Adding support for inheriting tag helpers from _ViewStart files

Fixes #1166

src/Microsoft.AspNet.Mvc.Razor.Host/Directives/ChunkInheritanceUtility.cs

@@ -4,7 +4,6 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
-using System.Linq;
 using Microsoft.AspNet.FileSystems;
 using Microsoft.AspNet.Razor;
 using Microsoft.AspNet.Razor.Generator.Compiler;
@@ -13,63 +12,57 @@ using Microsoft.AspNet.Razor.Parser;
 namespace Microsoft.AspNet.Mvc.Razor.Directives
 {
     /// <summary>
-    /// A utility type for supporting inheritance of chunks into a page from _ViewStart pages that apply to it.
+    /// A utility type for supporting inheritance of tag helpers and chunks into a page from applicable _ViewStart
+    /// pages.
     /// </summary>
     public class ChunkInheritanceUtility
     {
-        private readonly IReadOnlyList<Chunk> _defaultInheritedChunks;
+        private readonly Dictionary<string, CodeTree> _parsedCodeTrees;
+        private readonly MvcRazorHost _razorHost;
+        private readonly IFileSystem _fileSystem;
+        private readonly IEnumerable<Chunk> _defaultInheritedChunks;
 
         /// <summary>
-        /// Instantiates a new instance of <see cref="ChunkInheritanceUtility"/>.
+        /// Initializes a new instance of <see cref="ChunkInheritanceUtility"/>.
-        /// </summary>
-        /// <param name="codeTree">The <see cref="CodeTree"/> instance to add <see cref="Chunk"/>s to.</param>
-        /// <param name="defaultInheritedChunks">The list of <see cref="Chunk"/>s inherited by default.</param>
-        /// <param name="defaultModel">The model type used in the event no model is specified via the
-        /// <c>@model</c> keyword.</param>
-        public ChunkInheritanceUtility([NotNull] CodeTree codeTree,
-                                       [NotNull] IReadOnlyList<Chunk> defaultInheritedChunks,
-                                       [NotNull] string defaultModel)
-        {
-            CodeTree = codeTree;
-            _defaultInheritedChunks = defaultInheritedChunks;
-            ChunkMergers = GetMergerMappings(codeTree, defaultModel);
-        }
-
-        /// <summary>
-        /// Gets the CodeTree to add inherited <see cref="Chunk"/> instances to.
-        /// </summary>
-        public CodeTree CodeTree { get; private set; }
-
-        /// <summary>
-        /// Gets a dictionary mapping <see cref="Chunk"/> type to the <see cref="IChunkMerger"/> used to merge
-        /// chunks of that type.
-        /// </summary>
-        public IDictionary<Type, IChunkMerger> ChunkMergers { get; private set; }
-
-        /// <summary>
-        /// Gets the list of chunks that are to be inherited by a specified page.
-        /// Chunks are inherited from _ViewStarts that are applicable to the page.
         /// </summary>
         /// <param name="razorHost">The <see cref="MvcRazorHost"/> used to parse _ViewStart pages.</param>
         /// <param name="fileSystem">The filesystem that represents the application.</param>
+        /// <param name="defaultInheritedChunks">Sequence of <see cref="Chunk"/>s inherited by default.</param>
+        public ChunkInheritanceUtility([NotNull] MvcRazorHost razorHost,
+                                       [NotNull] IFileSystem fileSystem,
+                                       [NotNull] IEnumerable<Chunk> defaultInheritedChunks)
+        {
+            _razorHost = razorHost;
+            _fileSystem = fileSystem;
+            _defaultInheritedChunks = defaultInheritedChunks;
+            _parsedCodeTrees = new Dictionary<string, CodeTree>(StringComparer.Ordinal);
+        }
+
+        /// <summary>
+        /// Gets a <see cref="IReadOnlyList{T}"/> of <see cref="Chunk"/> containing parsed results of _ViewStart files
+        /// that are used for inheriting tag helpers and chunks to the page located at <paramref name="pagePath"/>.
+        /// </summary>
         /// <param name="pagePath">The path of the page to locate inherited chunks for.</param>
-        /// <returns>A list of chunks that are applicable to the given page.</returns>
+        /// <returns>A <see cref="IReadOnlyList{T}"/> of <see cref="Chunk"/> from _ViewStart pages.</returns>
-        public List<Chunk> GetInheritedChunks([NotNull] MvcRazorHost razorHost,
+        public IReadOnlyList<Chunk> GetInheritedChunks([NotNull] string pagePath)
-                                              [NotNull] IFileSystem fileSystem,
-                                              [NotNull] string pagePath)
         {
             var inheritedChunks = new List<Chunk>();
 
-            var templateEngine = new RazorTemplateEngine(razorHost);
+            var templateEngine = new RazorTemplateEngine(_razorHost);
-            foreach (var viewStart in ViewStartUtility.GetViewStartLocations(fileSystem, pagePath))
+            foreach (var viewStart in ViewStartUtility.GetViewStartLocations(_fileSystem, pagePath))
             {
+                CodeTree codeTree;
                 IFileInfo fileInfo;
-                if (fileSystem.TryGetFileInfo(viewStart, out fileInfo))
+
+                if (_parsedCodeTrees.TryGetValue(viewStart, out codeTree))
                 {
-                    var parsedTree = ParseViewFile(templateEngine, fileInfo);
+                    inheritedChunks.AddRange(codeTree.Chunks);
-                    var chunksToAdd = parsedTree.Chunks
+                }
-                                                .Where(chunk => ChunkMergers.ContainsKey(chunk.GetType()));
+                else if (_fileSystem.TryGetFileInfo(viewStart, out fileInfo))
-                    inheritedChunks.AddRange(chunksToAdd);
+                {
+                    codeTree = ParseViewFile(templateEngine, fileInfo);
+                    _parsedCodeTrees.Add(viewStart, codeTree);
+                    inheritedChunks.AddRange(codeTree.Chunks);
                 }
             }
 
@@ -79,19 +72,22 @@ namespace Microsoft.AspNet.Mvc.Razor.Directives
         }
 
         /// <summary>
-        /// Merges a list of chunks into the <see cref="CodeTree"/> instance.
+        /// Merges a list of chunks into the specified <paramref name="codeTree/>.
         /// </summary>
+        /// <param name="codeTree">The <see cref="CodeTree"/> to mere</param>
         /// <param name="inherited">The list of chunks to merge.</param>
-        public void MergeInheritedChunks(List<Chunk> inherited)
+        public void MergeInheritedChunks([NotNull] CodeTree codeTree,
+                                         [NotNull] IReadOnlyList<Chunk> inherited,
+                                         string defaultModel)
         {
-            var current = CodeTree.Chunks;
+            var mergerMappings = GetMergerMappings(codeTree, defaultModel);
             IChunkMerger merger;
 
             // We merge chunks into the codeTree in two passes. In the first pass, we traverse the CodeTree visiting
             // a mapped IChunkMerger for types that are registered.
-            foreach (var chunk in current)
+            foreach (var chunk in codeTree.Chunks)
             {
-                if (ChunkMergers.TryGetValue(chunk.GetType(), out merger))
+                if (mergerMappings.TryGetValue(chunk.GetType(), out merger))
                 {
                     merger.VisitChunk(chunk);
                 }
@@ -102,11 +98,11 @@ namespace Microsoft.AspNet.Mvc.Razor.Directives
             // rules.
             foreach (var chunk in inherited)
             {
-                if (ChunkMergers.TryGetValue(chunk.GetType(), out merger))
+                if (mergerMappings.TryGetValue(chunk.GetType(), out merger))
                 {
                     // TODO: When mapping chunks, we should remove mapping information since it would be incorrect
                     // to generate it in the page that inherits it. Tracked by #945
-                    merger.Merge(CodeTree, chunk);
+                    merger.Merge(codeTree, chunk);
                 }
             }
         }
@@ -123,14 +119,14 @@ namespace Microsoft.AspNet.Mvc.Razor.Directives
         }
 
         // TODO: This needs to be cached (#1016)
-        private CodeTree ParseViewFile(RazorTemplateEngine engine,
+        private static CodeTree ParseViewFile(RazorTemplateEngine engine,
-                                       IFileInfo fileInfo)
+                                              IFileInfo fileInfo)
         {
             using (var stream = fileInfo.CreateReadStream())
             {
                 using (var streamReader = new StreamReader(stream))
                 {
-                    var parseResults = engine.ParseTemplate(streamReader);
+                    var parseResults = engine.ParseTemplate(streamReader, fileInfo.PhysicalPath);
                     var className = ParserHelpers.SanitizeClassName(fileInfo.Name);
                     var language = engine.Host.CodeLanguage;
                     var codeGenerator = language.CreateCodeGenerator(className,
@@ -138,6 +134,7 @@ namespace Microsoft.AspNet.Mvc.Razor.Directives
                                                                      fileInfo.PhysicalPath,
                                                                      engine.Host);
                     codeGenerator.Visit(parseResults);
+
                     return codeGenerator.Context.CodeTreeBuilder.CodeTree;
                 }
             }

src/Microsoft.AspNet.Mvc.Razor.Host/MvcRazorHost.cs

@@ -36,6 +36,7 @@ namespace Microsoft.AspNet.Mvc.Razor
         // CodeGenerationContext.DefaultBaseClass is set to MyBaseType<dynamic>. 
         // This field holds the type name without the generic decoration (MyBaseType)
         private readonly string _baseType;
+        private ChunkInheritanceUtility _chunkInheritanceUtility;
 
 #if NET45
         /// <summary>
@@ -154,6 +155,20 @@ namespace Microsoft.AspNet.Mvc.Razor
             get { return "CreateModelExpression"; }
         }
 
+        private ChunkInheritanceUtility ChunkInheritanceUtility
+        {
+            get
+            {
+                if (_chunkInheritanceUtility == null)
+                {
+                    // This needs to be lazily evaluated to support DefaultInheritedChunks being virtual.
+                    _chunkInheritanceUtility = new ChunkInheritanceUtility(this, _fileSystem, DefaultInheritedChunks);
+                }
+
+                return _chunkInheritanceUtility;
+            }
+        }
+
         /// <inheritdoc />
         public GeneratorResults GenerateCode(string rootRelativePath, Stream inputStream)
         {
@@ -163,6 +178,13 @@ namespace Microsoft.AspNet.Mvc.Razor
             return engine.GenerateCode(inputStream, className, DefaultNamespace, rootRelativePath);
         }
 
+        /// <inheritdoc />
+        public override RazorParser DecorateRazorParser([NotNull] RazorParser razorParser, string sourceFileName)
+        {
+            var inheritedChunks = ChunkInheritanceUtility.GetInheritedChunks(sourceFileName);
+            return new MvcRazorParser(razorParser, inheritedChunks);
+        }
+
         /// <inheritdoc />
         public override ParserBase DecorateCodeParser([NotNull] ParserBase incomingCodeParser)
         {
@@ -173,7 +195,11 @@ namespace Microsoft.AspNet.Mvc.Razor
         public override CodeBuilder DecorateCodeBuilder([NotNull] CodeBuilder incomingBuilder,
                                                         [NotNull] CodeBuilderContext context)
         {
-            UpdateCodeBuilder(context);
+            var inheritedChunks = ChunkInheritanceUtility.GetInheritedChunks(context.SourceFile);
+
+            ChunkInheritanceUtility.MergeInheritedChunks(context.CodeTreeBuilder.CodeTree,
+                                                         inheritedChunks,
+                                                         DefaultModel);
 
             return new MvcCSharpCodeBuilder(context,
                                             DefaultModel,
@@ -184,14 +210,5 @@ namespace Microsoft.AspNet.Mvc.Razor
                                                 CreateModelExpressionMethodName = CreateModelExpressionMethod
                                             });
         }
-
-        private void UpdateCodeBuilder(CodeGeneratorContext context)
-        {
-            var chunkUtility = new ChunkInheritanceUtility(context.CodeTreeBuilder.CodeTree,
-                                                           DefaultInheritedChunks,
-                                                           DefaultModel);
-            var inheritedChunks = chunkUtility.GetInheritedChunks(this, _fileSystem, context.SourceFile);
-            chunkUtility.MergeInheritedChunks(inheritedChunks);
-        }
     }
 }

src/Microsoft.AspNet.Mvc.Razor.Host/MvcRazorParser.cs

@@ -0,0 +1,48 @@
+// Copyright (c) Microsoft Open Technologies, Inc. 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;
+using Microsoft.AspNet.Razor.Generator.Compiler;
+using Microsoft.AspNet.Razor.Parser;
+using Microsoft.AspNet.Razor.Parser.SyntaxTree;
+using Microsoft.AspNet.Razor.TagHelpers;
+
+namespace Microsoft.AspNet.Mvc.Razor
+{
+    /// <summary>
+    /// A subtype of <see cref="RazorParser"/> that <see cref="MvcRazorHost"/> uses to support inheritance of tag
+    /// helpers from _viewstart files.
+    /// </summary>
+    public class MvcRazorParser : RazorParser
+    {
+        private readonly IReadOnlyList<Chunk> _viewStartChunks;
+
+        /// <summary>
+        /// Initializes a new instance of <see cref="MvcRazorParser"/>.
+        /// </summary>
+        /// <param name="parser">The <see cref="RazorParser"/> to copy properties from.</param>
+        /// <param name="viewStartChunks">The <see cref="IReadOnlyList{T}"/> of <see cref="Chunk"/>s that are inherited
+        /// by parsed pages from _ViewStart files.</param>
+        public MvcRazorParser(RazorParser parser, IReadOnlyList<Chunk> viewStartChunks)
+            : base(parser)
+        {
+            _viewStartChunks = viewStartChunks;
+        }
+
+        /// <inheritdoc />
+        protected override IEnumerable<TagHelperDescriptor> GetTagHelperDescriptors([NotNull] Block documentRoot)
+        {
+            var descriptors = base.GetTagHelperDescriptors(documentRoot);
+
+            // TODO: https://github.com/aspnet/Razor/issues/112 Needs to support RemvoeHelperChunks too.
+
+            // Grab all the @addTagHelper chunks from view starts
+            var viewStartDescriptors = _viewStartChunks.OfType<AddTagHelperChunk>()
+                                                       .Select(c => c.LookupText)
+                                                       .SelectMany(TagHelperDescriptorResolver.Resolve);
+
+            return descriptors.Concat(viewStartDescriptors);
+        }
+    }
+}

test/Microsoft.AspNet.Mvc.FunctionalTests/TagHelpersTest.cs

@@ -51,5 +51,26 @@ namespace Microsoft.AspNet.Mvc.FunctionalTests
             Assert.Equal(expectedMediaType, response.Content.Headers.ContentType);
             Assert.Equal(expectedContent, responseContent);
         }
+
+        [Theory]
+        [InlineData("NestedViewStartTagHelper")]
+        [InlineData("ViewWithLayoutAndNestedTagHelper")]
+        public async Task TagHelpersAreInheritedFromViewStartPages(string action)
+        {
+            // Arrange
+            var expected = string.Join(Environment.NewLine,
+                                       "<root>root-content</root>",
+                                       "",
+                                       "",
+                                       "<nested>nested-content</nested>");
+            var server = TestServer.Create(_provider, _app);
+            var client = server.CreateClient();
+
+            // Act
+            var result = await client.GetStringAsync("http://localhost/Home/" + action);
+
+            // Assert
+            Assert.Equal(expected, result.Trim());
+        }
     }
 }

test/Microsoft.AspNet.Mvc.FunctionalTests/compiler/resources/TagHelpersWebSite.Home.About.html

@@ -34,6 +34,7 @@
 
 
 
+
 <div>
     <p>Hello, you've reached the about page.</p>
 

test/Microsoft.AspNet.Mvc.FunctionalTests/compiler/resources/TagHelpersWebSite.Home.Help.html

@@ -33,6 +33,7 @@
         
 
 
+
 <div>
     <p>Hello, you've reached the help page. If you're having troubles try visiting our <a href="/?approved=true">My Approved Home Page</a></p>
 </div>

test/Microsoft.AspNet.Mvc.FunctionalTests/compiler/resources/TagHelpersWebSite.Home.Index.html

@@ -41,6 +41,7 @@
 
 
 
+
 <div>
     <p>This website has <strong style="font-size: 1.25em;
                               text-decoration: underline;">not</strong> been approved yet. Visit <strong><a target="_blank" href="http://www.contoso.com">www.contoso.com</a></strong> for <strong>more</strong> information.</p>

test/Microsoft.AspNet.Mvc.Razor.Host.Test/Directives/ChunkInheritanceUtilityTest.cs

@@ -1,10 +1,7 @@
 // Copyright (c) Microsoft Open Technologies, Inc. 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 Microsoft.AspNet.Razor.Generator.Compiler;
-using Moq;
 using Xunit;
 
 namespace Microsoft.AspNet.Mvc.Razor.Directives
@@ -29,24 +26,30 @@ namespace Microsoft.AspNet.Mvc.Razor.Directives
 
 ");
             var host = new MvcRazorHost(fileSystem);
-            var utility = new ChunkInheritanceUtility(new CodeTree(), new Chunk[0], "dynamic");
+            var utility = new ChunkInheritanceUtility(host, fileSystem, new Chunk[0]);
 
             // Act
-            var chunks = utility.GetInheritedChunks(host,
+            var chunks = utility.GetInheritedChunks(@"x:\myapproot\views\home\Index.cshtml");
-                                                    fileSystem,
-                                                    @"x:\myapproot\views\home\Index.cshtml");
 
             // Assert
-            Assert.Equal(3, chunks.Count);
+            Assert.Equal(8, chunks.Count);
-            var usingChunk = Assert.IsType<UsingChunk>(chunks[0]);
+            Assert.IsType<LiteralChunk>(chunks[0]);
+
+            var usingChunk = Assert.IsType<UsingChunk>(chunks[1]);
             Assert.Equal("MyNamespace", usingChunk.Namespace);
 
-            var injectChunk = Assert.IsType<InjectChunk>(chunks[1]);
+            Assert.IsType<LiteralChunk>(chunks[2]);
+            Assert.IsType<LiteralChunk>(chunks[3]);
+
+            var injectChunk = Assert.IsType<InjectChunk>(chunks[4]);
             Assert.Equal("MyHelper<TModel>", injectChunk.TypeName);
             Assert.Equal("Helper", injectChunk.MemberName);
 
-            var setBaseTypeChunk = Assert.IsType<SetBaseTypeChunk>(chunks[2]);
+            var setBaseTypeChunk = Assert.IsType<SetBaseTypeChunk>(chunks[5]);
             Assert.Equal("MyBaseType", setBaseTypeChunk.TypeName);
+
+            Assert.IsType<StatementChunk>(chunks[6]);
+            Assert.IsType<LiteralChunk>(chunks[7]);
         }
 
         [Fact]
@@ -58,12 +61,10 @@ namespace Microsoft.AspNet.Mvc.Razor.Directives
             fileSystem.AddFile(@"x:\myapproot\views\_Layout.cshtml", string.Empty);
             fileSystem.AddFile(@"x:\myapproot\views\home\_not-viewstart.cshtml", string.Empty);
             var host = new MvcRazorHost(fileSystem);
-            var utility = new ChunkInheritanceUtility(new CodeTree(), new Chunk[0], "dynamic");
+            var utility = new ChunkInheritanceUtility(host, fileSystem, new Chunk[0]);
 
             // Act
-            var chunks = utility.GetInheritedChunks(host,
+            var chunks = utility.GetInheritedChunks(@"x:\myapproot\views\home\Index.cshtml");
-                                                    fileSystem,
-                                                    @"x:\myapproot\views\home\Index.cshtml");
 
             // Assert
             Assert.Empty(chunks);
@@ -75,73 +76,26 @@ namespace Microsoft.AspNet.Mvc.Razor.Directives
             // Arrange
             var fileSystem = new TestFileSystem();
             fileSystem.AddFile(@"x:\myapproot\views\_viewstart.cshtml",
-@"@inject DifferentHelper<TModel> Html
+                               "@inject DifferentHelper<TModel> Html");
-@using AppNamespace.Models
-@{
-    Layout = ""test.cshtml"";
-}
-
-");
             var host = new MvcRazorHost(fileSystem);
             var defaultChunks = new Chunk[]
             {
                 new InjectChunk("MyTestHtmlHelper", "Html"),
                 new UsingChunk { Namespace = "AppNamespace.Model" },
             };
-            var utility = new ChunkInheritanceUtility(new CodeTree(), defaultChunks, "dynamic");
+            var utility = new ChunkInheritanceUtility(host, fileSystem, defaultChunks);
 
             // Act
-            var chunks = utility.GetInheritedChunks(host,
+            var chunks = utility.GetInheritedChunks(@"x:\myapproot\views\home\Index.cshtml");
-                                                    fileSystem,
-                                                    @"x:\myapproot\views\home\Index.cshtml");
 
             // Assert
             Assert.Equal(4, chunks.Count);
-            var injectChunk = Assert.IsType<InjectChunk>(chunks[0]);
+            var injectChunk = Assert.IsType<InjectChunk>(chunks[1]);
             Assert.Equal("DifferentHelper<TModel>", injectChunk.TypeName);
             Assert.Equal("Html", injectChunk.MemberName);
 
-            var usingChunk = Assert.IsType<UsingChunk>(chunks[1]);
+            Assert.Same(defaultChunks[0], chunks[2]);
-            Assert.Equal("AppNamespace.Models", usingChunk.Namespace);
+            Assert.Same(defaultChunks[1], chunks[3]);
-
-            injectChunk = Assert.IsType<InjectChunk>(chunks[2]);
-            Assert.Equal("MyTestHtmlHelper", injectChunk.TypeName);
-            Assert.Equal("Html", injectChunk.MemberName);
-
-            usingChunk = Assert.IsType<UsingChunk>(chunks[3]);
-            Assert.Equal("AppNamespace.Model", usingChunk.Namespace);
-        }
-
-        [Fact]
-        public void MergeChunks_VisitsChunksPriorToMerging()
-        {
-            // Arrange
-            var codeTree = new CodeTree();
-            codeTree.Chunks.Add(new LiteralChunk());
-            codeTree.Chunks.Add(new ExpressionBlockChunk());
-            codeTree.Chunks.Add(new ExpressionBlockChunk());
-            
-            var merger = new Mock<IChunkMerger>();
-            var mockSequence = new MockSequence();
-            merger.InSequence(mockSequence)
-                  .Setup(m => m.VisitChunk(It.IsAny<LiteralChunk>()))
-                  .Verifiable();
-            merger.InSequence(mockSequence)
-                   .Setup(m => m.Merge(codeTree, It.IsAny<LiteralChunk>()))
-                   .Verifiable();
-            var inheritedChunks = new List<Chunk>
-            {
-                new CodeAttributeChunk(),
-                new LiteralChunk()
-            };
-            var utility = new ChunkInheritanceUtility(codeTree, inheritedChunks, "dynamic");
-
-            // Act
-            utility.ChunkMergers[typeof(LiteralChunk)] = merger.Object;
-            utility.MergeInheritedChunks(inheritedChunks);
-
-            // Assert
-            merger.Verify();
         }
     }
 }

test/Microsoft.AspNet.Mvc.Razor.Host.Test/TestFileSystem.cs

@@ -24,7 +24,7 @@ namespace Microsoft.AspNet.Mvc.Razor
         {
             var fileInfo = new Mock<IFileInfo>();
             fileInfo.Setup(f => f.CreateReadStream())
-                    .Returns(new MemoryStream(Encoding.UTF8.GetBytes(contents)));
+                    .Returns(() => new MemoryStream(Encoding.UTF8.GetBytes(contents)));
             fileInfo.SetupGet(f => f.PhysicalPath)
                     .Returns(path);
             fileInfo.SetupGet(f => f.Name)

test/WebSites/TagHelpersWebSite/Controllers/HomeController.cs

@@ -29,5 +29,15 @@ namespace TagHelpersWebSite.Controllers
         {
             return View();
         }
+
+        public ViewResult NestedViewStartTagHelper()
+        {
+            return View();
+        }
+
+        public ViewResult ViewWithLayoutAndNestedTagHelper()
+        {
+            return View();
+        }
     }
 }

test/WebSites/TagHelpersWebSite/TagHelpers/NestedViewStartTagHelper.cs

@@ -0,0 +1,18 @@
+// Copyright (c) Microsoft Open Technologies, Inc. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using Microsoft.AspNet.Razor.Runtime.TagHelpers;
+using Microsoft.AspNet.Razor.TagHelpers;
+
+namespace TagHelpersWebSite.TagHelpers
+{
+    [TagName("nested")]
+    [ContentBehavior(ContentBehavior.Modify)]
+    public class NestedViewStartTagHelper : TagHelper
+    {
+        public override void Process(TagHelperContext context, TagHelperOutput output)
+        {
+            output.Content = "nested-content";
+        }
+    }
+}

test/WebSites/TagHelpersWebSite/TagHelpers/RootViewStartTagHelper.cs

@@ -0,0 +1,18 @@
+// Copyright (c) Microsoft Open Technologies, Inc. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using Microsoft.AspNet.Razor.Runtime.TagHelpers;
+using Microsoft.AspNet.Razor.TagHelpers;
+
+namespace TagHelpersWebSite.TagHelpers
+{
+    [TagName("root")]
+    [ContentBehavior(ContentBehavior.Replace)]
+    public class RootViewStartTagHelper : TagHelper
+    {
+        public override void Process(TagHelperContext context, TagHelperOutput output)
+        {
+            output.Content = "root-content";
+        }
+    }
+}

test/WebSites/TagHelpersWebSite/Views/Home/NestedViewStartTagHelper.cshtml

@@ -0,0 +1,7 @@
+@{
+    Layout = null;
+}
+<root />
+
+
+<nested>some-content</nested>

test/WebSites/TagHelpersWebSite/Views/Home/_ViewStart.cshtml

@@ -0,0 +1 @@
+@addtaghelper "TagHelpersWebSite.TagHelpers.NestedViewStartTagHelper, TagHelpersWebSite"

test/WebSites/TagHelpersWebSite/Views/Shared/ViewWithLayoutAndNestedTagHelper.cshtml

@@ -0,0 +1,5 @@
+@{
+    Layout = "~/Views/Shared/_LayoutWithRootTagHelper.cshtml";
+}
+@addtaghelper "TagHelpersWebSite.TagHelpers.NestedViewStartTagHelper, TagHelpersWebSite"
+<nested>some-content</nested>

test/WebSites/TagHelpersWebSite/Views/Shared/_LayoutWithRootTagHelper.cshtml

@@ -0,0 +1,2 @@
+<root />
+@RenderBody()

test/WebSites/TagHelpersWebSite/Views/_ViewStart.cshtml

@@ -1,3 +1,4 @@
 @{
     Layout = "/Views/Shared/_Layout.cshtml";
 }
+@addtaghelper "TagHelpersWebSite.TagHelpers.RootViewStartTagHelper, TagHelpersWebSite"