngine.AddDefaultPhases(builder.Phases);
+            AddDefaultsFeatures(builder.Features);
 
-            if (configuration.DesignTime)
+            LoadExtensions(builder, configuration.Extensions);
-            {
-                AddDesignTimeDefaults(builder);
-            }
-            else
-            {
-                AddRuntimeDefaults(builder);
-            }
 
             configure?.Invoke(builder);
 
             return builder.Build();
         }
-
+        
-        private static void AddDefaults(RazorProjectEngineBuilder builder)
+        private static void AddDefaultsFeatures(ICollection<IRazorFeature> features)
         {
-            builder.Features.Add(new DefaultRazorImportFeature());
+            features.Add(new DefaultImportProjectFeature());
-        }
 
-        private static void AddDesignTimeDefaults(RazorProjectEngineBuilder builder)
+            // General extensibility
-        {
+            features.Add(new DefaultRazorDirectiveFeature());
-            var engineFeatures = new List<IRazorEngineFeature>();
+            features.Add(new DefaultMetadataIdentifierFeature());
-            RazorEngine.AddDefaultFeatures(engineFeatures);
-            RazorEngine.AddDefaultDesignTimeFeatures(builder.Configuration, engineFeatures);
 
-            AddEngineFeaturesAndPhases(builder, engineFeatures);
+            // Options features
-        }
+            features.Add(new DefaultRazorParserOptionsFactoryProjectFeature());
+            features.Add(new DefaultRazorCodeGenerationOptionsFactoryProjectFeature());
 
-        private static void AddRuntimeDefaults(RazorProjectEngineBuilder builder)
+            // Legacy options features
-        {
+            //
-            var engineFeatures = new List<IRazorEngineFeature>();
+            // These features are obsolete as of 2.1. Our code will resolve this but not invoke them.
-            RazorEngine.AddDefaultFeatures(engineFeatures);
+            features.Add(new DefaultRazorParserOptionsFeature(designTime: false, version: RazorLanguageVersion.Version_2_0));
-            RazorEngine.AddDefaultRuntimeFeatures(builder.Configuration, engineFeatures);
+            features.Add(new DefaultRazorCodeGenerationOptionsFeature(designTime: false));
 
-            AddEngineFeaturesAndPhases(builder, engineFeatures);
+            // Syntax Tree passes
-        }
+            features.Add(new DefaultDirectiveSyntaxTreePass());
+            features.Add(new HtmlNodeOptimizationPass());
+            features.Add(new PreallocatedTagHelperAttributeOptimizationPass());
 
-        private static void AddEngineFeaturesAndPhases(RazorProjectEngineBuilder builder, IReadOnlyList<IRazorEngineFeature> engineFeatures)
+            // Intermediate Node Passes
-        {
+            features.Add(new DefaultDocumentClassifierPass());
-            for (var i = 0; i < engineFeatures.Count; i++)
+            features.Add(new MetadataAttributePass());
+            features.Add(new DesignTimeDirectivePass());
+            features.Add(new DirectiveRemovalOptimizationPass());
+            features.Add(new DefaultTagHelperOptimizationPass());
+
+            // Default Code Target Extensions
+            var targetExtensionFeature = new DefaultRazorTargetExtensionFeature();
+            features.Add(targetExtensionFeature);
+            targetExtensionFeature.TargetExtensions.Add(new MetadataAttributeTargetExtension());
+            targetExtensionFeature.TargetExtensions.Add(new DefaultTagHelperTargetExtension());
+            targetExtensionFeature.TargetExtensions.Add(new PreallocatedAttributeTargetExtension());
+            targetExtensionFeature.TargetExtensions.Add(new DesignTimeDirectiveTargetExtension());
+
+            // Default configuration
+            var configurationFeature = new DefaultDocumentClassifierPassFeature();
+            features.Add(configurationFeature);
+            configurationFeature.ConfigureClass.Add((document, @class) =>
             {
-                var engineFeature = engineFeatures[i];
+                @class.ClassName = "Template";
-                builder.Features.Add(engineFeature);
+                @class.Modifiers.Add("public");
-            }
+            });
 
-            RazorEngine.AddDefaultPhases(builder.Phases);
+            configurationFeature.ConfigureNamespace.Add((document, @namespace) =>
+            {
+                @namespace.Content = "Razor";
+            });
+
+            configurationFeature.ConfigureMethod.Add((document, method) =>
+            {
+                method.MethodName = "ExecuteAsync";
+                method.ReturnType = $"global::{typeof(Task).FullName}";
+
+                method.Modifiers.Add("public");
+                method.Modifiers.Add("async");
+                method.Modifiers.Add("override");
+            });
+        }
+
+        private static void LoadExtensions(RazorProjectEngineBuilder builder, IReadOnlyList<RazorExtension> extensions)
+        {
+            for (var i = 0; i < extensions.Count; i++)
+            {
+                // For now we only handle AssemblyExtension - which is not user-constructable. We're keeping a tight
+                // lid on how things work until we add official support for extensibility everywhere. So, this is
+                // intentionally inflexible for the time being.
+                var extension = extensions[i] as AssemblyExtension;
+                var initializer = extension?.CreateInitializer();
+                initializer?.Initialize(builder);
+            }
         }
     }
 }

src/Microsoft.AspNetCore.Razor.Language/RazorProjectEngineBuilder.cs

@@ -7,14 +7,14 @@ namespace Microsoft.AspNetCore.Razor.Language
 {
     public abstract class RazorProjectEngineBuilder
     {
+        public abstract RazorConfiguration Configuration { get; }
+
         public abstract RazorProjectFileSystem FileSystem { get; }
 
         public abstract ICollection<IRazorFeature> Features { get; }
 
         public abstract IList<IRazorEnginePhase> Phases { get; }
 
-        public abstract RazorConfiguration Configuration { get; }
-
         public abstract RazorProjectEngine Build();
     }
 }

src/Microsoft.AspNetCore.Razor.Language/RazorProjectEngineBuilderExtensions.cs

@@ -2,14 +2,80 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.Collections.Generic;
+using System.IO;
 using System.Linq;
+using System.Text;
 using Microsoft.AspNetCore.Razor.Language.CodeGeneration;
+using Microsoft.AspNetCore.Razor.Language.Intermediate;
 
 namespace Microsoft.AspNetCore.Razor.Language
 {
     public static class RazorProjectEngineBuilderExtensions
     {
-        public static void SetImportFeature(this RazorProjectEngineBuilder builder, IRazorImportFeature feature)
+        /// <summary>
+        /// Registers a class configuration delegate that gets invoked during code generation.
+        /// </summary>
+        /// <param name="builder">The <see cref="RazorProjectEngineBuilder"/>.</param>
+        /// <param name="configureClass"><see cref="Action"/> invoked to configure 
+        /// <see cref="ClassDeclarationIntermediateNode"/> during code generation.</param>
+        /// <returns>The <see cref="RazorProjectEngineBuilder"/>.</returns>
+        public static RazorProjectEngineBuilder ConfigureClass(
+            this RazorProjectEngineBuilder builder,
+            Action<RazorCodeDocument, ClassDeclarationIntermediateNode> configureClass)
+        {
+            if (builder == null)
+            {
+                throw new ArgumentNullException(nameof(builder));
+            }
+
+            if (configureClass == null)
+            {
+                throw new ArgumentNullException(nameof(configureClass));
+            }
+
+            var configurationFeature = GetDefaultDocumentClassifierPassFeature(builder);
+            configurationFeature.ConfigureClass.Add(configureClass);
+            return builder;
+        }
+
+        /// <summary>
+        /// Sets the base type for generated types.
+        /// </summary>
+        /// <param name="builder">The <see cref="RazorProjectEngineBuilder"/>.</param>
+        /// <param name="baseType">The name of the base type.</param>
+        /// <returns>The <see cref="RazorProjectEngineBuilder"/>.</returns>
+        public static RazorProjectEngineBuilder SetBaseType(this RazorProjectEngineBuilder builder, string baseType)
+        {
+            if (builder == null)
+            {
+                throw new ArgumentNullException(nameof(builder));
+            }
+
+            var configurationFeature = GetDefaultDocumentClassifierPassFeature(builder);
+            configurationFeature.ConfigureClass.Add((document, @class) => @class.BaseType = baseType);
+            return builder;
+        }
+
+        /// <summary>
+        /// Sets the namespace for generated types.
+        /// </summary>
+        /// <param name="builder">The <see cref="RazorProjectEngineBuilder"/>.</param>
+        /// <param name="namespaceName">The name of the namespace.</param>
+        /// <returns>The <see cref="RazorProjectEngineBuilder"/>.</returns>
+        public static RazorProjectEngineBuilder SetNamespace(this RazorProjectEngineBuilder builder, string namespaceName)
+        {
+            if (builder == null)
+            {
+                throw new ArgumentNullException(nameof(builder));
+            }
+
+            var configurationFeature = GetDefaultDocumentClassifierPassFeature(builder);
+            configurationFeature.ConfigureNamespace.Add((document, @namespace) => @namespace.Content = namespaceName);
+            return builder;
+        }
+
+        public static void SetImportFeature(this RazorProjectEngineBuilder builder, IImportProjectFeature feature)
         {
             if (builder == null)
             {
@@ -22,7 +88,7 @@ namespace Microsoft.AspNetCore.Razor.Language
             }
 
             // Remove any existing import features in favor of the new one we're given.
-            var existingFeatures = builder.Features.OfType<IRazorImportFeature>().ToArray();
+            var existingFeatures = builder.Features.OfType<IImportProjectFeature>().ToArray();
             foreach (var existingFeature in existingFeatures)
             {
                 builder.Features.Remove(existingFeature);
@@ -79,6 +145,27 @@ namespace Microsoft.AspNetCore.Razor.Language
             return builder;
         }
 
+        /// <summary>
+        /// Adds the provided <see cref="RazorProjectItem" />s as imports to all project items processed
+        /// by the <see cref="RazorProjectEngine"/>.
+        /// </summary>
+        /// <param name="builder">The <see cref="RazorProjectEngineBuilder"/>.</param>
+        /// <param name="imports">The collection of imports.</param>
+        /// <returns>The <see cref="RazorProjectEngineBuilder"/>.</returns>
+        public static RazorProjectEngineBuilder AddDefaultImports(this RazorProjectEngineBuilder builder, params string[] imports)
+        {
+            if (builder == null)
+            {
+                throw new ArgumentNullException(nameof(builder));
+            }
+
+            var existingImportFeature = builder.Features.OfType<IImportProjectFeature>().First();
+            var testImportFeature = new AdditionalImportsProjectFeature(existingImportFeature, imports);
+            builder.SetImportFeature(testImportFeature);
+
+            return builder;
+        }
+
         private static IRazorDirectiveFeature GetDirectiveFeature(RazorProjectEngineBuilder builder)
         {
             var directiveFeature = builder.Features.OfType<IRazorDirectiveFeature>().FirstOrDefault();
@@ -102,5 +189,77 @@ namespace Microsoft.AspNetCore.Razor.Language
 
             return targetExtensionFeature;
         }
+
+        private static DefaultDocumentClassifierPassFeature GetDefaultDocumentClassifierPassFeature(RazorProjectEngineBuilder builder)
+        {
+            var configurationFeature = builder.Features.OfType<DefaultDocumentClassifierPassFeature>().FirstOrDefault();
+            if (configurationFeature == null)
+            {
+                configurationFeature = new DefaultDocumentClassifierPassFeature();
+                builder.Features.Add(configurationFeature);
+            }
+
+            return configurationFeature;
+        }
+
+        private class AdditionalImportsProjectFeature : RazorProjectEngineFeatureBase, IImportProjectFeature
+        {
+            private readonly IImportProjectFeature _existingImportFeature;
+            private readonly IEnumerable<RazorProjectItem> _imports;
+
+            public override RazorProjectEngine ProjectEngine
+            {
+                get => base.ProjectEngine;
+                set
+                {
+                    _existingImportFeature.ProjectEngine = value;
+                    base.ProjectEngine = value;
+                }
+            }
+
+            public AdditionalImportsProjectFeature(IImportProjectFeature existingImportFeature, params string[] imports)
+            {
+                _existingImportFeature = existingImportFeature;
+                _imports = imports.Select(import => new InMemoryProjectItem(import));
+            }
+
+            public IReadOnlyList<RazorProjectItem> GetImports(RazorProjectItem projectItem)
+            {
+                var imports = _existingImportFeature.GetImports(projectItem).ToList();
+                imports.AddRange(_imports);
+
+                return imports;
+            }
+
+            private class InMemoryProjectItem : RazorProjectItem
+            {
+                private readonly byte[] _importBytes;
+
+                public InMemoryProjectItem(string content)
+                {
+                    if (string.IsNullOrEmpty(content))
+                    {
+                        throw new ArgumentException(Resources.ArgumentCannotBeNullOrEmpty, nameof(content));
+                    }
+
+                    var preamble = Encoding.UTF8.GetPreamble();
+                    var contentBytes = Encoding.UTF8.GetBytes(content);
+
+                    _importBytes = new byte[preamble.Length + contentBytes.Length];
+                    preamble.CopyTo(_importBytes, 0);
+                    contentBytes.CopyTo(_importBytes, preamble.Length);
+                }
+
+                public override string BasePath => null;
+
+                public override string FilePath => null;
+
+                public override string PhysicalPath => null;
+
+                public override bool Exists => true;
+
+                public override Stream Read() => new MemoryStream(_importBytes);
+            }
+        }
     }
 }

src/Microsoft.AspNetCore.Razor.Language/RazorProjectFileSystem.cs

@@ -7,6 +7,8 @@ namespace Microsoft.AspNetCore.Razor.Language
 {
     public abstract class RazorProjectFileSystem : RazorProject
     {
+        internal static readonly RazorProjectFileSystem Empty = new EmptyProjectFileSystem();
+
         /// <summary>
         /// Create a Razor project file system based off of a root directory.
         /// </summary>

src/Microsoft.AspNetCore.Razor.Tasks/RazorGenerate.cs

@@ -9,9 +9,28 @@ namespace Microsoft.AspNetCore.Razor.Tasks
 {
     public class RazorGenerate : DotNetToolTask
     {
+        private static readonly string[] SourceRequiredMetadata = new string[]
+        {
+            FullPath,
+            GeneratedOutput,
+            TargetPath,
+        };
+
         private const string GeneratedOutput = "GeneratedOutput";
         private const string TargetPath = "TargetPath";
         private const string FullPath = "FullPath";
+        private const string Identity = "Identity";
+        private const string AssemblyName = "AssemblyName";
+        private const string AssemblyFilePath = "AssemblyFilePath";
+
+        [Required]
+        public string Version { get; set; }
+
+        [Required]
+        public ITaskItem[] Configuration { get; set; }
+
+        [Required]
+        public ITaskItem[] Extensions { get;  set; }
 
         [Required]
         public ITaskItem[] Sources { get; set; }
@@ -26,11 +45,28 @@ namespace Microsoft.AspNetCore.Razor.Tasks
 
         protected override bool ValidateParameters()
         {
+            if (Configuration.Length == 0)
+            {
+                Log.LogError("The project {0} must provide a value for {1}.", ProjectRoot, nameof(Configuration));
+                return false;
+            }
+
             for (var i = 0; i < Sources.Length; i++)
             {
                 if (!EnsureRequiredMetadata(Sources[i], FullPath) ||
                     !EnsureRequiredMetadata(Sources[i], GeneratedOutput) ||
                     !EnsureRequiredMetadata(Sources[i], TargetPath))
+                {
+                    Log.LogError("The Razor source item '{0}' is missing a required metadata entry. Required metadata are: '{1}'", Sources[i], SourceRequiredMetadata);
+                    return false;
+                }
+            }
+
+            for (var i = 0; i < Extensions.Length; i++)
+            {
+                if (!EnsureRequiredMetadata(Extensions[i], Identity) ||
+                    !EnsureRequiredMetadata(Extensions[i], AssemblyName) ||
+                    !EnsureRequiredMetadata(Extensions[i], AssemblyFilePath))
                 {
                     return false;
                 }
@@ -65,6 +101,21 @@ namespace Microsoft.AspNetCore.Razor.Tasks
             builder.AppendLine("-t");
             builder.AppendLine(TagHelperManifest);
 
+            builder.AppendLine("-v");
+            builder.AppendLine(Version);
+
+            builder.AppendLine("-c");
+            builder.AppendLine(Configuration[0].GetMetadata(Identity));
+
+            for (var i = 0; i < Extensions.Length; i++)
+            {
+                builder.AppendLine("-n");
+                builder.AppendLine(Extensions[i].GetMetadata(Identity));
+
+                builder.AppendLine("-e");
+                builder.AppendLine(Path.GetFullPath(Extensions[i].GetMetadata(AssemblyFilePath)));
+            }
+
             return builder.ToString();
         }
 

src/Microsoft.AspNetCore.Razor.Tasks/RazorTagHelper.cs

@@ -10,6 +10,19 @@ namespace Microsoft.AspNetCore.Razor.Tasks
 {
     public class RazorTagHelper : DotNetToolTask
     {
+        private const string Identity = "Identity";
+        private const string AssemblyName = "AssemblyName";
+        private const string AssemblyFilePath = "AssemblyFilePath";
+
+        [Required]
+        public string Version { get; set; }
+
+        [Required]
+        public ITaskItem[] Configuration { get; set; }
+
+        [Required]
+        public ITaskItem[] Extensions { get; set; }
+
         [Required]
         public string[] Assemblies { get; set; }
 
@@ -51,6 +64,21 @@ namespace Microsoft.AspNetCore.Razor.Tasks
             builder.AppendLine("-p");
             builder.AppendLine(ProjectRoot);
 
+            builder.AppendLine("-v");
+            builder.AppendLine(Version);
+
+            builder.AppendLine("-c");
+            builder.AppendLine(Configuration[0].GetMetadata(Identity));
+
+            for (var i = 0; i < Extensions.Length; i++)
+            {
+                builder.AppendLine("-n");
+                builder.AppendLine(Extensions[i].GetMetadata(Identity));
+
+                builder.AppendLine("-e");
+                builder.AppendLine(Path.GetFullPath(Extensions[i].GetMetadata(AssemblyFilePath)));
+            }
+
             return builder.ToString();
         }
     }

src/Microsoft.AspNetCore.Razor.Tools/Application.cs

@@ -6,15 +6,19 @@ using System.Collections.Generic;
 using System.IO;
 using System.Reflection;
 using System.Threading;
+using Microsoft.CodeAnalysis;
 using Microsoft.Extensions.CommandLineUtils;
 
 namespace Microsoft.AspNetCore.Razor.Tools
 {
     internal class Application : CommandLineApplication
     {
-        public Application(CancellationToken cancellationToken)
+        public Application(CancellationToken cancellationToken, ExtensionAssemblyLoader loader, ExtensionDependencyChecker checker, Func<string, MetadataReferenceProperties, PortableExecutableReference> assemblyReferenceProvider)
         {
             CancellationToken = cancellationToken;
+            Checker = checker;
+            Loader = loader;
+            AssemblyReferenceProvider = assemblyReferenceProvider;
 
             Name = "rzc";
             FullName = "Microsoft ASP.NET Core Razor CLI tool";
@@ -31,6 +35,12 @@ namespace Microsoft.AspNetCore.Razor.Tools
 
         public CancellationToken CancellationToken { get; }
 
+        public ExtensionAssemblyLoader Loader { get; }
+
+        public ExtensionDependencyChecker Checker { get; }
+
+        public Func<string, MetadataReferenceProperties, PortableExecutableReference> AssemblyReferenceProvider { get; }
+
         public new int Execute(params string[] args)
         {
             try

src/Microsoft.AspNetCore.Razor.Tools/CachingMetadataReference.cs

@@ -0,0 +1,32 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using Microsoft.CodeAnalysis;
+
+namespace Microsoft.AspNetCore.Razor.Tools
+{
+    internal sealed class CachingMetadataReference : PortableExecutableReference
+    {
+        private static readonly MetadataCache _metadataCache = new MetadataCache();
+
+        public CachingMetadataReference(string fullPath, MetadataReferenceProperties properties)
+            : base(properties, fullPath)
+        {
+        }
+
+        protected override DocumentationProvider CreateDocumentationProvider()
+        {
+            return DocumentationProvider.Default;
+        }
+
+        protected override Metadata GetMetadataImpl()
+        {
+            return _metadataCache.GetMetadata(FilePath);
+        }
+
+        protected override PortableExecutableReference WithPropertiesImpl(MetadataReferenceProperties properties)
+        {
+            return new CachingMetadataReference(FilePath, properties);
+        }
+    }
+}

src/Microsoft.AspNetCore.Razor.Tools/CompilerHost.cs

@@ -1,10 +1,12 @@
 // Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
+using System;
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
 using System.Threading;
+using Microsoft.CodeAnalysis;
 
 namespace Microsoft.AspNetCore.Razor.Tools
 {
@@ -19,6 +21,25 @@ namespace Microsoft.AspNetCore.Razor.Tools
 
         private class DefaultCompilerHost : CompilerHost
         {
+            public DefaultCompilerHost()
+            {
+                // The loader needs to live for the lifetime of the server. 
+                //
+                // This means that if a request tries to use a set of binaries that are inconsistent with what
+                // the server already has, then it will be rejected to try again on the client.
+                //
+                // We also check each set of extensions for missing depenencies individually, so that we can
+                // consistently reject a request that doesn't specify everything it needs. Otherwise the request
+                // could succeed sometimes if it relies on transient state.
+                Loader = new DefaultExtensionAssemblyLoader(Path.Combine(Path.GetTempPath(), "Razor-Server"));
+
+                AssemblyReferenceProvider = (path, properties) => new CachingMetadataReference(path, properties);
+            }
+
+            public Func<string, MetadataReferenceProperties, PortableExecutableReference> AssemblyReferenceProvider { get; }
+
+            public ExtensionAssemblyLoader Loader { get; }
+
             public override ServerResponse Execute(ServerRequest request, CancellationToken cancellationToken)
             {
                 if (!TryParseArguments(request, out var parsed))
@@ -28,28 +49,23 @@ namespace Microsoft.AspNetCore.Razor.Tools
 
                 var exitCode = 0;
                 var output = string.Empty;
-                var app = new Application(cancellationToken);
                 var commandArgs = parsed.args.ToArray();
 
+                var writer = ServerLogger.IsLoggingEnabled ? new StringWriter() : TextWriter.Null;
+
+                var checker = new DefaultExtensionDependencyChecker(Loader, writer, writer);
+                var app = new Application(cancellationToken, Loader, checker, AssemblyReferenceProvider)
+                {
+                    Out = writer,
+                    Error = writer,
+                };
+
+                exitCode = app.Execute(commandArgs);
+
                 if (ServerLogger.IsLoggingEnabled)
                 {
-                    using (var writer = new StringWriter())
+                    output = writer.ToString();
-                    {
+                    ServerLogger.Log(output);
-                        app.Out = writer;
-                        app.Error = writer;
-                        exitCode = app.Execute(commandArgs);
-                        output = writer.ToString();
-                        ServerLogger.Log(output);
-                    }
-                }
-                else
-                {
-                    using (var writer = new StreamWriter(Stream.Null))
-                    {
-                        app.Out = writer;
-                        app.Error = writer;
-                        exitCode = app.Execute(commandArgs);
-                    }
                 }
 
                 return new CompletedServerResponse(exitCode, utf8output: false, output: string.Empty);

src/Microsoft.AspNetCore.Razor.Tools/CompositeRazorProjectFileSystem.cs

@@ -3,24 +3,25 @@
 
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using Microsoft.AspNetCore.Razor.Language;
 
 namespace Microsoft.AspNetCore.Razor.Tools
 {
     internal class CompositeRazorProjectFileSystem : RazorProjectFileSystem
     {
-        public CompositeRazorProjectFileSystem(IReadOnlyList<RazorProjectFileSystem> projects)
+        public CompositeRazorProjectFileSystem(IReadOnlyList<RazorProjectFileSystem> fileSystems)
         {
-            Projects = projects ?? throw new ArgumentNullException(nameof(projects));
+            FileSystems = fileSystems ?? throw new ArgumentNullException(nameof(fileSystems));
         }
 
-        public IReadOnlyList<RazorProjectFileSystem> Projects { get; }
+        public IReadOnlyList<RazorProjectFileSystem> FileSystems { get; }
 
         public override IEnumerable<RazorProjectItem> EnumerateItems(string basePath)
         {
-            foreach (var project in Projects)
+            foreach (var fileSystem in FileSystems)
             {
-                foreach (var result in project.EnumerateItems(basePath))
+                foreach (var result in fileSystem.EnumerateItems(basePath))
                 {
                     yield return result;
                 }
@@ -30,9 +31,9 @@ namespace Microsoft.AspNetCore.Razor.Tools
         public override RazorProjectItem GetItem(string path)
         {
             RazorProjectItem razorProjectItem = null;
-            foreach (var project in Projects)
+            foreach (var fileSystem in FileSystems)
             {
-                razorProjectItem = project.GetItem(path);
+                razorProjectItem = fileSystem.GetItem(path);
                 if (razorProjectItem != null && razorProjectItem.Exists)
                 {
                     return razorProjectItem;

src/Microsoft.AspNetCore.Razor.Tools/ConcurrentLruCache.cs

@@ -0,0 +1,207 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+
+namespace Microsoft.AspNetCore.Razor.Tools
+{
+    /// <summary>
+    /// Cache with a fixed size that evicts the least recently used members.
+    /// Thread-safe.
+    /// This was taken from https://github.com/dotnet/roslyn/blob/749c0ec135d7d080658dc1aa794d15229c3d10d2/src/Compilers/Core/Portable/InternalUtilities/ConcurrentLruCache.cs.
+    /// </summary>
+    internal class ConcurrentLruCache<TKey, TValue>
+    {
+        private readonly int _capacity;
+
+        private readonly Dictionary<TKey, CacheValue> _cache;
+        private readonly LinkedList<TKey> _nodeList;
+        // This is a naive course-grained lock, it can probably be optimized
+        private readonly object _lockObject = new object();
+
+        public ConcurrentLruCache(int capacity)
+            : this (capacity, EqualityComparer<TKey>.Default)
+        {
+        }
+
+        public ConcurrentLruCache(int capacity, IEqualityComparer<TKey> comparer)
+        {
+            if (capacity <= 0)
+            {
+                throw new ArgumentOutOfRangeException(nameof(capacity));
+            }
+            _capacity = capacity;
+            _cache = new Dictionary<TKey, CacheValue>(capacity, comparer);
+            _nodeList = new LinkedList<TKey>();
+        }
+
+        /// <summary>
+        /// Create cache from an array. The cache capacity will be the size
+        /// of the array. All elements of the array will be added to the 
+        /// cache. If any duplicate keys are found in the array a
+        /// <see cref="ArgumentException"/> will be thrown.
+        /// </summary>
+        public ConcurrentLruCache(KeyValuePair<TKey, TValue>[] array)
+            : this(array.Length)
+        {
+            foreach (var kvp in array)
+            {
+                UnsafeAdd(kvp.Key, kvp.Value);
+            }
+        }
+
+        public int Count
+        {
+            get
+            {
+                lock (_lockObject)
+                {
+                    return _cache.Count;
+                }
+            }
+        }
+
+        public void Add(TKey key, TValue value)
+        {
+            lock (_lockObject)
+            {
+                UnsafeAdd(key, value);
+            }
+        }
+
+        public TValue GetOrAdd(TKey key, TValue value)
+        {
+            lock (_lockObject)
+            {
+                if (UnsafeTryGetValue(key, out var result))
+                {
+                    return result;
+                }
+                else
+                {
+                    UnsafeAdd(key, value);
+                    return value;
+                }
+            }
+        }
+
+        public bool TryGetValue(TKey key, out TValue value)
+        {
+            lock (_lockObject)
+            {
+                return UnsafeTryGetValue(key, out value);
+            }
+        }
+
+        public bool Remove(TKey key)
+        {
+            lock (_lockObject)
+            {
+                return UnsafeRemove(key);
+            }
+        }
+
+        /// <summary>
+        /// For testing. Very expensive.
+        /// </summary>
+        internal IEnumerable<KeyValuePair<TKey, TValue>> TestingEnumerable
+        {
+            get
+            {
+                lock (_lockObject)
+                {
+                    foreach (var key in _nodeList)
+                    {
+                        var kvp = new KeyValuePair<TKey, TValue>(key, _cache[key].Value);
+                        yield return kvp;
+                    }
+                }
+            }
+        }
+
+        /// <summary>
+        /// Doesn't lock.
+        /// </summary>
+        private bool UnsafeTryGetValue(TKey key, out TValue value)
+        {
+            if (_cache.TryGetValue(key, out var result))
+            {
+                MoveNodeToTop(result.Node);
+                value = result.Value;
+                return true;
+            }
+            else
+            {
+                value = default(TValue);
+                return false;
+            }
+        }
+
+        private void MoveNodeToTop(LinkedListNode<TKey> node)
+        {
+            if (!object.ReferenceEquals(_nodeList.First, node))
+            {
+                _nodeList.Remove(node);
+                _nodeList.AddFirst(node);
+            }
+        }
+
+        /// <summary>
+        /// Expects non-empty cache. Does not lock.
+        /// </summary>
+        private void UnsafeEvictLastNode()
+        {
+            Debug.Assert(_capacity > 0);
+            var lastNode = _nodeList.Last;
+            _nodeList.Remove(lastNode);
+            _cache.Remove(lastNode.Value);
+        }
+
+        private void UnsafeAddNodeToTop(TKey key, TValue value)
+        {
+            var node = new LinkedListNode<TKey>(key);
+            _cache.Add(key, new CacheValue(value, node));
+            _nodeList.AddFirst(node);
+        }
+
+        /// <summary>
+        /// Doesn't lock.
+        /// </summary>
+        private void UnsafeAdd(TKey key, TValue value)
+        {
+            if (_cache.TryGetValue(key, out var result))
+            {
+                throw new ArgumentException("Key already exists", nameof(key));
+            }
+            else
+            {
+                if (_cache.Count == _capacity)
+                {
+                    UnsafeEvictLastNode();
+                }
+                UnsafeAddNodeToTop(key, value);
+            }
+        }
+
+        private bool UnsafeRemove(TKey key)
+        {
+            _nodeList.Remove(key);
+            return _cache.Remove(key);
+        }
+
+        private struct CacheValue
+        {
+            public CacheValue(TValue value, LinkedListNode<TKey> node)
+            {
+                Value = value;
+                Node = node;
+            }
+
+            public TValue Value { get; }
+
+            public LinkedListNode<TKey> Node { get; }
+        }
+    }
+}

src/Microsoft.AspNetCore.Razor.Tools/DefaultExtensionAssemblyLoader.cs

@@ -0,0 +1,241 @@
+// Copyright(c) .NET Foundation.All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Collections.Generic;
+using System.Collections.Immutable;
+using System.IO;
+using System.Reflection;
+using System.Reflection.Metadata;
+using System.Reflection.PortableExecutable;
+using System.Runtime.Loader;
+using Microsoft.CodeAnalysis;
+
+namespace Microsoft.AspNetCore.Razor.Tools
+{
+    internal class DefaultExtensionAssemblyLoader : ExtensionAssemblyLoader
+    {
+        private readonly string _baseDirectory;
+
+        private readonly object _lock = new object();
+        private readonly Dictionary<string, (Assembly assembly, AssemblyIdentity identity)> _loadedByPath;
+        private readonly Dictionary<AssemblyIdentity, Assembly> _loadedByIdentity;
+        private readonly Dictionary<string, AssemblyIdentity> _identityCache;
+        private readonly Dictionary<string, List<string>> _wellKnownAssemblies;
+
+        private ShadowCopyManager _shadowCopyManager;
+
+        public DefaultExtensionAssemblyLoader(string baseDirectory)
+        {
+            _baseDirectory = baseDirectory;
+
+            _loadedByPath = new Dictionary<string, (Assembly assembly, AssemblyIdentity identity)>(StringComparer.OrdinalIgnoreCase);
+            _loadedByIdentity = new Dictionary<AssemblyIdentity, Assembly>();
+            _identityCache = new Dictionary<string, AssemblyIdentity>(StringComparer.OrdinalIgnoreCase);
+            _wellKnownAssemblies = new Dictionary<string, List<string>>(StringComparer.OrdinalIgnoreCase);
+
+            LoadContext = new ExtensionAssemblyLoadContext(AssemblyLoadContext.GetLoadContext(typeof(ExtensionAssemblyLoader).Assembly), this);
+        }
+
+        protected AssemblyLoadContext LoadContext { get; }
+
+        public override void AddAssemblyLocation(string filePath)
+        {
+            if (filePath == null)
+            {
+                throw new ArgumentNullException(nameof(filePath));
+            }
+
+            if (!Path.IsPathRooted(filePath))
+            {
+                throw new ArgumentException(nameof(filePath));
+            }
+
+            var assemblyName = Path.GetFileNameWithoutExtension(filePath);
+            lock (_lock)
+            {
+                if (!_wellKnownAssemblies.TryGetValue(assemblyName, out var paths))
+                {
+                    paths = new List<string>();
+                    _wellKnownAssemblies.Add(assemblyName, paths);
+                }
+
+                if (!paths.Contains(filePath))
+                {
+                    paths.Add(filePath);
+                }
+            }
+        }
+
+        public override Assembly Load(string assemblyName)
+        {
+            if (!AssemblyIdentity.TryParseDisplayName(assemblyName, out var identity))
+            {
+                return null;
+            }
+
+            lock (_lock)
+            {
+                // First, check if this loader already loaded the requested assembly:
+                if (_loadedByIdentity.TryGetValue(identity, out var assembly))
+                {
+                    return assembly;
+                }
+
+                // Second, check if an assembly file of the same simple name was registered with the loader:
+                if (_wellKnownAssemblies.TryGetValue(identity.Name, out var paths))
+                {
+                    // Multiple assemblies of the same simple name but different identities might have been registered.
+                    // Load the one that matches the requested identity (if any).
+                    foreach (var path in paths)
+                    {
+                        var candidateIdentity = GetIdentity(path);
+
+                        if (identity.Equals(candidateIdentity))
+                        {
+                            return LoadFromPathUnsafe(path, candidateIdentity);
+                        }
+                    }
+                }
+
+                // We only support loading by name from 'well-known' paths. If you need to load something by
+                // name and you get here, then that means we don't know where to look.
+                return null;
+            }
+        }
+
+        public override Assembly LoadFromPath(string filePath)
+        {
+            if (filePath == null)
+            {
+                throw new ArgumentNullException(nameof(filePath));
+            }
+
+            if (!Path.IsPathRooted(filePath))
+            {
+                throw new ArgumentException(nameof(filePath));
+            }
+
+            lock (_lock)
+            {
+                return LoadFromPathUnsafe(filePath, identity: null);
+            }
+        }
+
+        private Assembly LoadFromPathUnsafe(string filePath, AssemblyIdentity identity)
+        {
+            // If we've already loaded the assembly by path there should be nothing else to do,
+            // all of our data is up to date.
+            if (_loadedByPath.TryGetValue(filePath, out var entry))
+            {
+                return entry.assembly;
+            }
+
+            // If we've already loaded the assembly by identity, then we might has some updating
+            // to do.
+            identity = identity ?? GetIdentity(filePath);
+            if (identity != null && _loadedByIdentity.TryGetValue(identity, out var assembly))
+            {
+                // An assembly file might be replaced by another file with a different identity.
+                // Last one wins.
+                _loadedByPath[filePath] = (assembly, identity);
+                return assembly;
+            }
+
+            // Ok we don't have this cached. Let's actually try to load the assembly.
+            assembly = LoadFromPathUnsafeCore(CopyAssembly(filePath));
+
+            identity = identity ?? AssemblyIdentity.FromAssemblyDefinition(assembly);
+
+            // It's possible an assembly was loaded by two different paths. Just use the original then.
+            if (_loadedByIdentity.TryGetValue(identity, out var duplicate))
+            {
+                assembly = duplicate;
+            }
+            else
+            {
+                _loadedByIdentity.Add(identity, assembly);
+            }
+
+            _loadedByPath[filePath] = (assembly, identity);
+            return assembly;
+        }
+
+        private AssemblyIdentity GetIdentity(string filePath)
+        {
+            if (!_identityCache.TryGetValue(filePath, out var identity))
+            {
+                identity = ReadAssemblyIdentity(filePath);
+                _identityCache.Add(filePath, identity);
+            }
+
+            return identity;
+        }
+
+        protected virtual string CopyAssembly(string filePath)
+        {
+            if (_baseDirectory == null)
+            {
+                // Don't shadow-copy when base directory is null. This means we're running as a CLI not
+                // a server.
+                return filePath;
+            }
+
+            if (_shadowCopyManager == null)
+            {
+                _shadowCopyManager = new ShadowCopyManager(_baseDirectory);
+            }
+
+            return _shadowCopyManager.AddAssembly(filePath);
+        }
+
+        protected virtual Assembly LoadFromPathUnsafeCore(string filePath)
+        {
+            return LoadContext.LoadFromAssemblyPath(filePath);
+        }
+
+        private static AssemblyIdentity ReadAssemblyIdentity(string filePath)
+        {
+            try
+            {
+                using (var stream = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite | FileShare.Delete))
+                using (var reader = new PEReader(stream))
+                {
+                    var metadataReader = reader.GetMetadataReader();
+                    return metadataReader.GetAssemblyIdentity();
+                }
+            }
+            catch
+            {
+            }
+
+            return null;
+        }
+
+        private class ExtensionAssemblyLoadContext : AssemblyLoadContext
+        {
+            private readonly AssemblyLoadContext _parent;
+            private readonly DefaultExtensionAssemblyLoader _loader;
+
+            public ExtensionAssemblyLoadContext(AssemblyLoadContext parent, DefaultExtensionAssemblyLoader loader)
+            {
+                _parent = parent;
+                _loader = loader;
+            }
+
+            protected override Assembly Load(AssemblyName assemblyName)
+            {
+                // Try to load from well-known paths. This will be called when loading a dependency of an extension.
+                var assembly = _loader.Load(assemblyName.ToString());
+                if (assembly != null)
+                {
+                    return assembly;
+                }
+
+                // If we don't have an entry, then fall back to the default load context. This allows extensions
+                // to resolve assemblies that are provided by the host.
+                return _parent.LoadFromAssemblyName(assemblyName);
+            }
+        }
+    }
+}

src/Microsoft.AspNetCore.Razor.Tools/DefaultExtensionDependencyChecker.cs

@@ -0,0 +1,158 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Reflection;
+using System.Reflection.Metadata;
+using System.Reflection.PortableExecutable;
+using Microsoft.CodeAnalysis;
+
+namespace Microsoft.AspNetCore.Razor.Tools
+{
+    internal class DefaultExtensionDependencyChecker : ExtensionDependencyChecker
+    {
+        // These are treated as prefixes. So `Microsoft.CodeAnalysis.Razor` would be assumed to work.
+        private static readonly string[] DefaultIgnoredAssemblies = new string[]
+        {
+            "mscorlib",
+            "netstandard",
+            "System",
+            "Microsoft.CodeAnalysis",
+            "Microsoft.AspNetCore.Razor.Language",
+        };
+
+        private readonly ExtensionAssemblyLoader _loader;
+        private readonly TextWriter _output;
+        private readonly TextWriter _error;
+        private readonly string[] _ignoredAssemblies;
+
+        public DefaultExtensionDependencyChecker(
+            ExtensionAssemblyLoader loader,
+            TextWriter output,
+            TextWriter error,
+            string[] ignoredAssemblies = null)
+        {
+            _loader = loader;
+            _output = output;
+            _error = error;
+            _ignoredAssemblies = ignoredAssemblies ?? DefaultIgnoredAssemblies;
+        }
+
+        public override bool Check(IEnumerable<string> assmblyFilePaths)
+        {
+            try
+            {
+                return CheckCore(assmblyFilePaths);
+            }
+            catch (Exception ex)
+            {
+                _error.WriteLine("Exception performing Extension dependency check:");
+                _error.WriteLine(ex.ToString());
+                return false;
+            }
+        }
+
+        private bool CheckCore(IEnumerable<string> assemblyFilePaths)
+        {
+            var items = assemblyFilePaths.Select(a => ExtensionVerificationItem.Create(a)).ToArray();
+            var assemblies = new HashSet<AssemblyIdentity>(items.Select(i => i.Identity));
+
+            for (var i = 0; i < items.Length; i++)
+            {
+                var item = items[i];
+                _output.WriteLine($"Verifying assembly at {item.FilePath}");
+
+                if (!Path.IsPathRooted(item.FilePath))
+                {
+                    _error.WriteLine($"The file path '{item.FilePath}' is not a rooted path. File paths must be absolute and fully-qualified.");
+                    return false;
+                }
+
+                foreach (var reference in item.References)
+                {
+                    if (_ignoredAssemblies.Any(n => reference.Name.StartsWith(n)))
+                    {
+                        // This is on the allow list, keep going.
+                        continue;
+                    }
+
+                    if (assemblies.Contains(reference))
+                    {
+                        // This was also provided as a dependency, keep going.
+                        continue;
+                    }
+
+                    // If we get here we can't resolve this assembly. This is an error.
+                    _error.WriteLine($"Extension assembly '{item.Identity.Name}' depends on '{reference.ToString()} which is missing.");
+                    return false;
+                }
+            }
+
+            // Assuming we get this far, the set of assemblies we have is at least a coherent set (barring
+            // version conflicts). Register all of the paths with the loader so they can find each other by
+            // name.
+            for (var i = 0; i < items.Length; i++)
+            {
+                _loader.AddAssemblyLocation(items[i].FilePath);
+            }
+
+            // Now try to load everything. This has the side effect of resolving all of these items
+            // in the loader's caches.
+            for (var i = 0; i < items.Length; i++)
+            {
+                var item = items[i];
+                item.Assembly = _loader.LoadFromPath(item.FilePath);
+            }
+
+            // Third, check that the MVIDs of the files on disk match the MVIDs of the loaded assemblies.
+            for (var i = 0; i < items.Length; i++)
+            {
+                var item = items[i];
+                if (item.Mvid != item.Assembly.ManifestModule.ModuleVersionId)
+                {
+                    _error.WriteLine($"Extension assembly '{item.Identity.Name}' at '{item.FilePath}' has a different ModuleVersionId than loaded assembly '{item.Assembly.FullName}'");
+                    return false;
+                }
+            }
+
+            return true;
+        }
+
+        private class ExtensionVerificationItem
+        {
+            public static ExtensionVerificationItem Create(string filePath)
+            {
+                using (var peReader = new PEReader(new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.Read)))
+                {
+                    var metadataReader = peReader.GetMetadataReader();
+                    var identity = metadataReader.GetAssemblyIdentity();
+                    var mvid = metadataReader.GetGuid(metadataReader.GetModuleDefinition().Mvid);
+                    var references = metadataReader.GetReferencedAssembliesOrThrow();
+
+                    return new ExtensionVerificationItem(filePath, identity, mvid, references.ToArray());
+                }
+            }
+
+            private ExtensionVerificationItem(string filePath, AssemblyIdentity identity, Guid mvid, AssemblyIdentity[] references)
+            {
+                FilePath = filePath;
+                Identity = identity;
+                Mvid = mvid;
+                References = references;
+            }
+
+            public string FilePath { get; }
+
+            public Assembly Assembly { get; set; }
+
+            public AssemblyIdentity Identity { get; }
+
+            public Guid Mvid { get; }
+
+            public IReadOnlyList<AssemblyIdentity> References { get; }
+        }
+    }
+}

src/Microsoft.AspNetCore.Razor.Tools/DefaultRequestDispatcher.cs

@@ -383,6 +383,8 @@ namespace Microsoft.AspNetCore.Razor.Tools
                                 ServerLogger.Log("End writing response.");
 
                                 reason = ConnectionResult.Reason.CompilationCompleted;
+
+                                _eventBus.CompilationCompleted();
                             }
                             catch
                             {

src/Microsoft.AspNetCore.Razor.Tools/DiscoverCommand.cs

@@ -8,7 +8,6 @@ using System.Linq;
 using System.Security.Cryptography;
 using System.Text;
 using System.Threading.Tasks;
-using Microsoft.AspNetCore.Mvc.Razor.Extensions;
 using Microsoft.AspNetCore.Razor.Language;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.Razor;
@@ -26,6 +25,10 @@ namespace Microsoft.AspNetCore.Razor.Tools
             Assemblies = Argument("assemblies", "assemblies to search for tag helpers", multipleValues: true);
             TagHelperManifest = Option("-o", "output file", CommandOptionType.SingleValue);
             ProjectDirectory = Option("-p", "project root directory", CommandOptionType.SingleValue);
+            Version = Option("-v|--version", "Razor language version", CommandOptionType.SingleValue);
+            Configuration = Option("-c", "Razor configuration name", CommandOptionType.SingleValue);
+            ExtensionNames = Option("-n", "extension name", CommandOptionType.MultipleValue);
+            ExtensionFilePaths = Option("-e", "extension file path", CommandOptionType.MultipleValue);
         }
 
         public CommandArgument Assemblies { get; }
@@ -34,6 +37,14 @@ namespace Microsoft.AspNetCore.Razor.Tools
 
         public CommandOption ProjectDirectory { get; }
 
+        public CommandOption Version { get; }
+
+        public CommandOption Configuration { get; }
+
+        public CommandOption ExtensionNames { get; }
+
+        public CommandOption ExtensionFilePaths { get; }
+
         protected override bool ValidateArguments()
         {
             if (string.IsNullOrEmpty(TagHelperManifest.Value()))
@@ -53,12 +64,61 @@ namespace Microsoft.AspNetCore.Razor.Tools
                 ProjectDirectory.Values.Add(Environment.CurrentDirectory);
             }
 
+            if (string.IsNullOrEmpty(Version.Value()))
+            {
+                Error.WriteLine($"{Version.ValueName} must be specified.");
+                return false;
+            }
+            else if (!RazorLanguageVersion.TryParse(Version.Value(), out _))
+            {
+                Error.WriteLine($"{Version.ValueName} is not a valid language version.");
+                return false;
+            }
+
+            if (string.IsNullOrEmpty(Configuration.Value()))
+            {
+                Error.WriteLine($"{Configuration.ValueName} must be specified.");
+                return false;
+            }
+
+            if (ExtensionNames.Values.Count != ExtensionFilePaths.Values.Count)
+            {
+                Error.WriteLine($"{ExtensionNames.ValueName} and {ExtensionFilePaths.ValueName} should have the same number of values.");
+            }
+
+            foreach (var filePath in ExtensionFilePaths.Values)
+            {
+                if (!Path.IsPathRooted(filePath))
+                {
+                    Error.WriteLine($"Extension file paths must be fully-qualified, absolute paths.");
+                    return false;
+                }
+            }
+
+            if (!Parent.Checker.Check(ExtensionFilePaths.Values))
+            {
+                Error.WriteLine($"Extenions could not be loaded. See output for details.");
+                return false;
+            }
+
             return true;
         }
 
         protected override Task<int> ExecuteCoreAsync()
         {
+            // Loading all of the extensions should succeed as the dependency checker will have already
+            // loaded them.
+            var extensions = new RazorExtension[ExtensionNames.Values.Count];
+            for (var i = 0; i < ExtensionNames.Values.Count; i++)
+            {
+                extensions[i] = new AssemblyExtension(ExtensionNames.Values[i], Parent.Loader.LoadFromPath(ExtensionFilePaths.Values[i]));
+            }
+
+            var version = RazorLanguageVersion.Parse(Version.Value());
+            var configuration = new RazorConfiguration(version, Configuration.Value(), extensions);
+
             var result = ExecuteCore(
+                configuration: configuration,
                 projectDirectory: ProjectDirectory.Value(),
                 outputFilePath: TagHelperManifest.Value(),
                 assemblies: Assemblies.Values.ToArray());
@@ -66,36 +126,31 @@ namespace Microsoft.AspNetCore.Razor.Tools
             return Task.FromResult(result);
         }
 
-        private int ExecuteCore(string projectDirectory, string outputFilePath, string[] assemblies)
+        private int ExecuteCore(RazorConfiguration configuration, string projectDirectory, string outputFilePath, string[] assemblies)
         {
             outputFilePath = Path.Combine(projectDirectory, outputFilePath);
 
             var metadataReferences = new MetadataReference[assemblies.Length];
             for (var i = 0; i < assemblies.Length; i++)
             {
-                metadataReferences[i] = MetadataReference.CreateFromFile(assemblies[i]);
+                metadataReferences[i] = Parent.AssemblyReferenceProvider(assemblies[i], default(MetadataReferenceProperties));
             }
 
-            var engine = RazorEngine.Create((b) =>
+            var engine = RazorProjectEngine.Create(configuration, RazorProjectFileSystem.Empty, b =>
             {
-                RazorExtensions.Register(b);
-
                 b.Features.Add(new DefaultMetadataReferenceFeature() { References = metadataReferences });
                 b.Features.Add(new CompilationTagHelperFeature());
-
-                // TagHelperDescriptorProviders (actually do tag helper discovery)
                 b.Features.Add(new DefaultTagHelperDescriptorProvider());
-                b.Features.Add(new ViewComponentTagHelperDescriptorProvider());
             });
 
-            var feature = engine.Features.OfType<ITagHelperFeature>().Single();
+            var feature = engine.Engine.Features.OfType<ITagHelperFeature>().Single();
             var tagHelpers = feature.GetDescriptors();
 
             using (var stream = new MemoryStream())
             {
                 Serialize(stream, tagHelpers);
 
-                stream.Position = 0L;
+                stream.Position = 0;
 
                 var newHash = Hash(stream);
                 var existingHash = Hash(outputFilePath);
@@ -103,7 +158,7 @@ namespace Microsoft.AspNetCore.Razor.Tools
                 if (!HashesEqual(newHash, existingHash))
                 {
                     stream.Position = 0;
-                    using (var output = File.OpenWrite(outputFilePath))
+                    using (var output = File.Open(outputFilePath, FileMode.Create))
                     {
                         stream.CopyTo(output);
                     }

src/Microsoft.AspNetCore.Razor.Tools/EventBus.cs

@@ -37,6 +37,13 @@ namespace Microsoft.AspNetCore.Razor.Tools
         public virtual void ConnectionCompleted(int count)
         {
         }
+        
+        /// <summary>
+        /// Called when a compilation is completed successfully and the response is written to the stream.
+        /// </summary>
+        public virtual void CompilationCompleted()
+        {
+        }
 
         /// <summary>
         /// Called when a bad client connection was detected and the server will be shutting down as a 

src/Microsoft.AspNetCore.Razor.Tools/ExtensionAssemblyLoader.cs

@@ -0,0 +1,16 @@
+// Copyright(c) .NET Foundation.All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Reflection;
+
+namespace Microsoft.AspNetCore.Razor.Tools
+{
+    internal abstract class ExtensionAssemblyLoader
+    {
+        public abstract void AddAssemblyLocation(string filePath);
+
+        public abstract Assembly Load(string assemblyName);
+
+        public abstract Assembly LoadFromPath(string filePath);
+    }
+}

src/Microsoft.AspNetCore.Razor.Tools/ExtensionDependencyChecker.cs

@@ -0,0 +1,12 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Collections.Generic;
+
+namespace Microsoft.AspNetCore.Razor.Tools
+{
+    internal abstract class ExtensionDependencyChecker
+    {
+        public abstract bool Check(IEnumerable<string> extensionFilePaths);
+    }
+}

src/Microsoft.AspNetCore.Razor.Tools/GenerateCommand.cs

@@ -3,9 +3,9 @@
 
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.IO;
 using System.Threading.Tasks;
-using Microsoft.AspNetCore.Mvc.Razor.Extensions;
 using Microsoft.AspNetCore.Razor.Language;
 using Microsoft.Extensions.CommandLineUtils;
 using Microsoft.VisualStudio.LanguageServices.Razor;
@@ -13,13 +13,6 @@ using Newtonsoft.Json;
 
 namespace Microsoft.AspNetCore.Razor.Tools
 {
-    internal class Builder<T>
-    {
-        public static Builder<T> Make(CommandBase result) => null;
-
-        public static Builder<T> Make(T result) => null;
-    }
-
     internal class GenerateCommand : CommandBase
     {
         public GenerateCommand(Application parent)
@@ -30,6 +23,10 @@ namespace Microsoft.AspNetCore.Razor.Tools
             RelativePaths = Option("-r", "Relative path", CommandOptionType.MultipleValue);
             ProjectDirectory = Option("-p", "project root directory", CommandOptionType.SingleValue);
             TagHelperManifest = Option("-t", "tag helper manifest file", CommandOptionType.SingleValue);
+            Version = Option("-v|--version", "Razor language version", CommandOptionType.SingleValue);
+            Configuration = Option("-c", "Razor configuration name", CommandOptionType.SingleValue);
+            ExtensionNames = Option("-n", "extension name", CommandOptionType.MultipleValue);
+            ExtensionFilePaths = Option("-e", "extension file path", CommandOptionType.MultipleValue);
         }
 
         public CommandOption Sources { get; }
@@ -42,9 +39,29 @@ namespace Microsoft.AspNetCore.Razor.Tools
 
         public CommandOption TagHelperManifest { get; }
 
+        public CommandOption Version { get; }
+
+        public CommandOption Configuration { get; }
+
+        public CommandOption ExtensionNames { get; }
+
+        public CommandOption ExtensionFilePaths { get; }
+
         protected override Task<int> ExecuteCoreAsync()
         {
+            // Loading all of the extensions should succeed as the dependency checker will have already
+            // loaded them.
+            var extensions = new RazorExtension[ExtensionNames.Values.Count];
+            for (var i = 0; i < ExtensionNames.Values.Count; i++)
+            {
+                extensions[i] = new AssemblyExtension(ExtensionNames.Values[i], Parent.Loader.LoadFromPath(ExtensionFilePaths.Values[i]));
+            }
+
+            var version = RazorLanguageVersion.Parse(Version.Value());
+            var configuration = new RazorConfiguration(version, Configuration.Value(), extensions);
+
             var result = ExecuteCore(
+                configuration: configuration,
                 projectDirectory: ProjectDirectory.Value(),
                 tagHelperManifest: TagHelperManifest.Value(),
                 sources: Sources.Values,
@@ -77,10 +94,48 @@ namespace Microsoft.AspNetCore.Razor.Tools
                 ProjectDirectory.Values.Add(Environment.CurrentDirectory);
             }
 
+            if (string.IsNullOrEmpty(Version.Value()))
+            {
+                Error.WriteLine($"{Version.ValueName} must be specified.");
+                return false;
+            }
+            else if (!RazorLanguageVersion.TryParse(Version.Value(), out _))
+            {
+                Error.WriteLine($"{Version.ValueName} is not a valid language version.");
+                return false;
+            }
+
+            if (string.IsNullOrEmpty(Configuration.Value()))
+            {
+                Error.WriteLine($"{Configuration.ValueName} must be specified.");
+                return false;
+            }
+
+            if (ExtensionNames.Values.Count != ExtensionFilePaths.Values.Count)
+            {
+                Error.WriteLine($"{ExtensionNames.ValueName} and {ExtensionFilePaths.ValueName} should have the same number of values.");
+            }
+
+            foreach (var filePath in ExtensionFilePaths.Values)
+            {
+                if (!Path.IsPathRooted(filePath))
+                {
+                    Error.WriteLine($"Extension file paths must be fully-qualified, absolute paths.");
+                    return false;
+                }
+            }
+
+            if (!Parent.Checker.Check(ExtensionFilePaths.Values))
+            {
+                Error.WriteLine($"Extensions could not be loaded. See output for details.");
+                return false;
+            }
+
             return true;
         }
 
         private int ExecuteCore(
+            RazorConfiguration configuration,
             string projectDirectory,
             string tagHelperManifest,
             List<string> sources,
@@ -90,26 +145,19 @@ namespace Microsoft.AspNetCore.Razor.Tools
             tagHelperManifest = Path.Combine(projectDirectory, tagHelperManifest);
 
             var tagHelpers = GetTagHelpers(tagHelperManifest);
-
+            var inputItems = GetInputItems(projectDirectory, sources, outputs, relativePaths);
-            var engine = RazorEngine.Create(b =>
+            var compositeFileSystem = new CompositeRazorProjectFileSystem(new[]
+            {
+                GetVirtualRazorProjectSystem(inputItems),
+                RazorProjectFileSystem.Create(projectDirectory),
+            });
+            
+            var engine = RazorProjectEngine.Create(configuration, compositeFileSystem, b =>
             {
-                RazorExtensions.Register(b);
-
                 b.Features.Add(new StaticTagHelperFeature() { TagHelpers = tagHelpers, });
             });
 
-
+            var results = GenerateCode(engine, inputItems);
-            var inputItems = GetInputItems(projectDirectory, sources, outputs, relativePaths);
-            var compositeProject = new CompositeRazorProjectFileSystem(
-                new[]
-                {
-                    GetVirtualRazorProjectSystem(inputItems),
-                    RazorProjectFileSystem.Create(projectDirectory),
-                });
-
-            var templateEngine = new MvcRazorTemplateEngine(engine, compositeProject);
-
-            var results = GenerateCode(templateEngine, inputItems);
 
             var success = true;
 
@@ -180,14 +228,15 @@ namespace Microsoft.AspNetCore.Razor.Tools
             return items;
         }
 
-        private OutputItem[] GenerateCode(RazorTemplateEngine templateEngine, SourceItem[] inputs)
+        private OutputItem[] GenerateCode(RazorProjectEngine engine, SourceItem[] inputs)
         {
             var outputs = new OutputItem[inputs.Length];
-            Parallel.For(0, outputs.Length, new ParallelOptions() { MaxDegreeOfParallelism = 4 }, i =>
+            Parallel.For(0, outputs.Length, new ParallelOptions() { MaxDegreeOfParallelism = Debugger.IsAttached ? 1 : 4 }, i =>
             {
                 var inputItem = inputs[i];
 
-                var csharpDocument = templateEngine.GenerateCode(inputItem.FilePath);
+                var codeDocument = engine.Process(engine.FileSystem.GetItem(inputItem.FilePath));
+                var csharpDocument = codeDocument.GetCSharpDocument();
                 outputs[i] = new OutputItem(inputItem, csharpDocument);
             });
 

src/Microsoft.AspNetCore.Razor.Tools/MetadataCache.cs

@@ -0,0 +1,86 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Diagnostics;
+using System.IO;
+using System.Reflection.PortableExecutable;
+using Microsoft.CodeAnalysis;
+
+namespace Microsoft.AspNetCore.Razor.Tools
+{
+    internal class MetadataCache
+    {
+        // Store 1000 entries -- arbitrary number
+        private const int CacheSize = 1000;
+        private readonly ConcurrentLruCache<string, MetadataCacheEntry> _metadataCache =
+            new ConcurrentLruCache<string, MetadataCacheEntry>(CacheSize, StringComparer.OrdinalIgnoreCase);
+
+        // For testing purposes only.
+        internal ConcurrentLruCache<string, MetadataCacheEntry> Cache => _metadataCache;
+
+        internal Metadata GetMetadata(string fullPath)
+        {
+            var timestamp = GetFileTimeStamp(fullPath);
+
+            // Check if we have an entry in the dictionary.
+            if (_metadataCache.TryGetValue(fullPath, out var entry))
+            {
+                if (timestamp.HasValue && timestamp.Value == entry.Timestamp)
+                {
+                    // The file has not changed since we cached it. Return the cached entry.
+                    return entry.Metadata;
+                }
+                else
+                {
+                    // The file has changed recently. Remove the cache entry.
+                    _metadataCache.Remove(fullPath);
+                }
+            }
+
+            Metadata metadata;
+            using (var fileStream = File.OpenRead(fullPath))
+            {
+                metadata = AssemblyMetadata.CreateFromStream(fileStream, PEStreamOptions.PrefetchMetadata);
+            }
+
+            _metadataCache.GetOrAdd(fullPath, new MetadataCacheEntry(timestamp.Value, metadata));
+
+            return metadata;
+        }
+
+        private static DateTime? GetFileTimeStamp(string fullPath)
+        {
+            try
+            {
+                Debug.Assert(Path.IsPathRooted(fullPath));
+
+                return File.GetLastWriteTimeUtc(fullPath);
+            }
+            catch (Exception e)
+            {
+                // There are several exceptions that can occur here: NotSupportedException or PathTooLongException
+                // for a bad path, UnauthorizedAccessException for access denied, etc. Rather than listing them all,
+                // just catch all exceptions and log.
+                ServerLogger.LogException(e, $"Error getting timestamp of file {fullPath}.");
+
+                return null;
+            }
+        }
+
+        internal struct MetadataCacheEntry
+        {
+            public MetadataCacheEntry(DateTime timestamp, Metadata metadata)
+            {
+                Debug.Assert(timestamp.Kind == DateTimeKind.Utc);
+
+                Timestamp = timestamp;
+                Metadata = metadata;
+            }
+
+            public DateTime Timestamp { get; }
+
+            public Metadata Metadata { get; }
+        }
+    }
+}

src/Microsoft.AspNetCore.Razor.Tools/MetadataReaderExtensions.cs

@@ -0,0 +1,93 @@
+// Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
+
+using System;
+using System.Collections.Generic;
+using System.Collections.Immutable;
+using System.Reflection;
+using System.Reflection.Metadata;
+using Microsoft.CodeAnalysis;
+
+namespace Microsoft.AspNetCore.Razor.Tools
+{
+    internal static class MetadataReaderExtensions
+    {
+        internal static AssemblyIdentity GetAssemblyIdentity(this MetadataReader reader)
+        {
+            if (!reader.IsAssembly)
+            {
+                throw new BadImageFormatException();
+            }
+
+            var definition = reader.GetAssemblyDefinition();
+
+            return CreateAssemblyIdentity(
+                reader,
+                definition.Version,
+                definition.Flags,
+                definition.PublicKey,
+                definition.Name,
+                definition.Culture,
+                isReference: false);
+        }
+        
+        internal static AssemblyIdentity[] GetReferencedAssembliesOrThrow(this MetadataReader reader)
+        {
+            var references = new List<AssemblyIdentity>();
+
+            foreach (var referenceHandle in reader.AssemblyReferences)
+            {
+                var reference = reader.GetAssemblyReference(referenceHandle);
+                references.Add(CreateAssemblyIdentity(
+                    reader,
+                    reference.Version,
+                    reference.Flags,
+                    reference.PublicKeyOrToken,
+                    reference.Name,
+                    reference.Culture,
+                    isReference: true));
+            }
+
+            return references.ToArray();
+        }
+        
+        private static AssemblyIdentity CreateAssemblyIdentity(
+            MetadataReader reader,
+            Version version,
+            AssemblyFlags flags,
+            BlobHandle publicKey,
+            StringHandle name,
+            StringHandle culture,
+            bool isReference)
+        {
+            var publicKeyOrToken = reader.GetBlobContent(publicKey);
+            bool hasPublicKey;
+
+            if (isReference)
+            {
+                hasPublicKey = (flags & AssemblyFlags.PublicKey) != 0;
+            }
+            else
+            {
+                // Assembly definitions never contain a public key token, they only can have a full key or nothing,
+                // so the flag AssemblyFlags.PublicKey does not make sense for them and is ignored.
+                // See Ecma-335, Partition II Metadata, 22.2 "Assembly : 0x20".
+                // This also corresponds to the behavior of the native C# compiler and sn.exe tool.
+                hasPublicKey = !publicKeyOrToken.IsEmpty;
+            }
+
+            if (publicKeyOrToken.IsEmpty)
+            {
+                publicKeyOrToken = default;
+            }
+
+            return new AssemblyIdentity(
+                name: reader.GetString(name),
+                version: version,
+                cultureName: culture.IsNil ? null : reader.GetString(culture),
+                publicKeyOrToken: publicKeyOrToken,
+                hasPublicKey: hasPublicKey,
+                isRetargetable: (flags & AssemblyFlags.Retargetable) != 0,
+                contentType: (AssemblyContentType)((int)(flags & AssemblyFlags.ContentTypeMask) >> 9));
+        }
+    }
+}

src/Microsoft.AspNetCore.Razor.Tools/Microsoft.AspNetCore.Razor.Tools.csproj

@@ -27,7 +27,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <ProjectReference Include="..\Microsoft.AspNetCore.Mvc.Razor.Extensions\Microsoft.AspNetCore.Mvc.Razor.Extensions.csproj" />
+    <ProjectReference Include="..\Microsoft.CodeAnalysis.Razor\Microsoft.CodeAnalysis.Razor.csproj" />
   </ItemGroup>
 
   <!-- This makes it so that the runtimeconfig.json is included as part of the build output of the project that references this project. -->

src/Microsoft.AspNetCore.Razor.Tools/Program.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Threading;
+using Microsoft.CodeAnalysis;
 
 namespace Microsoft.AspNetCore.Razor.Tools
 {
@@ -15,7 +16,16 @@ namespace Microsoft.AspNetCore.Razor.Tools
             var cancel = new CancellationTokenSource();
             Console.CancelKeyPress += (sender, e) => { cancel.Cancel(); };
 
-            var application = new Application(cancel.Token);
+            // Prevent shadow copying.
+            var loader = new DefaultExtensionAssemblyLoader(baseDirectory: null);
+            var checker = new DefaultExtensionDependencyChecker(loader, Console.Out, Console.Error);
+
+            var application = new Application(
+                cancel.Token,
+                loader,
+                checker,
+                (path, properties) => MetadataReference.CreateFromFile(path, properties));
+
             return application.Execute(args);
         }
     }

src/Microsoft.AspNetCore.Razor.Tools/ServerCommand.cs

@@ -57,6 +57,7 @@ namespace Microsoft.AspNetCore.Razor.Tools
                     }
 
                     var host = ConnectionHost.Create(Pipe.Value());
+
                     var compilerHost = CompilerHost.Create();
                     ExecuteServerCore(host, compilerHost, Cancelled, eventBus: null, keepAlive: keepAlive);
                 }

src/Microsoft.AspNetCore.Razor.Tools/ServerProtocol/ServerRequest.cs

@@ -170,7 +170,7 @@ namespace Microsoft.AspNetCore.Razor.Tools
         /// <summary>
         /// Write a Request to the stream.
         /// </summary>
-        public async Task WriteAsync(Stream outStream, CancellationToken cancellationToken = default)
+        public async Task WriteAsync(Stream outStream, CancellationToken cancellationToken = default(CancellationToken))
         {
             using (var memoryStream = new MemoryStream())
             using (var writer = new BinaryWriter(memoryStream, Encoding.Unicode))

src/Microsoft.AspNetCore.Razor.Tools/ShadowCopyManager.cs

@@ -0,0 +1,169 @@
+// Copyright(c) .NET Foundation.All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Microsoft.AspNetCore.Razor.Tools
+{
+    // Note that this class has no thread-safety guarantees. The caller should use a lock 
+    // if concurrency is required.
+    internal class ShadowCopyManager : IDisposable
+    {
+        // Note that this class uses the *existance* of the Mutex to lock a directory.
+        //
+        // Nothing in this code actually ever acquires the Mutex, we just try to see if it exists
+        // already.
+        private readonly Mutex _mutex;
+
+        private int _counter;
+
+        public ShadowCopyManager(string baseDirectory = null)
+        {
+            BaseDirectory = baseDirectory ?? Path.Combine(Path.GetTempPath(), "Razor", "ShadowCopy");
+
+            var guid = Guid.NewGuid().ToString("N").ToLowerInvariant();
+            UniqueDirectory = Path.Combine(BaseDirectory, guid);
+
+            _mutex = new Mutex(initiallyOwned: false, name: guid);
+
+            Directory.CreateDirectory(UniqueDirectory);
+        }
+
+        public string BaseDirectory { get; }
+
+        public string UniqueDirectory { get; }
+
+        public string AddAssembly(string filePath)
+        {
+            var assemblyDirectory = CreateUniqueDirectory();
+
+            var destination = Path.Combine(assemblyDirectory, Path.GetFileName(filePath));
+            CopyFile(filePath, destination);
+            
+            var fileNameWithoutExtension = Path.GetFileNameWithoutExtension(filePath);
+            var resourcesNameWithoutExtension = fileNameWithoutExtension + ".resources";
+            var resourcesNameWithExtension = resourcesNameWithoutExtension + ".dll";
+
+            foreach (var directory in Directory.EnumerateDirectories(Path.GetDirectoryName(filePath)))
+            {
+                var directoryName = Path.GetFileName(directory);
+
+                var resourcesPath = Path.Combine(directory, resourcesNameWithExtension);
+                if (File.Exists(resourcesPath))
+                {
+                    var resourcesShadowCopyPath = Path.Combine(assemblyDirectory, directoryName, resourcesNameWithExtension);
+                    CopyFile(resourcesPath, resourcesShadowCopyPath);
+                }
+
+                resourcesPath = Path.Combine(directory, resourcesNameWithoutExtension, resourcesNameWithExtension);
+                if (File.Exists(resourcesPath))
+                {
+                    var resourcesShadowCopyPath = Path.Combine(assemblyDirectory, directoryName, resourcesNameWithoutExtension, resourcesNameWithExtension);
+                    CopyFile(resourcesPath, resourcesShadowCopyPath);
+                }
+            }
+
+            return destination;
+        }
+
+        public void Dispose()
+        {
+            _mutex.ReleaseMutex();
+        }
+
+        public Task PurgeUnusedDirectoriesAsync()
+        {
+            return Task.Run((Action)PurgeUnusedDirectories);
+        }
+
+        private string CreateUniqueDirectory()
+        {
+            var id = _counter++;
+
+            var directory = Path.Combine(UniqueDirectory, id.ToString());
+            Directory.CreateDirectory(directory);
+            return directory;
+        }
+
+        private void CopyFile(string originalPath, string shadowCopyPath)
+        {
+            var directory = Path.GetDirectoryName(shadowCopyPath);
+            Directory.CreateDirectory(directory);
+
+            File.Copy(originalPath, shadowCopyPath);
+
+            MakeWritable(new FileInfo(shadowCopyPath));
+        }
+
+        private void MakeWritable(string directoryPath)
+        {
+            var directory = new DirectoryInfo(directoryPath);
+
+            foreach (var file in directory.EnumerateFiles(searchPattern: "*", searchOption: SearchOption.AllDirectories))
+            {
+                MakeWritable(file);
+            }
+        }
+
+        private void MakeWritable(FileInfo file)
+        {
+            try
+            {
+                if (file.IsReadOnly)
+                {
+                    file.IsReadOnly = false;
+                }
+            }
+            catch
+            {
+                // There are many reasons this could fail. Ignore it and keep going.
+            }
+        }
+
+        private void PurgeUnusedDirectories()
+        {
+            IEnumerable<string> directories;
+            try
+            {
+                directories = Directory.EnumerateDirectories(BaseDirectory);
+            }
+            catch (DirectoryNotFoundException)
+            {
+                return;
+            }
+
+            foreach (var directory in directories)
+            {
+                Mutex mutex = null;
+                try
+                {
+                    // We only want to try deleting the directory if no-one else is currently using it.
+                    //
+                    // Note that the mutex name is the name of the directory. This is OK because we're using
+                    // GUIDs as directory/mutex names.
+                    if (!Mutex.TryOpenExisting(Path.GetFileName(directory).ToLowerInvariant(), out mutex))
+                    {
+                        MakeWritable(directory);
+                        Directory.Delete(directory, recursive: true);
+                    }
+                }
+                catch
+                {
+                    // If something goes wrong we will leave it to the next run to clean up.
+                    // Just swallow the exception and move on.
+                }
+                finally
+                {
+                    if (mutex != null)
+                    {
+                        mutex.Dispose();
+                    }
+                }
+            }
+        }
+    }
+}

src/Microsoft.AspNetCore.Razor.Tools/ShutdownCommand.cs

@@ -57,19 +57,21 @@ namespace Microsoft.AspNetCore.Razor.Tools
                             var process = Process.GetProcessById(response.ServerProcessId);
                             process.WaitForExit();
                         }
-                        catch (Exception)
+                        catch (Exception ex)
                         {
                             // There is an inherent race here with the server process.  If it has already shutdown
-                            // by the time we try to access it then the operation has succeed.
+                            // by the time we try to access it then the operation has succeeded.
+                            Error.Write(ex);
                         }
 
                         Out.Write("Server pid:{0} shut down", response.ServerProcessId);
                     }
                 }
             }
-            catch (Exception) when (IsServerRunning())
+            catch (Exception ex) when (IsServerRunning())
             {
                 // Ignore an exception that occurred while the server was shutting down.
+                Error.Write(ex);
             }
 
             return 0;

src/Microsoft.CodeAnalysis.Razor.Workspaces/ExportCustomProjectEngineFactoryAttribute.cs

@@ -0,0 +1,28 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Composition;
+
+namespace Microsoft.CodeAnalysis.Razor
+{
+    [MetadataAttribute]
+    [AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = true)]
+    public class ExportCustomProjectEngineFactoryAttribute : ExportAttribute, ICustomProjectEngineFactoryMetadata
+    {
+        public ExportCustomProjectEngineFactoryAttribute(string configurationName)
+            : base(typeof(IProjectEngineFactory))
+        {
+            if (configurationName == null)
+            {
+                throw new ArgumentNullException(nameof(configurationName));
+            }
+
+            ConfigurationName = configurationName;
+        }
+
+        public string ConfigurationName { get; }
+
+        public bool SupportsSerialization { get; set; }
+    }
+}

src/Microsoft.CodeAnalysis.Razor.Workspaces/ICustomProjectEngineFactoryMetadata.cs

@@ -0,0 +1,12 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+namespace Microsoft.CodeAnalysis.Razor
+{
+    public interface ICustomProjectEngineFactoryMetadata
+    {
+        string ConfigurationName { get; }
+
+        bool SupportsSerialization { get; }
+    }
+}

src/Microsoft.CodeAnalysis.Razor.Workspaces/IProjectEngineFactory.cs

@@ -0,0 +1,13 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using Microsoft.AspNetCore.Razor.Language;
+
+namespace Microsoft.CodeAnalysis.Razor
+{
+    public interface IProjectEngineFactory
+    {
+        RazorProjectEngine Create(RazorConfiguration configuration, RazorProjectFileSystem fileSystem, Action<RazorProjectEngineBuilder> configure);
+    }
+}

src/Microsoft.CodeAnalysis.Razor.Workspaces/RazorProjectEngineFactoryService.cs

@@ -7,8 +7,8 @@ using Microsoft.CodeAnalysis.Host;
 
 namespace Microsoft.CodeAnalysis.Razor
 {
-    internal abstract class RazorTemplateEngineFactoryService : ILanguageService
+    internal abstract class RazorProjectEngineFactoryService : ILanguageService
     {
-        public abstract RazorTemplateEngine Create(string projectPath, Action<IRazorEngineBuilder> configure);
+        public abstract RazorProjectEngine Create(string projectPath, Action<RazorProjectEngineBuilder> configure);
     }
 }

src/Microsoft.NET.Sdk.Razor/Sdk/Sdk.props

@@ -10,7 +10,16 @@ Copyright (c) .NET Foundation. All rights reserved.
 ***********************************************************************************************
 -->
 <Project ToolsVersion="14.0">
+  <PropertyGroup>
+    <!-- Determines if the Razor Sdk is responsible for importing Microsoft.NET.Sdk. Microsoft.NET.Sdk.Web may have previously imported this. -->
+    <_RazorSdkImportsMicrosoftNetSdk Condition="'$(UsingMicrosoftNETSdk)' != 'true'">true</_RazorSdkImportsMicrosoftNetSdk>
+  </PropertyGroup>
 
-  <Import Project="$(MSBuildThisFileDirectory)..\build\netstandard2.0\Sdk.Razor.CurrentVersion.props" />
+  <Import Sdk="Microsoft.NET.Sdk" Project="Sdk.props" Condition="'$(_RazorSdkImportsMicrosoftNetSdk)' == 'true'" />
 
+  <PropertyGroup>
+    <RazorSdkCurrentVersionProps Condition="'$(RazorSdkCurrentVersionProps)' == ''">$(MSBuildThisFileDirectory)..\build\netstandard2.0\Sdk.Razor.CurrentVersion.props</RazorSdkCurrentVersionProps>
+  </PropertyGroup>
+
+  <Import Project="$(RazorSdkCurrentVersionProps)" />
 </Project>

src/Microsoft.NET.Sdk.Razor/Sdk/Sdk.targets

@@ -11,11 +11,13 @@ Copyright (c) .NET Foundation. All rights reserved.
 -->
 <Project ToolsVersion="14.0">
 
-  <PropertyGroup Condition="'$(RazorSdkTargetsPath)' == ''">
+  <Import Sdk="Microsoft.NET.Sdk" Project="Sdk.targets" Condition="'$(_RazorSdkImportsMicrosoftNetSdk)' == 'true'" />
-    <RazorSdkTargetsPath Condition="'$(IsCrossTargetingBuild)' == 'true'">$(MSBuildThisFileDirectory)..\buildMultiTargeting\Sdk.Razor.CurrentVersion.MultiTargeting.targets</RazorSdkTargetsPath>
+
-    <RazorSdkTargetsPath Condition="'$(IsCrossTargetingBuild)' != 'true'">$(MSBuildThisFileDirectory)..\build\netstandard2.0\Sdk.Razor.CurrentVersion.targets</RazorSdkTargetsPath>
+  <PropertyGroup Condition="'$(RazorSdkCurrentVersionTargets)' == ''">
+    <RazorSdkCurrentVersionTargets Condition="'$(IsCrossTargetingBuild)' == 'true'">$(MSBuildThisFileDirectory)..\buildMultiTargeting\Sdk.Razor.CurrentVersion.MultiTargeting.targets</RazorSdkCurrentVersionTargets>
+    <RazorSdkCurrentVersionTargets Condition="'$(IsCrossTargetingBuild)' != 'true'">$(MSBuildThisFileDirectory)..\build\netstandard2.0\Sdk.Razor.CurrentVersion.targets</RazorSdkCurrentVersionTargets>
   </PropertyGroup>
 
-  <Import Project="$(RazorSdkTargetsPath)" />
+  <Import Project="$(RazorSdkCurrentVersionTargets)" />
 
 </Project>

src/Microsoft.NET.Sdk.Razor/build/netstandard2.0/Microsoft.NET.Sdk.Razor.DesignTime.targets

@@ -0,0 +1,48 @@
+<!--
+***********************************************************************************************
+Microsoft.NET.Sdk.Razor.DesignTime.targets
+
+WARNING:  DO NOT MODIFY this file unless you are knowledgeable about MSBuild and have
+          created a backup copy.  Incorrect changes to this file will make it
+          impossible to load or build your projects from the command-line or the IDE.
+
+Copyright (c) .NET Foundation. All rights reserved.
+***********************************************************************************************
+-->
+
+<Project ToolsVersion="14.0">
+
+  <ItemGroup>
+    <!--
+      Defines the generic .NET Core 'Razor' capability.
+
+      Note that we don't define any capabilities here that depend on the version of the runtime/toolset
+      in use by the project. Those capabilities are defined by the relevant runtime packages so that
+      we use the lack of the capability to detect downlevel scenarios.
+    -->
+    <ProjectCapability Include="DotNetCoreRazor"/>
+  </ItemGroup>
+
+  <!--
+    WebSdk imports these capabilities for nesting in DotNetCoreWeb projects.
+    Conditinally import these capabilities if the project isn't targeting the WebSdk.
+   -->
+  <ItemGroup Condition="'$(UsingMicrosoftNETSdkWeb)'==''">
+    <ProjectCapability Include="SupportHierarchyContextSvc" />
+    <ProjectCapability Include="DynamicDependentFile" />
+    <ProjectCapability Include="DynamicFileNesting" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <PropertyPageSchema Include="$(MSBuildThisFileDirectory)Rules\RazorConfiguration.xaml">
+      <Context>File</Context>
+    </PropertyPageSchema>
+    <PropertyPageSchema Include="$(MSBuildThisFileDirectory)Rules\RazorExtension.xaml">
+      <Context>File</Context>
+    </PropertyPageSchema>
+    <PropertyPageSchema Include="$(MSBuildThisFileDirectory)Rules\RazorGeneral.xaml">
+      <Context>Project</Context>
+    </PropertyPageSchema>
+  </ItemGroup>
+
+</Project>

src/Microsoft.NET.Sdk.Razor/build/netstandard2.0/Microsoft.NET.Sdk.Razor.props

@@ -10,10 +10,8 @@ Copyright (c) .NET Foundation. All rights reserved.
 ***********************************************************************************************
 -->
 <Project ToolsVersion="14.0">
-  <Import Project="Sdk.Razor.CurrentVersion.props" />
-
   <PropertyGroup>
-    <!-- Redirect the SDK to use the targets from the current package -->
+    <RazorSdkCurrentVersionProps>$(MSBuildThisFileDirectory)Sdk.Razor.CurrentVersion.props</RazorSdkCurrentVersionProps>
-    <RazorSdkTargetsPath>$(MSBuildThisFileDirectory)Sdk.Razor.CurrentVersion.targets</RazorSdkTargetsPath>
+    <RazorSdkCurrentVersionTargets>$(MSBuildThisFileDirectory)Sdk.Razor.CurrentVersion.targets</RazorSdkCurrentVersionTargets>
   </PropertyGroup>
 </Project>

src/Microsoft.NET.Sdk.Razor/build/netstandard2.0/Microsoft.NET.Sdk.Razor.targets

@@ -1,14 +0,0 @@
-<!--
-***********************************************************************************************
-Microsoft.NET.Sdk.Razor.targets
-
-WARNING:  DO NOT MODIFY this file unless you are knowledgeable about MSBuild and have
-          created a backup copy.  Incorrect changes to this file will make it
-          impossible to load or build your projects from the command-line or the IDE.
-
-Copyright (c) .NET Foundation. All rights reserved.
-***********************************************************************************************
--->
-<Project ToolsVersion="14.0">
-  <Import Project="Sdk.Razor.CurrentVersion.targets" />
-</Project>

src/Microsoft.NET.Sdk.Razor/build/netstandard2.0/Sdk.Razor.CurrentVersion.props

@@ -18,6 +18,12 @@ Copyright (c) .NET Foundation. All rights reserved.
     Default properties for common Razor SDK behavior.
   -->
   <PropertyGroup>
+    <!--
+    Set to true to automatically include certain file types, such as .cshtml files, as content in the project.
+    When referenced via Microsoft.NET.Sdk.Web, this additionally includes all files under wwwroot, and any config files.
+    -->
+    <EnableDefaultContentItems Condition="'$(EnableDefaultContentItems)'==''">true</EnableDefaultContentItems>
+
     <!--
       Set to true to automatically include Razor (.cshtml) files in @(RazorGenerate) from @(Content).
     -->
@@ -46,37 +52,24 @@ Copyright (c) .NET Foundation. All rights reserved.
     <RazorCompileToolset>Implicit</RazorCompileToolset>
 
     <!--
-    Set to true to allow a Razor code generation to use a persistent build server process.
+    Configures whether all Razor content items (.cshtml files) will be marked to be included in the produced NuGet package as content.
+
+    All Content items are included in a NuGet package as content files. This setting can be used to control this behavior for Razor content items.
     -->
-    <UseRazorBuildServer Condition="'$(UseRazorBuildServer)'==''">false</UseRazorBuildServer>
+    <IncludeRazorContentInPack Condition="'$(IncludeRazorContentInPack)'==''">false</IncludeRazorContentInPack>
+
+    <!--
+    Set to false to disable Razor code generation from using a persistent build server process.
+    -->
+    <UseRazorBuildServer Condition="'$(UseRazorBuildServer)'==''">true</UseRazorBuildServer>
   </PropertyGroup>
 
-  <ItemGroup Condition="'$(_WebSdkHeartsRazorSdk)' == 'true' AND '$(EnableDefaultItems)' == 'true' And '$(EnableDefaultContentItems)' == 'true'">
+  <ItemGroup Condition="'$(EnableDefaultItems)' == 'true' And '$(EnableDefaultContentItems)' == 'true'">
-    <Content Include="**\*.cshtml" CopyToPublishDirectory="PreserveNewest" Exclude="$(DefaultItemExcludes);$(DefaultExcludesInProjectFolder);$(DefaultWebContentItemExcludes)" />
+    <Content Include="**\*.cshtml" CopyToPublishDirectory="PreserveNewest" Exclude="$(DefaultItemExcludes);$(DefaultExcludesInProjectFolder);$(DefaultWebContentItemExcludes)">
+      <Pack>$(IncludeRazorContentInPack)</Pack>
+    </Content>
+
     <None Remove="**\*.cshtml" />
   </ItemGroup>
 
-  <ItemGroup>
-    <!-- 
-      Defines the generic .NET Core 'Razor' capability.
-
-      Note that we don't define any capabilities here that depend on the version of the runtime/toolset
-      in use by the project. Those capabilities are defined by the relevant runtime packages so that
-      we use the lack of the capability to detect downlevel scenarios.
-    -->
-    <ProjectCapability Include="DotNetCoreRazor"/>
-  </ItemGroup>
-
-  <ItemGroup> 
-    <PropertyPageSchema Include="$(MSBuildThisFileDirectory)Rules\RazorConfiguration.xaml">
-      <Context>File</Context>
-    </PropertyPageSchema>
-    <PropertyPageSchema Include="$(MSBuildThisFileDirectory)Rules\RazorExtension.xaml">
-      <Context>File</Context>
-    </PropertyPageSchema>
-    <PropertyPageSchema Include="$(MSBuildThisFileDirectory)Rules\RazorGeneral.xaml">
-      <Context>Project</Context>
-    </PropertyPageSchema>
-  </ItemGroup>
-
 </Project>

src/Microsoft.NET.Sdk.Razor/build/netstandard2.0/Sdk.Razor.CurrentVersion.targets

@@ -34,13 +34,13 @@ Copyright (c) .NET Foundation. All rights reserved.
       ResolveRazorGenerateInputs;
       AssignRazorGenerateTargetPaths;
       ResolveAssemblyReferenceRazorGenerateInputs;
-      _EnsureRazorCompilerReferenced;
+      _CheckForMissingRazorCompiler;
       ResolveTagHelperRazorGenerateInputs
     </PrepareForRazorGenerateDependsOn>
 
     <RazorGenerateDependsOn>
       PrepareForRazorGenerate;
-      _EnsureRazorCompilerReferenced;
+      _CheckForMissingRazorCompiler;
       RazorCoreGenerate
     </RazorGenerateDependsOn>
   
@@ -50,20 +50,36 @@ Copyright (c) .NET Foundation. All rights reserved.
     </PrepareForRazorCompileDependsOn>
 
     <ResolveRazorCompileInputsDependsOn>
-        ResolveRazorEmbeddedResources
+      ResolveRazorEmbeddedResources
     </ResolveRazorCompileInputsDependsOn>
 
     <RazorCompileDependsOn>
       PrepareForRazorCompile;
       RazorCoreCompile
     </RazorCompileDependsOn>
+
+    <BuiltProjectOutputGroupDependsOn>
+      $(BuiltProjectOutputGroupDependsOn);
+      _RazorAddBuiltProjectOutputGroupOutput
+    </BuiltProjectOutputGroupDependsOn>
+
+    <DebugSymbolsProjectOutputGroupDependsOn>
+      $(DebugSymbolsProjectOutputGroupDependsOn);
+      _RazorAddDebugSymbolsProjectOutputGroupOutput
+    </DebugSymbolsProjectOutputGroupDependsOn>
+
+    <PrepareForBuildDependsOn>
+      _InitializePreserveCompilationContext;
+      $(PrepareForBuildDependsOn)
+    </PrepareForBuildDependsOn>
+
   </PropertyGroup>
 
   <!-- 
     Default values for properties that affect Razor targets to the standard build lifecycle.
   -->
   <PropertyGroup Condition="'$(RazorCompileOnBuild)'==''">
-    <RazorCompileOnBuild>false</RazorCompileOnBuild>
+    <RazorCompileOnBuild>true</RazorCompileOnBuild>
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(RazorCompileOnPublish)'==''">
@@ -83,9 +99,12 @@ Copyright (c) .NET Foundation. All rights reserved.
   <PropertyGroup>
     <!-- Output directory used for generated files -->
     <RazorGenerateIntermediateOutputPath Condition="'$(RazorGenerateIntermediateOutputPath)'==''">$(IntermediateOutputPath)Razor\</RazorGenerateIntermediateOutputPath>
+
+    <!-- Suffix appended to $(TargetName) to produce $(RazorTargetName), the name of the assembly produced by Razor -->
+    <RazorTargetNameSuffix Condition="'$(RazorTargetNameSuffix)' == ''">.Razor</RazorTargetNameSuffix>
     
     <!-- File name (without extension) of the assembly produced by Razor -->
-    <RazorTargetName Condition="'$(RazorTargetName)'==''">$(TargetName).PrecompiledViews</RazorTargetName>
+    <RazorTargetName Condition="'$(RazorTargetName)'==''">$(TargetName)$(RazorTargetNameSuffix)</RazorTargetName>
 
     <!--
       The compatibility zone - these properties were provided by the MVC Precompilation tool and they
@@ -129,7 +148,7 @@ Copyright (c) .NET Foundation. All rights reserved.
   </PropertyGroup>
 
   <!-- Resolve the toolset to use -->
-  <PropertyGroup Condition="'$(_WebSdkHeartsRazorSdk)' == 'true'">
+  <PropertyGroup>
     <!-- Default value for the property 'MvcRazorCompileOnPublish' is empty. If it has been explicitly enabled, continue using precompilation. -->
     <ResolvedRazorCompileToolset Condition="'$(MvcRazorCompileOnPublish)' == 'true'">PrecompilationTool</ResolvedRazorCompileToolset>
 
@@ -154,6 +173,8 @@ Copyright (c) .NET Foundation. All rights reserved.
     <RazorIntermediateAssembly Condition="'$(RazorIntermediateAssembly)'==''" Include="$(IntermediateOutputPath)$(RazorTargetName).dll" />
     <!-- Used in Compilation.targets -->   
     <_RazorDebugSymbolsIntermediatePath Condition="'$(_RazorDebugSymbolsProduced)'=='true'" Include="$(IntermediateOutputPath)$(RazorTargetName).pdb" />
+    <!-- Add all cshtml files to UpToDateCheckInput - a collection used by the IDE's project system to determine if a project needs to be rebuilt -->
+    <UpToDateCheckInput Condition="'$(RazorCompileOnBuild)'=='true'" Include="@(Content->WithMetadataValue('Extension', '.cshtml'))" />
   </ItemGroup>
 
   <!--
@@ -182,6 +203,16 @@ Copyright (c) .NET Foundation. All rights reserved.
   <Target Name="RazorCompile" DependsOnTargets="$(RazorCompileDependsOn)">
   </Target>
 
+  <!--
+    PreserveCompilationContext needs to enabled by default only for "applications" (OutputType = exe) which have one or more Razor files.
+    We cannot only inspect ItemGroups globally, therefore setting this must be done inside a target. We wire this up to run before Build.
+  -->
+  <Target Name="_InitializePreserveCompilationContext">
+    <PropertyGroup Condition="'$(PreserveCompilationContext)' == '' AND '$(OutputType)' == 'exe' AND '@(Content->AnyHaveMetadataValue('Extension', '.cshtml'))' == 'true'">
+      <PreserveCompilationContext>true</PreserveCompilationContext>
+    </PropertyGroup>
+  </Target>
+
   <!-- 
     Computes the applicable @(ResolvedRazorConfiguration) and @(ResolvedRazorExtension) items that match the project's
     configuration. 
@@ -316,16 +347,22 @@ Copyright (c) .NET Foundation. All rights reserved.
   -->
   <Target 
     Name="_RazorAddBuiltProjectOutputGroupOutput"
-    DependsOnTargets="ResolveRazorGenerateInputs"
+    DependsOnTargets="_ResolveRazorTargetPath;ResolveRazorGenerateInputs"
-    BeforeTargets="BuiltProjectOutputGroup"
     Condition="'$(ResolvedRazorCompileToolset)'=='RazorSdk' and '$(RazorCompileOnBuild)'=='true'">
 
-    <PropertyGroup>
-      <RazorOutputPath Condition="'$(RazorOutputPath)'==''">$([MSBuild]::EnsureTrailingSlash('$(OutDir)'))</RazorOutputPath>
-    </PropertyGroup>
-
     <ItemGroup Condition="'@(RazorGenerate)'!= ''">
-      <BuiltProjectOutputGroupOutput Include="@(RazorIntermediateAssembly)" FinalOutputPath="$(RazorOutputPath)$(RazorTargetName).dll" />
+      <BuiltProjectOutputGroupOutput Include="%(RazorIntermediateAssembly.FullPath)" FinalOutputPath="$(RazorTargetPath)" />
+    </ItemGroup>
+
+  </Target>
+
+  <Target
+    Name="_RazorAddDebugSymbolsProjectOutputGroupOutput"
+    DependsOnTargets="_ResolveRazorTargetPath;ResolveRazorGenerateInputs"
+    Condition="'$(ResolvedRazorCompileToolset)'=='RazorSdk' and '$(RazorCompileOnBuild)'=='true'">
+
+    <ItemGroup Condition="Exists('@(_RazorDebugSymbolsIntermediatePath)')">
+      <DebugSymbolsProjectOutputGroupOutput Include="%(_RazorDebugSymbolsIntermediatePath.FullPath)" FinalOutputPath="$(RazorTargetDir)$(RazorTargetName).pdb" />
     </ItemGroup>
     
   </Target>
@@ -393,18 +430,14 @@ Copyright (c) .NET Foundation. All rights reserved.
   -->
   <Target 
     Name="_RazorCopyFilesToOutputDirectory" 
-    DependsOnTargets="RazorCompile"
+    DependsOnTargets="_ResolveRazorTargetPath;RazorCompile"
     AfterTargets="CopyFilesToOutputDirectory"
     Condition="'$(ResolvedRazorCompileToolset)'=='RazorSdk' and '$(RazorCompileOnBuild)'=='true'">
 
-    <PropertyGroup>
-      <RazorOutputPath Condition="'$(RazorOutputPath)'==''">$([MSBuild]::EnsureTrailingSlash('$(OutDir)'))</RazorOutputPath>
-    </PropertyGroup>
-
     <!-- Copy the Razor dll  -->
     <Copy
       SourceFiles="@(RazorIntermediateAssembly)"
-      DestinationFolder="$(RazorOutputPath)"
+      DestinationFiles="$(RazorTargetPath)"
       SkipUnchangedFiles="$(SkipCopyUnchangedFiles)"
       OverwriteReadOnlyFiles="$(OverwriteReadOnlyFiles)"
       Retries="$(CopyRetryCount)"
@@ -478,11 +511,29 @@ Copyright (c) .NET Foundation. All rights reserved.
 
   </Target>
 
-  <Target Name="_EnsureRazorCompilerReferenced" Condition="'$(IsRazorCompilerReferenced)' != 'true'">
+  <Target Name="_CheckForMissingRazorCompiler" Condition="'$(IsRazorCompilerReferenced)' != 'true'">
-    <Error Text="A reference to Microsoft.AspNetCore.Razor.Design is required for this target to succeed. See todo:fwlink for instructions on how to resolve this." />
+    <Error
-
+      Text="A PackageReference for 'Microsoft.AspNetCore.Razor.Design' was not included in your project. This package is required to compile Razor files. Typically, a
+      transitive reference to 'Microsoft.AspNetCore.Razor.Design' and references required to compile Razor files are obtained by adding a PackageReference
+      for 'Microsoft.AspNetCore.Mvc' in your project. For more information, see https://go.microsoft.com/fwlink/?linkid=868374." />
   </Target>
 
+  <Target Name="_ResolveRazorTargetPath">
+    <PropertyGroup>
+      <RazorOutputPath Condition="'$(RazorOutputPath)'==''">$([MSBuild]::EnsureTrailingSlash('$(OutDir)'))</RazorOutputPath>
+      <RazorTargetDir>$([MSBuild]::Escape($([MSBuild]::EnsureTrailingSlash($([System.IO.Path]::GetFullPath('$([System.IO.Path]::Combine('$(MSBuildProjectDirectory)', '$(RazorOutputPath)'))'))))))</RazorTargetDir>
+      <!-- Example, c:\MyProjects\MyProject\bin\debug\MyAssembly.Views.dll -->
+      <RazorTargetPath Condition=" '$(RazorTargetPath)' == '' ">$(RazorTargetDir)$(RazorTargetName).dll</RazorTargetPath>
+    </PropertyGroup>
+  </Target>
+
+  <PropertyGroup Condition="'$(RazorDesignTimeTargets)'==''">
+    <RazorDesignTimeTargets>$(MSBuildExtensionsPath)\Microsoft\VisualStudio\Razor\Microsoft.NET.Sdk.Razor.DesignTime.targets</RazorDesignTimeTargets>
+    <RazorDesignTimeTargets Condition="!Exists('$(RazorDesignTimeTargets)')">$(MSBuildThisFileDirectory)Microsoft.NET.Sdk.Razor.DesignTime.targets</RazorDesignTimeTargets>
+  </PropertyGroup>
+
+  <Import Project="$(RazorDesignTimeTargets)" />
+
   <!--
     This is a hook to import a set of targets after the Razor targets. By default this is unused.
   -->

src/Microsoft.NET.Sdk.Razor/buildMultiTargeting/Microsoft.NET.Sdk.Razor.props

@@ -1,6 +1,6 @@
 <!--
 ***********************************************************************************************
-Microsoft.NET.Sdk.Razor.targets
+Microsoft.Net.Sdk.Razor.props
 
 WARNING:  DO NOT MODIFY this file unless you are knowledgeable about MSBuild and have
           created a backup copy.  Incorrect changes to this file will make it
@@ -10,5 +10,8 @@ Copyright (c) .NET Foundation. All rights reserved.
 ***********************************************************************************************
 -->
 <Project ToolsVersion="14.0">
-  <Import Project="Sdk.Razor.CurrentVersion.MultiTargeting.targets" />
+  <PropertyGroup>
+    <RazorSdkCurrentVersionProps>$(MSBuildThisFileDirectory)..\build\netstandard2.0\Sdk.Razor.CurrentVersion.props</RazorSdkCurrentVersionProps>
+    <RazorSdkCurrentVersionTargets>$(MSBuildThisFileDirectory)Sdk.Razor.CurrentVersion.MultiTargeting.targets</RazorSdkCurrentVersTargetsFile>
+  </PropertyGroup>
 </Project>

src/Microsoft.VisualStudio.Editor.Razor/BackgroundParser.cs

@@ -6,6 +6,7 @@ using System.Collections.Generic;
 using System.Diagnostics;
 using System.IO;
 using System.Linq;
+using System.Text;
 using System.Threading;
 using Microsoft.AspNetCore.Razor.Language;
 using Microsoft.AspNetCore.Razor.Language.Legacy;
@@ -18,10 +19,10 @@ namespace Microsoft.VisualStudio.Editor.Razor
         private MainThreadState _main;
         private BackgroundThread _bg;
 
-        public BackgroundParser(RazorTemplateEngine templateEngine, string filePath)
+        public BackgroundParser(RazorProjectEngine projectEngine, string filePath, string projectDirectory)
         {
             _main = new MainThreadState(filePath);
-            _bg = new BackgroundThread(_main, templateEngine, filePath);
+            _bg = new BackgroundThread(_main, projectEngine, filePath, projectDirectory);
 
             _main.ResultsReady += (sender, args) => OnResultsReady(args);
         }
@@ -233,22 +234,25 @@ namespace Microsoft.VisualStudio.Editor.Razor
 
         private class BackgroundThread : ThreadStateBase
         {
+            private readonly string _filePath;
+            private readonly string _relativeFilePath;
+            private readonly string _projectDirectory;
             private MainThreadState _main;
             private Thread _backgroundThread;
             private CancellationToken _shutdownToken;
-            private RazorTemplateEngine _templateEngine;
+            private RazorProjectEngine _projectEngine;
-            private string _filePath;
             private RazorSyntaxTree _currentSyntaxTree;
             private IList<Edit> _previouslyDiscarded = new List<Edit>();
 
-            public BackgroundThread(MainThreadState main, RazorTemplateEngine templateEngine, string fileName)
+            public BackgroundThread(MainThreadState main, RazorProjectEngine projectEngine, string filePath, string projectDirectory)
             {
                 // Run on MAIN thread!
                 _main = main;
                 _shutdownToken = _main.CancelToken;
-                _templateEngine = templateEngine;
+                _projectEngine = projectEngine;
-                _filePath = fileName;
+                _filePath = filePath;
-
+                _relativeFilePath = GetNormalizedRelativeFilePath(filePath, projectDirectory);
+                _projectDirectory = projectDirectory;
                 _backgroundThread = new Thread(WorkerLoop);
                 SetThreadId(_backgroundThread.ManagedThreadId);
             }
@@ -262,8 +266,6 @@ namespace Microsoft.VisualStudio.Editor.Razor
             // **** BACKGROUND THREAD ****
             private void WorkerLoop()
             {
-                var fileNameOnly = Path.GetFileName(_filePath);
-
                 try
                 {
                     EnsureOnThread();
@@ -347,14 +349,31 @@ namespace Microsoft.VisualStudio.Editor.Razor
             {
                 EnsureOnThread();
 
-                var sourceDocument = new TextSnapshotSourceDocument(snapshot, _filePath);
+                var projectItem = new TextSnapshotProjectItem(snapshot, _projectDirectory, _relativeFilePath, _filePath);
-                var imports = _templateEngine.GetImports(_filePath);
+                var codeDocument = _projectEngine.ProcessDesignTime(projectItem);
 
-                var codeDocument = RazorCodeDocument.Create(sourceDocument, imports);
-
-                _templateEngine.GenerateCode(codeDocument);
                 return codeDocument;
             }
+
+            private string GetNormalizedRelativeFilePath(string filePath, string projectDirectory)
+            {
+                if (filePath.StartsWith(projectDirectory, StringComparison.OrdinalIgnoreCase))
+                {
+                    filePath = filePath.Substring(projectDirectory.Length);
+                }
+
+                if (filePath.Length > 1)
+                {
+                    filePath = filePath.Replace('\\', '/');
+
+                    if (filePath[0] != '/')
+                    {
+                        filePath = "/" + filePath;
+                    }
+                }
+
+                return filePath;
+            }
         }
 
         private class WorkParcel

src/Microsoft.VisualStudio.Editor.Razor/DefaultImportDocumentManager.cs

@@ -5,6 +5,7 @@ using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.IO;
+using System.Linq;
 using Microsoft.AspNetCore.Razor.Language;
 using Microsoft.CodeAnalysis.Razor;
 
@@ -15,7 +16,7 @@ namespace Microsoft.VisualStudio.Editor.Razor
         private readonly FileChangeTrackerFactory _fileChangeTrackerFactory;
         private readonly ForegroundDispatcher _foregroundDispatcher;
         private readonly ErrorReporter _errorReporter;
-        private readonly RazorTemplateEngineFactoryService _templateEngineFactoryService;
+        private readonly RazorProjectEngineFactoryService _projectEngineFactoryService;
         private readonly Dictionary<string, ImportTracker> _importTrackerCache;
 
         public override event EventHandler<ImportChangedEventArgs> Changed;
@@ -24,7 +25,7 @@ namespace Microsoft.VisualStudio.Editor.Razor
             ForegroundDispatcher foregroundDispatcher,
             ErrorReporter errorReporter,
             FileChangeTrackerFactory fileChangeTrackerFactory,
-            RazorTemplateEngineFactoryService templateEngineFactoryService)
+            RazorProjectEngineFactoryService projectEngineFactoryService)
         {
             if (foregroundDispatcher == null)
             {
@@ -41,15 +42,15 @@ namespace Microsoft.VisualStudio.Editor.Razor
                 throw new ArgumentNullException(nameof(fileChangeTrackerFactory));
             }
 
-            if (templateEngineFactoryService == null)
+            if (projectEngineFactoryService == null)
             {
-                throw new ArgumentNullException(nameof(templateEngineFactoryService));
+                throw new ArgumentNullException(nameof(projectEngineFactoryService));
             }
 
             _foregroundDispatcher = foregroundDispatcher;
             _errorReporter = errorReporter;
             _fileChangeTrackerFactory = fileChangeTrackerFactory;
-            _templateEngineFactoryService = templateEngineFactoryService;
+            _projectEngineFactoryService = projectEngineFactoryService;
             _importTrackerCache = new Dictionary<string, ImportTracker>(StringComparer.OrdinalIgnoreCase);
         }
 
@@ -115,10 +116,17 @@ namespace Microsoft.VisualStudio.Editor.Razor
         private IEnumerable<RazorProjectItem> GetImportItems(VisualStudioDocumentTracker tracker)
         {
             var projectDirectory = Path.GetDirectoryName(tracker.ProjectPath);
-            var templateEngine = _templateEngineFactoryService.Create(projectDirectory, _ => { });
+            var projectEngine = _projectEngineFactoryService.Create(projectDirectory, _ => { });
-            var imports = templateEngine.GetImportItems(tracker.FilePath);
+            var trackerItem = projectEngine.FileSystem.GetItem(tracker.FilePath);
+            var importFeature = projectEngine.ProjectFeatures.OfType<IImportProjectFeature>().FirstOrDefault();
 
-            return imports;
+            // There should always be an import feature unless someone has misconfigured their RazorProjectEngine.
+            // In that case once we attempt to parse the Razor file we'll explode and give the a user a decent
+            // error message; for now, lets just be extra protective and assume 0 imports to not give a bad error.
+            var importItems = importFeature?.GetImports(trackerItem) ?? Enumerable.Empty<RazorProjectItem>();
+            var physicalImports = importItems.Where(import => import.FilePath != null);
+
+            return physicalImports;
         }
 
         private void OnChanged(ImportTracker importTracker, FileChangeKind changeKind)

src/Microsoft.VisualStudio.Editor.Razor/DefaultImportDocumentManagerFactory.cs

@@ -35,13 +35,13 @@ namespace Microsoft.VisualStudio.Editor.Razor
 
             var errorReporter = languageServices.WorkspaceServices.GetRequiredService<ErrorReporter>();
             var fileChangeTrackerFactory = languageServices.GetRequiredService<FileChangeTrackerFactory>();
-            var templateEngineFactoryService = languageServices.GetRequiredService<RazorTemplateEngineFactoryService>();
+            var projectEngineFactoryService = languageServices.GetRequiredService<RazorProjectEngineFactoryService>();
 
             return new DefaultImportDocumentManager(
                 _foregroundDispatcher,
                 errorReporter,
                 fileChangeTrackerFactory,
-                templateEngineFactoryService);
+                projectEngineFactoryService);
         }
     }
 }