Support _ViewImports.cshtml files hierarchically

src/Microsoft.AspNetCore.Blazor.Build/Core/RazorCompilation/BlazorProjectItem.cs

@@ -0,0 +1,35 @@
+// 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;
+using System.IO;
+
+namespace Microsoft.AspNetCore.Blazor.Build.Core.RazorCompilation
+{
+    internal class BlazorProjectItem : RazorProjectItem
+    {
+        private readonly string _projectBasePath;
+        private readonly string _itemFullPhysicalPath;
+        private readonly Stream _itemContents;
+
+        public BlazorProjectItem(
+            string projectBasePath,
+            string itemFullPhysicalPath,
+            Stream itemFileContents)
+        {
+            _projectBasePath = projectBasePath;
+            _itemFullPhysicalPath = itemFullPhysicalPath;
+            _itemContents = itemFileContents;
+        }
+
+        public override string BasePath => _projectBasePath;
+
+        public override string FilePath => _itemFullPhysicalPath;
+
+        public override string PhysicalPath => _itemFullPhysicalPath;
+
+        public override bool Exists => true;
+
+        public override Stream Read() => _itemContents;
+    }
+}

src/Microsoft.AspNetCore.Blazor.Build/Core/RazorCompilation/RazorCompiler.cs

@@ -94,13 +94,14 @@ namespace Microsoft.AspNetCore.Blazor.Build.Core.RazorCompilation
                 // invocations.
 
                 var engine = new BlazorRazorEngine();
-
-                var sourceDoc = RazorSourceDocument.ReadFrom(inputFileContents, inputFilePath);
-                var codeDoc = RazorCodeDocument.Create(sourceDoc);
+                var blazorTemplateEngine = new BlazorTemplateEngine(
+                    engine.Engine,
+                    RazorProject.Create(inputRootPath));
+                var codeDoc = blazorTemplateEngine.CreateCodeDocument(
+                    new BlazorProjectItem(inputRootPath, inputFilePath, inputFileContents));
                 codeDoc.Items[BlazorCodeDocItems.Namespace] = combinedNamespace;
                 codeDoc.Items[BlazorCodeDocItems.ClassName] = itemClassName;
-                engine.Process(codeDoc);
-                var csharpDocument = codeDoc.GetCSharpDocument();
+                var csharpDocument = blazorTemplateEngine.GenerateCode(codeDoc);
                 var generatedCode = csharpDocument.GeneratedCode;
 
                 // Add parameters to the primary method via string manipulation because

src/Microsoft.AspNetCore.Blazor.Razor.Extensions/BlazorRazorEngine.cs

@@ -15,6 +15,8 @@ namespace Microsoft.AspNetCore.Blazor.Razor
         private readonly RazorEngine _engine;
         private readonly RazorCodeGenerationOptions _codegenOptions;
 
+        public RazorEngine Engine => _engine;
+
         public BlazorRazorEngine()
         {
             _codegenOptions = RazorCodeGenerationOptions.CreateDefault();
@@ -39,8 +41,5 @@ namespace Microsoft.AspNetCore.Blazor.Razor
                 });
             });
         }
-
-        public void Process(RazorCodeDocument document)
-            => _engine.Process(document);
     }
 }

src/Microsoft.AspNetCore.Blazor.Razor.Extensions/BlazorTemplateEngine.cs

@@ -0,0 +1,41 @@
+// 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.IO;
+using Microsoft.AspNetCore.Razor.Language;
+using System.Text;
+
+namespace Microsoft.AspNetCore.Blazor.Razor
+{
+    /// <summary>
+    /// A <see cref="RazorTemplateEngine"/> for Blazor components.
+    /// </summary>
+    public class BlazorTemplateEngine : RazorTemplateEngine
+    {
+        public BlazorTemplateEngine(RazorEngine engine, RazorProject project)
+            : base(engine, project)
+        {
+            Options.ImportsFileName = "_ViewImports.cshtml";
+            Options.DefaultImports = GetDefaultImports();
+        }
+
+        private static RazorSourceDocument GetDefaultImports()
+        {
+            using (var stream = new MemoryStream())
+            using (var writer = new StreamWriter(stream, Encoding.UTF8))
+            {
+                // TODO: Add other commonly-used Blazor namespaces here. Can't do so yet
+                // because the tooling wouldn't know about it, so it would still look like
+                // an error if you hadn't explicitly imported them.
+                writer.WriteLine("@using System");
+                writer.WriteLine("@using System.Collections.Generic");
+                writer.WriteLine("@using System.Linq");
+                writer.WriteLine("@using System.Threading.Tasks");
+                writer.Flush();
+
+                stream.Position = 0;
+                return RazorSourceDocument.ReadFrom(stream, fileName: null, encoding: Encoding.UTF8);
+            }
+        }
+    }
+}

test/Microsoft.AspNetCore.Blazor.Build.Test/RazorCompilerTest.cs

@@ -496,7 +496,9 @@ namespace Microsoft.AspNetCore.Blazor.Build.Test
             {
                 compilation.Emit(peStream);
 
-                var diagnostics = compilation.GetDiagnostics();
+                var diagnostics = compilation
+                    .GetDiagnostics()
+                    .Where(d => d.Severity != DiagnosticSeverity.Hidden);
                 return new CompileToAssemblyResult
                 {
                     Diagnostics = diagnostics,

test/Microsoft.AspNetCore.Blazor.E2ETest/Tests/ComponentRenderingTest.cs

@@ -3,8 +3,11 @@
 
 using System;
 using System.Collections.Generic;
+using System.Configuration.Assemblies;
 using System.Linq;
+using System.Numerics;
 using BasicTestApp;
+using BasicTestApp.HierarchicalImportsTest.Subdir;
 using Microsoft.AspNetCore.Blazor.Components;
 using Microsoft.AspNetCore.Blazor.E2ETest.Infrastructure;
 using Microsoft.AspNetCore.Blazor.E2ETest.Infrastructure.ServerFixtures;
@@ -189,6 +192,17 @@ namespace Microsoft.AspNetCore.Blazor.E2ETest.Tests
             Assert.Empty(fragmentElements);
         }
 
+        [Fact]
+        public void CanUseViewImportsHierarchically()
+        {
+            // The component is able to compile and output these type names only because
+            // of the _ViewImports.cshtml files at the same and ancestor levels
+            var appElement = MountTestComponent<ComponentUsingImports>();
+            Assert.Collection(appElement.FindElements(By.TagName("p")),
+                elem => Assert.Equal(typeof(Complex).FullName, elem.Text),
+                elem => Assert.Equal(typeof(AssemblyHashAlgorithm).FullName, elem.Text));
+        }
+
         private IWebElement MountTestComponent<TComponent>() where TComponent: IComponent
         {
             var componentTypeName = typeof(TComponent).FullName;

test/testapps/BasicTestApp/HierarchicalImportsTest/Subdir/ComponentUsingImports.cshtml

@@ -0,0 +1,5 @@
+The following two outputs rely on "using" directives in _ViewImports files at the current and ancestor levels.
+
+<p>@(typeof(Complex).FullName)</p>
+
+<p>@(typeof(AssemblyHashAlgorithm).FullName)</p>

test/testapps/BasicTestApp/HierarchicalImportsTest/Subdir/_ViewImports.cshtml

@@ -0,0 +1 @@
+@using System.Configuration.Assemblies

test/testapps/BasicTestApp/HierarchicalImportsTest/_ViewImports.cshtml

@@ -0,0 +1 @@
+@using System.Numerics