Add extensibility point to Razor compilation

src/Microsoft.AspNet.Mvc.Razor/Compilation/RoslynCompilationContext.cs

@@ -0,0 +1,33 @@
+// 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.CodeAnalysis.CSharp;
+
+namespace Microsoft.AspNet.Mvc.Razor.Compilation
+{
+    /// <summary>
+    /// Context object used to pass information about the current Razor page compilation.
+    /// </summary>
+    public class RoslynCompilationContext
+    {
+        /// <summary>
+        /// Constructs a new instance of the <see cref="RoslynCompilationContext"/> type.
+        /// </summary>
+        /// <param name="compilation"><see cref="CSharpCompilation"/> to be set to <see cref="Compilation"/> property.</param>
+        public RoslynCompilationContext(CSharpCompilation compilation)
+        {
+            if (compilation == null)
+            {
+                throw new ArgumentNullException(nameof(compilation));
+            }
+
+            Compilation = compilation;
+        }
+
+        /// <summary>
+        /// Gets or sets the <see cref="CSharpCompilation"/> used for current source file compilation.
+        /// </summary>
+        public CSharpCompilation Compilation { get; set; }
+    }
+}

src/Microsoft.AspNet.Mvc.Razor/Compilation/RoslynCompilationService.cs

@@ -40,6 +40,7 @@ namespace Microsoft.AspNet.Mvc.Razor.Compilation
         private readonly Lazy<List<MetadataReference>> _applicationReferences;
         private readonly string _classPrefix;
         private readonly string _configuration;
+        private Action<RoslynCompilationContext> _compilationCallback;
 
 #if DOTNET5_5
         private readonly RazorLoadContext _razorLoadContext;
@@ -71,6 +72,7 @@ namespace Microsoft.AspNet.Mvc.Razor.Compilation
             _fileProvider = optionsAccessor.Value.FileProvider;
             _classPrefix = host.MainClassNamePrefix;
             _configuration = optionsAccessor.Value.Configuration;
+            _compilationCallback = optionsAccessor.Value.CompilationCallback;
 
 #if DOTNET5_5
             _razorLoadContext = new RazorLoadContext();
@@ -111,6 +113,10 @@ namespace Microsoft.AspNet.Mvc.Razor.Compilation
 
             compilation = Rewrite(compilation);
 
+            var compilationContext = new RoslynCompilationContext(compilation);
+            _compilationCallback(compilationContext);
+            compilation = compilationContext.Compilation;
+
             using (var ms = new MemoryStream())
             {
                 using (var pdb = new MemoryStream())

src/Microsoft.AspNet.Mvc.Razor/RazorViewEngineOptions.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using Microsoft.AspNet.FileProviders;
+using Microsoft.AspNet.Mvc.Razor.Compilation;
 
 namespace Microsoft.AspNet.Mvc.Razor
 {
@@ -16,6 +17,8 @@ namespace Microsoft.AspNet.Mvc.Razor
 
         private string _configuration;
 
+        private Action<RoslynCompilationContext> _compilationCallback = c => { };
+
         /// <summary>
         /// Get a <see cref="IList{IViewLocationExpander}"/> used by the <see cref="RazorViewEngine"/>.
         /// </summary>
@@ -65,5 +68,23 @@ namespace Microsoft.AspNet.Mvc.Razor
                 _configuration = value;
             }
         }
+
+        /// <summary>
+        /// Gets or sets the callback that is used to customize Razor compilation
+        /// to change compilation settings you can update <see cref="RoslynCompilationContext.Compilation"/> property.
+        /// Customizations made here would not reflect in tooling (Intellisense).
+        /// </summary>
+        public Action<RoslynCompilationContext> CompilationCallback
+        {
+            get { return _compilationCallback; }
+            set
+            {
+                if (value == null)
+                {
+                    throw new ArgumentNullException(nameof(value));
+                }
+                _compilationCallback = value;
+            }
+        }
     }
 }

src/Microsoft.AspNet.Mvc.Razor/RazorViewEngineOptionsSetup.cs

@@ -4,6 +4,8 @@
 using Microsoft.AspNet.FileProviders;
 using Microsoft.AspNet.Hosting;
 using Microsoft.AspNet.Mvc.Razor;
+using Microsoft.AspNet.Mvc.Razor.Compilation;
+using Microsoft.Extensions.CompilationAbstractions;
 using Microsoft.Extensions.OptionsModel;
 using Microsoft.Extensions.PlatformAbstractions;
 
@@ -19,13 +21,13 @@ namespace Microsoft.AspNet.Mvc
         /// </summary>
         /// <param name="applicationEnvironment"><see cref="IApplicationEnvironment"/> for the application.</param>
         /// <param name="hostingEnvironment"><see cref="IHostingEnvironment"/> for the application.</param>
-        public RazorViewEngineOptionsSetup(IApplicationEnvironment applicationEnvironment, IHostingEnvironment hostingEnvironment)
+        public RazorViewEngineOptionsSetup(IApplicationEnvironment applicationEnvironment,
+            IHostingEnvironment hostingEnvironment)
             : base(options => ConfigureRazor(options, applicationEnvironment, hostingEnvironment))
         {
         }
 
-        private static void ConfigureRazor(
-            RazorViewEngineOptions razorOptions,
+        private static void ConfigureRazor(RazorViewEngineOptions razorOptions,
             IApplicationEnvironment applicationEnvironment,
             IHostingEnvironment hostingEnvironment)
         {

test/Microsoft.AspNet.Mvc.Razor.Test/Compilation/RoslynCompilationServiceTest.cs

@@ -362,6 +362,45 @@ public class NotRazorPrefixType {}";
                 });
         }
 
+        [Fact]
+        public void Compile_RunsCallback()
+        {
+            var content = "public class MyTestType  {}";
+            var applicationEnvironment = PlatformServices.Default.Application;
+            var libraryExporter = CompilationServices.Default.LibraryExporter;
+
+            var compilerOptionsProvider = new Mock<ICompilerOptionsProvider>();
+            compilerOptionsProvider
+                .Setup(p => p.GetCompilerOptions(
+                    applicationEnvironment.ApplicationName,
+                    applicationEnvironment.RuntimeFramework,
+                    ConfigurationName))
+                .Returns(new CompilerOptions());
+
+            RoslynCompilationContext usedCompilation = null;
+            var mvcRazorHost = new Mock<IMvcRazorHost>();
+            mvcRazorHost.SetupGet(m => m.MainClassNamePrefix)
+                        .Returns(string.Empty);
+
+            var compilationService = new RoslynCompilationService(
+                applicationEnvironment,
+                libraryExporter,
+                compilerOptionsProvider.Object,
+                mvcRazorHost.Object,
+                GetOptions(callback: c => usedCompilation = c));
+
+            var relativeFileInfo = new RelativeFileInfo(
+                new TestFileInfo { PhysicalPath = "SomePath" },
+                "some-relative-path");
+
+            // Act
+            var result = compilationService.Compile(relativeFileInfo, content);
+
+            Assert.NotNull(usedCompilation);
+            Assert.NotNull(usedCompilation.Compilation);
+            Assert.Equal(1, usedCompilation.Compilation.SyntaxTrees.Length);
+        }
+
         private static DiagnosticDescriptor GetDiagnosticDescriptor(string messageFormat)
         {
             return new DiagnosticDescriptor(
@@ -373,12 +412,14 @@ public class NotRazorPrefixType {}";
                 isEnabledByDefault: true);
         }
 
-        private static IOptions<RazorViewEngineOptions> GetOptions(IFileProvider fileProvider = null)
+        private static IOptions<RazorViewEngineOptions> GetOptions(IFileProvider fileProvider = null,
+            Action<RoslynCompilationContext> callback = null)
         {
             var razorViewEngineOptions = new RazorViewEngineOptions
             {
                 FileProvider = fileProvider ?? new TestFileProvider(),
-                Configuration = ConfigurationName
+                Configuration = ConfigurationName,
+                CompilationCallback = callback ?? (c => { })
             };
             var options = new Mock<IOptions<RazorViewEngineOptions>>();
             options