Add `TagHelperOutput.GetChildContentAsync()` overloads including an `HtmlEncoder`

- #643 part 1
- change is viral and requires an update to `RazorPage.StartTagHelperWritingScope()`
  - memoize `GetChildContentAsync()` per-encoder
  - update generation tests to match and to test new behaviour
- note `HtmlEncoder`s used elsewhere e.g. in other `RazorPage` instances are unaffected

Add `NullHtmlEncoder`

Nits:
- generally clean up affected doc comments and make them more consistent
- remove unused `using`s in files I had open

src/Microsoft.AspNet.Razor.Runtime/Runtime/TagHelpers/TagHelperExecutionContext.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Text.Encodings.Web;
 using System.Threading.Tasks;
 using Microsoft.AspNet.Razor.TagHelpers;
 
@@ -15,9 +16,10 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
     {
         private readonly List<ITagHelper> _tagHelpers;
         private readonly Func<Task> _executeChildContentAsync;
-        private readonly Action _startTagHelperWritingScope;
+        private readonly Action<HtmlEncoder> _startTagHelperWritingScope;
         private readonly Func<TagHelperContent> _endTagHelperWritingScope;
         private TagHelperContent _childContent;
+        private Dictionary<HtmlEncoder, TagHelperContent> _perEncoderChildContent;
 
         /// <summary>
         /// Internal for testing purposes only.
@@ -28,7 +30,7 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                    items: new Dictionary<object, object>(),
                    uniqueId: string.Empty,
                    executeChildContentAsync: async () => await Task.FromResult(result: true),
-                   startTagHelperWritingScope: () => { },
+                   startTagHelperWritingScope: _ => { },
                    endTagHelperWritingScope: () => new DefaultTagHelperContent())
         {
         }
@@ -42,7 +44,10 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
         /// <see cref="ITagHelper"/>s</param>
         /// <param name="uniqueId">An identifier unique to the HTML element this context is for.</param>
         /// <param name="executeChildContentAsync">A delegate used to execute the child content asynchronously.</param>
-        /// <param name="startTagHelperWritingScope">A delegate used to start a writing scope in a Razor page.</param>
+        /// <param name="startTagHelperWritingScope">
+        /// A delegate used to start a writing scope in a Razor page and optionally override the page's
+        /// <see cref="HtmlEncoder"/> within that scope.
+        /// </param>
         /// <param name="endTagHelperWritingScope">A delegate used to end a writing scope in a Razor page.</param>
         public TagHelperExecutionContext(
             string tagName,
@@ -50,7 +55,7 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
             IDictionary<object, object> items,
             string uniqueId,
             Func<Task> executeChildContentAsync,
-            Action startTagHelperWritingScope,
+            Action<HtmlEncoder> startTagHelperWritingScope,
             Func<TagHelperContent> endTagHelperWritingScope)
         {
             if (tagName == null)
@@ -224,23 +229,57 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
         }
 
         /// <summary>
-        /// Execute and retrieve the rendered child content asynchronously.
+        /// Executes children asynchronously with the given <paramref name="encoder"/> in scope and returns their
+        /// rendered content.
         /// </summary>
+        /// <param name="useCachedResult">
+        /// If <c>true</c>, multiple calls with the same <see cref="HtmlEncoder"/> will not cause children to
+        /// re-execute; returns cached content.
+        /// </param>
+        /// <param name="encoder">
+        /// The <see cref="HtmlEncoder"/> to use when the page handles
+        /// non-<see cref="Microsoft.AspNet.Html.IHtmlContent"/> C# expressions. If <c>null</c>, executes children with
+        /// the page's current <see cref="HtmlEncoder"/>.
+        /// </param>
         /// <returns>A <see cref="Task"/> that on completion returns the rendered child content.</returns>
-        /// <remarks>
-        /// Child content is only executed once. Successive calls to this method or successive executions of the
-        /// returned <see cref="Task{TagHelperContent}"/> return a cached result.
-        /// </remarks>
-        public async Task<TagHelperContent> GetChildContentAsync(bool useCachedResult)
+        public async Task<TagHelperContent> GetChildContentAsync(bool useCachedResult, HtmlEncoder encoder)
         {
-            if (!useCachedResult || _childContent == null)
+            // Get cached content for this encoder.
+            TagHelperContent childContent;
+            if (encoder == null)
             {
-                _startTagHelperWritingScope();
-                await _executeChildContentAsync();
-                _childContent = _endTagHelperWritingScope();
+                childContent = _childContent;
+            }
+            else
+            {
+                if (_perEncoderChildContent == null)
+                {
+                    childContent = null;
+                    _perEncoderChildContent = new Dictionary<HtmlEncoder, TagHelperContent>();
+                }
+                else
+                {
+                    _perEncoderChildContent.TryGetValue(encoder, out childContent);
+                }
             }
 
-            return new DefaultTagHelperContent().SetContent(_childContent);
+            if (!useCachedResult || childContent == null)
+            {
+                _startTagHelperWritingScope(encoder);
+                await _executeChildContentAsync();
+                childContent = _endTagHelperWritingScope();
+
+                if (encoder == null)
+                {
+                    _childContent = childContent;
+                }
+                else
+                {
+                    _perEncoderChildContent[encoder] = childContent;
+                }
+            }
+
+            return new DefaultTagHelperContent().SetContent(childContent);
         }
     }
 }

src/Microsoft.AspNet.Razor.Runtime/Runtime/TagHelpers/TagHelperRunner.cs

@@ -3,7 +3,6 @@
 
 using System;
 using System.Collections.Generic;
-using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.AspNet.Razor.TagHelpers;
 

src/Microsoft.AspNet.Razor.Runtime/Runtime/TagHelpers/TagHelperScopeManager.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Text.Encodings.Web;
 using System.Threading.Tasks;
 using Microsoft.AspNet.Razor.TagHelpers;
 using Microsoft.Extensions.Internal;
@@ -31,7 +32,10 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
         /// <param name="tagMode">HTML syntax of the element in the Razor source.</param>
         /// <param name="uniqueId">An identifier unique to the HTML element this scope is for.</param>
         /// <param name="executeChildContentAsync">A delegate used to execute the child content asynchronously.</param>
-        /// <param name="startTagHelperWritingScope">A delegate used to start a writing scope in a Razor page.</param>
+        /// <param name="startTagHelperWritingScope">
+        /// A delegate used to start a writing scope in a Razor page and optionally override the page's
+        /// <see cref="HtmlEncoder"/> within that scope.
+        /// </param>
         /// <param name="endTagHelperWritingScope">A delegate used to end a writing scope in a Razor page.</param>
         /// <returns>A <see cref="TagHelperExecutionContext"/> to use.</returns>
         public TagHelperExecutionContext Begin(
@@ -39,7 +43,7 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
             TagMode tagMode,
             string uniqueId,
             Func<Task> executeChildContentAsync,
-            Action startTagHelperWritingScope,
+            Action<HtmlEncoder> startTagHelperWritingScope,
             Func<TagHelperContent> endTagHelperWritingScope)
         {
             if (tagName == null)

src/Microsoft.AspNet.Razor.Runtime/TagHelpers/NullHtmlEncoder.cs

@@ -0,0 +1,120 @@
+// 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.IO;
+using System.Text.Encodings.Web;
+
+namespace Microsoft.AspNet.Razor.TagHelpers
+{
+    /// <summary>
+    /// A <see cref="HtmlEncoder"/> that does not encode. Should not be used when writing directly to a response
+    /// expected to contain valid HTML.
+    /// </summary>
+    public class NullHtmlEncoder : HtmlEncoder
+    {
+        /// <summary>
+        /// Initializes a <see cref="NullHtmlEncoder"/> instance.
+        /// </summary>
+        protected NullHtmlEncoder()
+        {
+        }
+
+        /// <summary>
+        /// A <see cref="HtmlEncoder"/> instance that does not encode. Should not be used when writing directly to a
+        /// response expected to contain valid HTML.
+        /// </summary>
+        public static new NullHtmlEncoder Default { get; } = new NullHtmlEncoder();
+
+        /// <inheritdoc />
+        public override int MaxOutputCharactersPerInputCharacter
+        {
+            get
+            {
+                return 1;
+            }
+        }
+
+        /// <inheritdoc />
+        public override string Encode(string value)
+        {
+            if (value == null)
+            {
+                throw new ArgumentNullException(nameof(value));
+            }
+
+            return value;
+        }
+
+        /// <inheritdoc />
+        public override void Encode(TextWriter output, char[] value, int startIndex, int characterCount)
+        {
+            if (output == null)
+            {
+                throw new ArgumentNullException(nameof(output));
+            }
+
+            if (value == null)
+            {
+                throw new ArgumentNullException(nameof(value));
+            }
+
+            if (characterCount == 0)
+            {
+                return;
+            }
+
+            output.Write(value, startIndex, characterCount);
+        }
+
+        /// <inheritdoc />
+        public override void Encode(TextWriter output, string value, int startIndex, int characterCount)
+        {
+            if (output == null)
+            {
+                throw new ArgumentNullException(nameof(output));
+            }
+
+            if (value == null)
+            {
+                throw new ArgumentNullException(nameof(value));
+            }
+
+            if (characterCount == 0)
+            {
+                return;
+            }
+
+            output.Write(value.Substring(startIndex, characterCount));
+        }
+
+        /// <inheritdoc />
+        public override unsafe int FindFirstCharacterToEncode(char* text, int textLength)
+        {
+            return -1;
+        }
+
+        /// <inheritdoc />
+        public override unsafe bool TryEncodeUnicodeScalar(
+            int unicodeScalar,
+            char* buffer,
+            int bufferLength,
+            out int numberOfCharactersWritten)
+        {
+            if (buffer == null)
+            {
+                throw new ArgumentNullException(nameof(buffer));
+            }
+
+            numberOfCharactersWritten = 0;
+
+            return false;
+        }
+
+        /// <inheritdoc />
+        public override bool WillEncode(int unicodeScalar)
+        {
+            return false;
+        }
+    }
+}

src/Microsoft.AspNet.Razor.Runtime/TagHelpers/TagHelperOutput.cs

@@ -14,7 +14,7 @@ namespace Microsoft.AspNet.Razor.TagHelpers
     /// </summary>
     public class TagHelperOutput : IHtmlContent
     {
-        private readonly Func<bool, Task<TagHelperContent>> _getChildContentAsync;
+        private readonly Func<bool, HtmlEncoder, Task<TagHelperContent>> _getChildContentAsync;
         private TagHelperContent _preElement;
         private TagHelperContent _preContent;
         private TagHelperContent _content;
@@ -27,7 +27,7 @@ namespace Microsoft.AspNet.Razor.TagHelpers
             : this(
                 tagName,
                 new TagHelperAttributeList(),
-                (cachedResult) => Task.FromResult<TagHelperContent>(new DefaultTagHelperContent()))
+                (useCachedResult, encoder) => Task.FromResult<TagHelperContent>(new DefaultTagHelperContent()))
         {
         }
 
@@ -36,12 +36,14 @@ namespace Microsoft.AspNet.Razor.TagHelpers
         /// </summary>
         /// <param name="tagName">The HTML element's tag name.</param>
         /// <param name="attributes">The HTML attributes.</param>
-        /// <param name="getChildContentAsync">A delegate used to execute and retrieve the rendered child content
-        /// asynchronously.</param>
+        /// <param name="getChildContentAsync">
+        /// A delegate used to execute children asynchronously with the given <see cref="HtmlEncoder"/> in scope and
+        /// return their rendered content.
+        /// </param>
         public TagHelperOutput(
             string tagName,
             TagHelperAttributeList attributes,
-            Func<bool, Task<TagHelperContent>> getChildContentAsync)
+            Func<bool, HtmlEncoder, Task<TagHelperContent>> getChildContentAsync)
         {
             if (attributes == null)
             {
@@ -206,24 +208,65 @@ namespace Microsoft.AspNet.Razor.TagHelpers
         }
 
         /// <summary>
-        /// A delegate used to execute children asynchronously.
+        /// Executes children asynchronously and returns their rendered content.
         /// </summary>
         /// <returns>A <see cref="Task"/> that on completion returns content rendered by children.</returns>
-        /// <remarks>This method is memoized.</remarks>
+        /// <remarks>
+        /// This method is memoized. Multiple calls will not cause children to re-execute with the page's original
+        /// <see cref="HtmlEncoder"/>.
+        /// </remarks>
         public Task<TagHelperContent> GetChildContentAsync()
         {
-            return GetChildContentAsync(useCachedResult: true);
+            return GetChildContentAsync(useCachedResult: true, encoder: null);
         }
 
         /// <summary>
-        /// A delegate used to execute children asynchronously.
+        /// Executes children asynchronously and returns their rendered content.
         /// </summary>
-        /// <param name="useCachedResult">If <c>true</c> multiple calls to this method will not cause re-execution
-        /// of child content; cached content will be returned.</param>
+        /// <param name="useCachedResult">
+        /// If <c>true</c>, multiple calls will not cause children to re-execute with the page's original
+        /// <see cref="HtmlEncoder"/>; returns cached content.
+        /// </param>
         /// <returns>A <see cref="Task"/> that on completion returns content rendered by children.</returns>
         public Task<TagHelperContent> GetChildContentAsync(bool useCachedResult)
         {
-            return _getChildContentAsync(useCachedResult);
+            return GetChildContentAsync(useCachedResult, encoder: null);
+        }
+
+        /// <summary>
+        /// Executes children asynchronously with the given <paramref name="encoder"/> in scope and returns their
+        /// rendered content.
+        /// </summary>
+        /// <param name="encoder">
+        /// The <see cref="HtmlEncoder"/> to use when the page handles non-<see cref="IHtmlContent"/> C# expressions.
+        /// If <c>null</c>, executes children with the page's current <see cref="HtmlEncoder"/>.
+        /// </param>
+        /// <returns>A <see cref="Task"/> that on completion returns content rendered by children.</returns>
+        /// <remarks>
+        /// This method is memoized. Multiple calls with the same <see cref="HtmlEncoder"/> instance will not cause
+        /// children to re-execute with that encoder in scope.
+        /// </remarks>
+        public Task<TagHelperContent> GetChildContentAsync(HtmlEncoder encoder)
+        {
+            return GetChildContentAsync(useCachedResult: true, encoder: encoder);
+        }
+
+        /// <summary>
+        /// Executes children asynchronously with the given <paramref name="encoder"/> in scope and returns their
+        /// rendered content.
+        /// </summary>
+        /// <param name="useCachedResult">
+        /// If <c>true</c>, multiple calls with the same <see cref="HtmlEncoder"/> will not cause children to
+        /// re-execute; returns cached content.
+        /// </param>
+        /// <param name="encoder">
+        /// The <see cref="HtmlEncoder"/> to use when the page handles non-<see cref="IHtmlContent"/> C# expressions.
+        /// If <c>null</c>, executes children with the page's current <see cref="HtmlEncoder"/>.
+        /// </param>
+        /// <returns>A <see cref="Task"/> that on completion returns content rendered by children.</returns>
+        public Task<TagHelperContent> GetChildContentAsync(bool useCachedResult, HtmlEncoder encoder)
+        {
+            return _getChildContentAsync(useCachedResult, encoder);
         }
 
         /// <inheritdoc />

src/Microsoft.AspNet.Razor.Runtime/project.json

@@ -2,8 +2,9 @@
   "description": "Runtime components for rendering Razor pages.",
   "version": "4.0.0-*",
   "compilationOptions": {
-    "warningsAsErrors": true,
-    "keyFile": "../../tools/Key.snk"
+    "allowUnsafe": true,
+    "keyFile": "../../tools/Key.snk",
+    "warningsAsErrors": true
   },
   "repository": {
     "type": "git",

src/Microsoft.AspNet.Razor/CodeGenerators/CSharpTagHelperCodeRenderer.cs

@@ -8,8 +8,8 @@ using System.Globalization;
 using System.Linq;
 using Microsoft.AspNet.Razor.Chunks;
 using Microsoft.AspNet.Razor.CodeGenerators.Visitors;
-using Microsoft.AspNet.Razor.TagHelpers;
 using Microsoft.AspNet.Razor.Compilation.TagHelpers;
+using Microsoft.AspNet.Razor.TagHelpers;
 
 namespace Microsoft.AspNet.Razor.CodeGenerators
 {
@@ -659,7 +659,7 @@ namespace Microsoft.AspNet.Razor.CodeGenerators
                 // Scopes are a runtime feature.
                 if (!_designTimeMode)
                 {
-                    _writer.WriteMethodInvocation(_tagHelperContext.StartTagHelperWritingScopeMethodName);
+                    _writer.WriteMethodInvocation(_tagHelperContext.StartTagHelperWritingScopeMethodName, "null");
                 }
 
                 var visitor = htmlEncodeValues ? _bodyVisitor : _literalBodyVisitor;
@@ -668,8 +668,9 @@ namespace Microsoft.AspNet.Razor.CodeGenerators
                 // Scopes are a runtime feature.
                 if (!_designTimeMode)
                 {
-                    _writer.WriteStartAssignment(StringValueBufferVariableName)
-                           .WriteMethodInvocation(_tagHelperContext.EndTagHelperWritingScopeMethodName);
+                    _writer
+                        .WriteStartAssignment(StringValueBufferVariableName)
+                        .WriteMethodInvocation(_tagHelperContext.EndTagHelperWritingScopeMethodName);
                 }
             }
             finally
@@ -708,7 +709,7 @@ namespace Microsoft.AspNet.Razor.CodeGenerators
                     StringValueBufferVariableName,
                     _tagHelperContext.TagHelperContentGetContentMethodName,
                     endLine: false,
-                    parameters: new string[] { _tagHelperContext.HtmlEncoderPropertyName });
+                    parameters: _tagHelperContext.HtmlEncoderPropertyName);
             }
         }
 

test/Microsoft.AspNet.Razor.Runtime.Test/Runtime/TagHelpers/TagHelperExecutionContextTest.cs

@@ -1,9 +1,9 @@
 // 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.Linq;
+using System.Text.Encodings.Web;
 using System.Threading.Tasks;
 using Microsoft.AspNet.Razor.TagHelpers;
 using Microsoft.Extensions.WebEncoders.Testing;
@@ -43,7 +43,7 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                 items: expectedItems,
                 uniqueId: string.Empty,
                 executeChildContentAsync: async () => await Task.FromResult(result: true),
-                startTagHelperWritingScope: () => { },
+                startTagHelperWritingScope: _ => { },
                 endTagHelperWritingScope: () => new DefaultTagHelperContent());
 
             // Assert
@@ -51,13 +51,47 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
             Assert.Same(expectedItems, executionContext.Items);
         }
 
-        [Fact]
-        public async Task GetChildContentAsync_CachesValue()
+        public static TheoryData<HtmlEncoder> HtmlEncoderData
+        {
+            get
+            {
+                return new TheoryData<HtmlEncoder>
+                {
+                    null,
+                    HtmlEncoder.Default,
+                    NullHtmlEncoder.Default,
+                    new HtmlTestEncoder(),
+                };
+            }
+        }
+
+        public static TheoryData<HtmlEncoder> HtmlEncoderDataLessNull
+        {
+            get
+            {
+                var data = new TheoryData<HtmlEncoder>();
+                foreach (var row in HtmlEncoderData)
+                {
+                    var encoder = (HtmlEncoder)(row[0]);
+                    if (encoder != null)
+                    {
+                        data.Add(encoder);
+                    }
+                }
+
+                return data;
+            }
+        }
+
+        [Theory]
+        [MemberData(nameof(HtmlEncoderData))]
+        public async Task GetChildContentAsync_ReturnsExpectedContent(HtmlEncoder encoder)
         {
             // Arrange
-            var defaultTagHelperContent = new DefaultTagHelperContent();
-            var content = string.Empty;
-            var expectedContent = string.Empty;
+            var tagHelperContent = new DefaultTagHelperContent();
+            var executionCount = 0;
+            var content = "Hello from child content";
+            var expectedContent = $"HtmlEncode[[{content}]]";
             var executionContext = new TagHelperExecutionContext(
                 "p",
                 tagMode: TagMode.StartTagAndEndTag,
@@ -65,30 +99,46 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                 uniqueId: string.Empty,
                 executeChildContentAsync: () =>
                 {
-                    if (string.IsNullOrEmpty(expectedContent))
-                    {
-                        content = "Hello from child content: " + Guid.NewGuid().ToString();
-                        expectedContent = $"HtmlEncode[[{content}]]";
-                    }
-
-                    defaultTagHelperContent.SetContent(content);
+                    executionCount++;
+                    tagHelperContent.SetContent(content);
 
                     return Task.FromResult(result: true);
                 },
-                startTagHelperWritingScope: () => { },
-                endTagHelperWritingScope: () => defaultTagHelperContent);
+                startTagHelperWritingScope: _ => { },
+                endTagHelperWritingScope: () => tagHelperContent);
 
             // Act
-            var content1 = await executionContext.GetChildContentAsync(useCachedResult: true);
-            var content2 = await executionContext.GetChildContentAsync(useCachedResult: true);
+            var actualContent = await executionContext.GetChildContentAsync(useCachedResult: true, encoder: encoder);
 
             // Assert
-            Assert.Equal(expectedContent, content1.GetContent(new HtmlTestEncoder()));
-            Assert.Equal(expectedContent, content2.GetContent(new HtmlTestEncoder()));
+            Assert.Equal(expectedContent, actualContent.GetContent(new HtmlTestEncoder()));
         }
 
-        [Fact]
-        public async Task GetChildContentAsync_CanExecuteChildrenMoreThanOnce()
+        [Theory]
+        [MemberData(nameof(HtmlEncoderData))]
+        public async Task GetChildContentAsync_StartsWritingScopeWithGivenEncoder(HtmlEncoder encoder)
+        {
+            // Arrange
+            HtmlEncoder passedEncoder = null;
+            var executionContext = new TagHelperExecutionContext(
+                "p",
+                tagMode: TagMode.StartTagAndEndTag,
+                items: new Dictionary<object, object>(),
+                uniqueId: string.Empty,
+                executeChildContentAsync: () => Task.FromResult(result: true),
+                startTagHelperWritingScope: encoderArgument => passedEncoder = encoderArgument,
+                endTagHelperWritingScope: () => new DefaultTagHelperContent());
+
+            // Act
+            await executionContext.GetChildContentAsync(useCachedResult: true, encoder: encoder);
+
+            // Assert
+            Assert.Same(encoder, passedEncoder);
+        }
+
+        [Theory]
+        [MemberData(nameof(HtmlEncoderData))]
+        public async Task GetChildContentAsync_CachesValue(HtmlEncoder encoder)
         {
             // Arrange
             var executionCount = 0;
@@ -100,15 +150,98 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                 executeChildContentAsync: () =>
                 {
                     executionCount++;
-
                     return Task.FromResult(result: true);
                 },
-                startTagHelperWritingScope: () => { },
+                startTagHelperWritingScope: _ => { },
                 endTagHelperWritingScope: () => new DefaultTagHelperContent());
 
             // Act
-            await executionContext.GetChildContentAsync(useCachedResult: false);
-            await executionContext.GetChildContentAsync(useCachedResult: false);
+            var content1 = await executionContext.GetChildContentAsync(useCachedResult: true, encoder: encoder);
+            var content2 = await executionContext.GetChildContentAsync(useCachedResult: true, encoder: encoder);
+
+            // Assert
+            Assert.Equal(1, executionCount);
+        }
+
+        [Theory]
+        [MemberData(nameof(HtmlEncoderDataLessNull))]
+        public async Task GetChildContentAsync_CachesValuePerEncoder(HtmlEncoder encoder)
+        {
+            // Arrange
+            var executionCount = 0;
+            var executionContext = new TagHelperExecutionContext(
+                "p",
+                tagMode: TagMode.StartTagAndEndTag,
+                items: new Dictionary<object, object>(),
+                uniqueId: string.Empty,
+                executeChildContentAsync: () =>
+                {
+                    executionCount++;
+                    return Task.FromResult(result: true);
+                },
+                startTagHelperWritingScope: _ => { },
+                endTagHelperWritingScope: () => new DefaultTagHelperContent());
+
+            // Act
+            var content1 = await executionContext.GetChildContentAsync(useCachedResult: true, encoder: null);
+            var content2 = await executionContext.GetChildContentAsync(useCachedResult: true, encoder: encoder);
+
+            // Assert
+            Assert.Equal(2, executionCount);
+        }
+
+        [Theory]
+        [MemberData(nameof(HtmlEncoderData))]
+        public async Task GetChildContentAsync_CachesValuePerEncoderInstance(HtmlEncoder encoder)
+        {
+            // Arrange
+            var executionCount = 0;
+            var executionContext = new TagHelperExecutionContext(
+                "p",
+                tagMode: TagMode.StartTagAndEndTag,
+                items: new Dictionary<object, object>(),
+                uniqueId: string.Empty,
+                executeChildContentAsync: () =>
+                {
+                    executionCount++;
+                    return Task.FromResult(result: true);
+                },
+                startTagHelperWritingScope: _ => { },
+                endTagHelperWritingScope: () => new DefaultTagHelperContent());
+
+            // HtmlEncoderData includes another HtmlTestEncoder instance but method compares HtmlEncoder instances.
+            var firstEncoder = new HtmlTestEncoder();
+
+            // Act
+            var content1 = await executionContext.GetChildContentAsync(useCachedResult: true, encoder: firstEncoder);
+            var content2 = await executionContext.GetChildContentAsync(useCachedResult: true, encoder: encoder);
+
+            // Assert
+            Assert.Equal(2, executionCount);
+        }
+
+        [Theory]
+        [MemberData(nameof(HtmlEncoderData))]
+        public async Task GetChildContentAsync_CanExecuteChildrenMoreThanOnce(HtmlEncoder encoder)
+        {
+            // Arrange
+            var executionCount = 0;
+            var executionContext = new TagHelperExecutionContext(
+                "p",
+                tagMode: TagMode.StartTagAndEndTag,
+                items: new Dictionary<object, object>(),
+                uniqueId: string.Empty,
+                executeChildContentAsync: () =>
+                {
+                    executionCount++;
+                    return Task.FromResult(result: true);
+                },
+                startTagHelperWritingScope: _ => { },
+                endTagHelperWritingScope: () => new DefaultTagHelperContent());
+
+            // Act
+            await executionContext.GetChildContentAsync(useCachedResult: false, encoder: encoder);
+            await executionContext.GetChildContentAsync(useCachedResult: false, encoder: encoder);
 
             // Assert
             Assert.Equal(2, executionCount);
@@ -120,27 +253,21 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
         public async Task GetChildContentAsync_ReturnsNewObjectEveryTimeItIsCalled(bool useCachedResult)
         {
             // Arrange
-            var defaultTagHelperContent = new DefaultTagHelperContent();
             var executionContext = new TagHelperExecutionContext(
                 "p",
                 tagMode: TagMode.StartTagAndEndTag,
                 items: new Dictionary<object, object>(),
                 uniqueId: string.Empty,
-                executeChildContentAsync: () => { return Task.FromResult(result: true); },
-                startTagHelperWritingScope: () => { },
-                endTagHelperWritingScope: () => defaultTagHelperContent);
+                executeChildContentAsync: () => Task.FromResult(result: true),
+                startTagHelperWritingScope: _ => { },
+                endTagHelperWritingScope: () => new DefaultTagHelperContent());
 
             // Act
-            var content1 = await executionContext.GetChildContentAsync(useCachedResult);
-            content1.Append("Hello");
-            var content2 = await executionContext.GetChildContentAsync(useCachedResult);
-            content2.Append("World!");
+            var content1 = await executionContext.GetChildContentAsync(useCachedResult, encoder: null);
+            var content2 = await executionContext.GetChildContentAsync(useCachedResult, encoder: null);
 
             // Assert
             Assert.NotSame(content1, content2);
-
-            var content3 = await executionContext.GetChildContentAsync(useCachedResult);
-            Assert.Empty(content3.GetContent(new HtmlTestEncoder()));
         }
 
         public static TheoryData<string, string> DictionaryCaseTestingData
@@ -314,7 +441,6 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
             Assert.Equal(2, tagHelpers.Length);
             Assert.Same(tagHelper1, tagHelpers[0]);
             Assert.Same(tagHelper2, tagHelpers[1]);
-
         }
 
         private class PTagHelper : TagHelper

test/Microsoft.AspNet.Razor.Runtime.Test/Runtime/TagHelpers/TagHelperOutputTest.cs

@@ -1,698 +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.Threading.Tasks;
-using Microsoft.AspNet.Razor.Runtime.TagHelpers;
-using Microsoft.AspNet.Razor.TagHelpers;
-using Microsoft.Extensions.WebEncoders.Testing;
-using Xunit;
-
-namespace Microsoft.AspNet.Razor.Runtime.Test.Runtime.TagHelpers
-{
-    public class TagHelperOutputTest
-    {
-        public static TheoryData<TagHelperOutput, string> WriteTagHelper_InputData
-        {
-            get
-            {
-                // parameters: TagHelperOutput, expectedOutput
-                return new TheoryData<TagHelperOutput, string>
-                {
-                    {
-                        // parameters: TagName, Attributes, SelfClosing, PreContent, Content, PostContent
-                        GetTagHelperOutput(
-                            tagName:     "div",
-                            attributes:  new TagHelperAttributeList(),
-                            tagMode: TagMode.StartTagAndEndTag,
-                            preElement:  null,
-                            preContent:  null,
-                            content:     "Hello World!",
-                            postContent: null,
-                            postElement: null),
-                        "<div>Hello World!</div>"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     string.Empty,
-                            attributes:  new TagHelperAttributeList(),
-                            tagMode: TagMode.StartTagAndEndTag,
-                            preElement:  null,
-                            preContent:  null,
-                            content:     "Hello World!",
-                            postContent: null,
-                            postElement: null),
-                        "Hello World!"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "  ",
-                            attributes:  new TagHelperAttributeList(),
-                            tagMode: TagMode.StartTagAndEndTag,
-                            preElement:  null,
-                            preContent:  null,
-                            content:     "Hello World!",
-                            postContent: null,
-                            postElement: null),
-                        "Hello World!"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "p",
-                            attributes:  new TagHelperAttributeList() { { "test", "testVal" } },
-                            tagMode: TagMode.StartTagAndEndTag,
-                            preElement:  null,
-                            preContent:  null,
-                            content:     "Hello World!",
-                            postContent: null,
-                            postElement: null),
-                        "<p test=\"HtmlEncode[[testVal]]\">Hello World!</p>"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "p",
-                            attributes:  new TagHelperAttributeList()
-                            {
-                                { "test", "testVal" },
-                                { "something", "  spaced  " }
-                            },
-                            tagMode: TagMode.StartTagAndEndTag,
-                            preElement:  null,
-                            preContent:  null,
-                            content:     "Hello World!",
-                            postContent: null,
-                            postElement: null),
-                        "<p test=\"HtmlEncode[[testVal]]\" something=\"HtmlEncode[[  spaced  ]]\">Hello World!</p>"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "p",
-                            attributes:  new TagHelperAttributeList()
-                            {
-                                ["test"] = new TagHelperAttribute
-                                {
-                                    Name = "test",
-                                    Minimized = true
-                                },
-                            },
-                            tagMode: TagMode.StartTagAndEndTag,
-                            preElement:  null,
-                            preContent:  null,
-                            content:     "Hello World!",
-                            postContent: null,
-                            postElement: null),
-                        "<p test>Hello World!</p>"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "p",
-                            attributes:  new TagHelperAttributeList()
-                            {
-                                ["test"] = new TagHelperAttribute
-                                {
-                                    Name = "test",
-                                    Minimized = true
-                                },
-                                ["test2"] = new TagHelperAttribute
-                                {
-                                    Name = "test2",
-                                    Minimized = true
-                                },
-                            },
-                            tagMode: TagMode.StartTagAndEndTag,
-                            preElement:  null,
-                            preContent:  null,
-                            content:     "Hello World!",
-                            postContent: null,
-                            postElement: null),
-                        "<p test test2>Hello World!</p>"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "p",
-                            attributes:  new TagHelperAttributeList()
-                            {
-                                ["first"] = "unminimized",
-                                ["test"] = new TagHelperAttribute
-                                {
-                                    Name = "test",
-                                    Minimized = true
-                                },
-                            },
-                            tagMode: TagMode.StartTagAndEndTag,
-                            preElement:  null,
-                            preContent:  null,
-                            content:     "Hello World!",
-                            postContent: null,
-                            postElement: null),
-                        "<p first=\"HtmlEncode[[unminimized]]\" test>Hello World!</p>"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "p",
-                            attributes:  new TagHelperAttributeList()
-                            {
-                                ["test"] = new TagHelperAttribute
-                                {
-                                    Name = "test",
-                                    Minimized = true
-                                },
-                                ["last"] = "unminimized",
-                            },
-                            tagMode: TagMode.StartTagAndEndTag,
-                            preElement:  null,
-                            preContent:  null,
-                            content:     "Hello World!",
-                            postContent: null,
-                            postElement: null),
-                        "<p test last=\"HtmlEncode[[unminimized]]\">Hello World!</p>"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "p",
-                            attributes:  new TagHelperAttributeList() { { "test", "testVal" } },
-                            tagMode: TagMode.SelfClosing,
-                            preElement:  null,
-                            preContent:  null,
-                            content:     "Hello World!",
-                            postContent: null,
-                            postElement: null),
-                        "<p test=\"HtmlEncode[[testVal]]\" />"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "p",
-                            attributes:  new TagHelperAttributeList()
-                            {
-                                { "test", "testVal" },
-                                { "something", "  spaced  " }
-                            },
-                            tagMode: TagMode.SelfClosing,
-                            preElement:  null,
-                            preContent:  null,
-                            content:     "Hello World!",
-                            postContent: null,
-                            postElement: null),
-                        "<p test=\"HtmlEncode[[testVal]]\" something=\"HtmlEncode[[  spaced  ]]\" />"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "p",
-                            attributes:  new TagHelperAttributeList() { { "test", "testVal" } },
-                            tagMode: TagMode.StartTagOnly,
-                            preElement:  null,
-                            preContent:  null,
-                            content:     "Hello World!",
-                            postContent: null,
-                            postElement: null),
-                        "<p test=\"HtmlEncode[[testVal]]\">"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "p",
-                            attributes:  new TagHelperAttributeList()
-                            {
-                                { "test", "testVal" },
-                                { "something", "  spaced  " }
-                            },
-                            tagMode: TagMode.StartTagOnly,
-                            preElement:  null,
-                            preContent:  null,
-                            content:     "Hello World!",
-                            postContent: null,
-                            postElement: null),
-                        "<p test=\"HtmlEncode[[testVal]]\" something=\"HtmlEncode[[  spaced  ]]\">"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "p",
-                            attributes:  new TagHelperAttributeList(),
-                            tagMode: TagMode.StartTagAndEndTag,
-                            preElement:  null,
-                            preContent:  "Hello World!",
-                            content:     null,
-                            postContent: null,
-                            postElement: null),
-                        "<p>Hello World!</p>"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "p",
-                            attributes:  new TagHelperAttributeList(),
-                            tagMode: TagMode.StartTagAndEndTag,
-                            preElement:  null,
-                            preContent:  null,
-                            content:     "Hello World!",
-                            postContent: null,
-                            postElement: null),
-                        "<p>Hello World!</p>"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "p",
-                            attributes:  new TagHelperAttributeList(),
-                            tagMode: TagMode.StartTagAndEndTag,
-                            preElement:  null,
-                            preContent:  null,
-                            content:     null,
-                            postContent: "Hello World!",
-                            postElement: null),
-                        "<p>Hello World!</p>"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "p",
-                            attributes:  new TagHelperAttributeList(),
-                            tagMode: TagMode.StartTagAndEndTag,
-                            preElement:  null,
-                            preContent:  "Hello",
-                            content:     "Test",
-                            postContent: "World!",
-                            postElement: null),
-                        "<p>HelloTestWorld!</p>"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "p",
-                            attributes:  new TagHelperAttributeList(),
-                            tagMode: TagMode.SelfClosing,
-                            preElement:  null,
-                            preContent:  "Hello",
-                            content:     "Test",
-                            postContent: "World!",
-                            postElement: null),
-                        "<p />"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "p",
-                            attributes:  new TagHelperAttributeList(),
-                            tagMode: TagMode.StartTagOnly,
-                            preElement:  null,
-                            preContent:  "Hello",
-                            content:     "Test",
-                            postContent: "World!",
-                            postElement: null),
-                        "<p>"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "custom",
-                            attributes:  new TagHelperAttributeList(),
-                            tagMode: TagMode.StartTagAndEndTag,
-                            preElement:  null,
-                            preContent:  "Hello",
-                            content:     "Test",
-                            postContent: "World!",
-                            postElement: null),
-                        "<custom>HelloTestWorld!</custom>"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "random",
-                            attributes:  new TagHelperAttributeList(),
-                            tagMode: TagMode.SelfClosing,
-                            preElement:  null,
-                            preContent:  "Hello",
-                            content:     "Test",
-                            postContent: "World!",
-                            postElement: null),
-                        "<random />"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "random",
-                            attributes:  new TagHelperAttributeList(),
-                            tagMode: TagMode.StartTagOnly,
-                            preElement:  null,
-                            preContent:  "Hello",
-                            content:     "Test",
-                            postContent: "World!",
-                            postElement: null),
-                        "<random>"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "custom",
-                            attributes:  new TagHelperAttributeList(),
-                            tagMode: TagMode.StartTagAndEndTag,
-                            preElement:  "Before",
-                            preContent:  null,
-                            content:     null,
-                            postContent: null,
-                            postElement: null),
-                        "Before<custom></custom>"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     string.Empty,
-                            attributes:  new TagHelperAttributeList(),
-                            tagMode: TagMode.StartTagAndEndTag,
-                            preElement:  "Before",
-                            preContent:  null,
-                            content:     null,
-                            postContent: null,
-                            postElement: null),
-                        "Before"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     string.Empty,
-                            attributes:  new TagHelperAttributeList { { "test", "testVal" } },
-                            tagMode: TagMode.SelfClosing,
-                            preElement:  "Before",
-                            preContent:  null,
-                            content:     null,
-                            postContent: null,
-                            postElement: null),
-                        "Before"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "custom",
-                            attributes:  new TagHelperAttributeList { { "test", "testVal" } },
-                            tagMode: TagMode.SelfClosing,
-                            preElement:  "Before",
-                            preContent:  null,
-                            content:     null,
-                            postContent: null,
-                            postElement: null),
-                        "Before<custom test=\"HtmlEncode[[testVal]]\" />"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "custom",
-                            attributes:  new TagHelperAttributeList(),
-                            tagMode: TagMode.SelfClosing,
-                            preElement:  "Before",
-                            preContent:  null,
-                            content:     null,
-                            postContent: null,
-                            postElement: null),
-                        "Before<custom />"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     string.Empty,
-                            attributes:  new TagHelperAttributeList { { "test", "testVal" } },
-                            tagMode: TagMode.StartTagOnly,
-                            preElement:  "Before",
-                            preContent:  null,
-                            content:     null,
-                            postContent: null,
-                            postElement: null),
-                        "Before"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "custom",
-                            attributes:  new TagHelperAttributeList { { "test", "testVal" } },
-                            tagMode: TagMode.StartTagOnly,
-                            preElement:  "Before",
-                            preContent:  null,
-                            content:     null,
-                            postContent: null,
-                            postElement: null),
-                        "Before<custom test=\"HtmlEncode[[testVal]]\">"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "custom",
-                            attributes:  new TagHelperAttributeList(),
-                            tagMode: TagMode.StartTagOnly,
-                            preElement:  "Before",
-                            preContent:  null,
-                            content:     null,
-                            postContent: null,
-                            postElement: null),
-                        "Before<custom>"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "custom",
-                            attributes:  new TagHelperAttributeList(),
-                            tagMode: TagMode.StartTagAndEndTag,
-                            preElement:  null,
-                            preContent:  null,
-                            content:     null,
-                            postContent: null,
-                            postElement: "After"),
-                        "<custom></custom>After"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     string.Empty,
-                            attributes:  new TagHelperAttributeList(),
-                            tagMode: TagMode.StartTagAndEndTag,
-                            preElement:  null,
-                            preContent:  null,
-                            content:     null,
-                            postContent: null,
-                            postElement: "After"),
-                        "After"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     string.Empty,
-                            attributes:  new TagHelperAttributeList { { "test", "testVal" } },
-                            tagMode: TagMode.SelfClosing,
-                            preElement:  null,
-                            preContent:  null,
-                            content:     null,
-                            postContent: null,
-                            postElement: "After"),
-                        "After"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "custom",
-                            attributes:  new TagHelperAttributeList { { "test", "testVal" } },
-                            tagMode: TagMode.SelfClosing,
-                            preElement:  null,
-                            preContent:  null,
-                            content:     null,
-                            postContent: null,
-                            postElement: "After"),
-                        "<custom test=\"HtmlEncode[[testVal]]\" />After"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "custom",
-                            attributes:  new TagHelperAttributeList(),
-                            tagMode: TagMode.SelfClosing,
-                            preElement:  null,
-                            preContent:  null,
-                            content:     null,
-                            postContent: null,
-                            postElement: "After"),
-                        "<custom />After"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     string.Empty,
-                            attributes:  new TagHelperAttributeList { { "test", "testVal" } },
-                            tagMode: TagMode.StartTagOnly,
-                            preElement:  null,
-                            preContent:  null,
-                            content:     null,
-                            postContent: null,
-                            postElement: "After"),
-                        "After"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "custom",
-                            attributes:  new TagHelperAttributeList { { "test", "testVal" } },
-                            tagMode: TagMode.StartTagOnly,
-                            preElement:  null,
-                            preContent:  null,
-                            content:     null,
-                            postContent: null,
-                            postElement: "After"),
-                        "<custom test=\"HtmlEncode[[testVal]]\">After"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "custom",
-                            attributes:  new TagHelperAttributeList(),
-                            tagMode: TagMode.StartTagOnly,
-                            preElement:  null,
-                            preContent:  null,
-                            content:     null,
-                            postContent: null,
-                            postElement: "After"),
-                        "<custom>After"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "custom",
-                            attributes:  new TagHelperAttributeList(),
-                            tagMode: TagMode.StartTagAndEndTag,
-                            preElement:  "Before",
-                            preContent:  "Hello",
-                            content:     "Test",
-                            postContent: "World!",
-                            postElement: "After"),
-                        "Before<custom>HelloTestWorld!</custom>After"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "custom",
-                            attributes:  new TagHelperAttributeList { { "test", "testVal" } },
-                            tagMode: TagMode.StartTagAndEndTag,
-                            preElement:  "Before",
-                            preContent:  "Hello",
-                            content:     "Test",
-                            postContent: "World!",
-                            postElement: "After"),
-                        "Before<custom test=\"HtmlEncode[[testVal]]\">HelloTestWorld!</custom>After"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "custom",
-                            attributes:  new TagHelperAttributeList(),
-                            tagMode: TagMode.SelfClosing,
-                            preElement:  "Before",
-                            preContent:  "Hello",
-                            content:     "Test",
-                            postContent: "World!",
-                            postElement: "After"),
-                        "Before<custom />After"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     string.Empty,
-                            attributes:  new TagHelperAttributeList(),
-                            tagMode: TagMode.SelfClosing,
-                            preElement:  "Before",
-                            preContent:  "Hello",
-                            content:     "Test",
-                            postContent: "World!",
-                            postElement: "After"),
-                        "BeforeHelloTestWorld!After"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     "custom",
-                            attributes:  new TagHelperAttributeList(),
-                            tagMode: TagMode.StartTagOnly,
-                            preElement:  "Before",
-                            preContent:  "Hello",
-                            content:     "Test",
-                            postContent: "World!",
-                            postElement: "After"),
-                        "Before<custom>After"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     string.Empty,
-                            attributes:  new TagHelperAttributeList(),
-                            tagMode: TagMode.StartTagOnly,
-                            preElement:  "Before",
-                            preContent:  "Hello",
-                            content:     "Test",
-                            postContent: "World!",
-                            postElement: "After"),
-                        "BeforeHelloTestWorld!After"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     string.Empty,
-                            attributes:  new TagHelperAttributeList(),
-                            tagMode: TagMode.StartTagAndEndTag,
-                            preElement:  "Before",
-                            preContent:  "Hello",
-                            content:     "Test",
-                            postContent: "World!",
-                            postElement: "After"),
-                        "BeforeHelloTestWorld!After"
-                    },
-                    {
-                        GetTagHelperOutput(
-                            tagName:     string.Empty,
-                            attributes:  new TagHelperAttributeList { { "test", "testVal" } },
-                            tagMode: TagMode.StartTagAndEndTag,
-                            preElement:  "Before",
-                            preContent:  "Hello",
-                            content:     "Test",
-                            postContent: "World!",
-                            postElement: "After"),
-                        "BeforeHelloTestWorld!After"
-                    },
-                };
-            }
-        }
-
-        [Theory]
-        [MemberData(nameof(WriteTagHelper_InputData))]
-        public void TagHelperOutput_WritesFormattedTagHelper(TagHelperOutput output, string expected)
-        {
-            // Arrange
-            var writer = new StringWriter();
-            var tagHelperExecutionContext = new TagHelperExecutionContext(
-                tagName: output.TagName,
-                tagMode: output.TagMode,
-                items: new Dictionary<object, object>(),
-                uniqueId: string.Empty,
-                executeChildContentAsync: () => Task.FromResult(result: true),
-                startTagHelperWritingScope: () => { },
-                endTagHelperWritingScope: () => new DefaultTagHelperContent());
-            tagHelperExecutionContext.Output = output;
-            var testEncoder = new HtmlTestEncoder();
-
-            // Act
-            output.WriteTo(writer, testEncoder);
-
-            // Assert
-            Assert.Equal(expected, writer.ToString(), StringComparer.Ordinal);
-        }
-
-        private static TagHelperOutput GetTagHelperOutput(
-            string tagName,
-            TagHelperAttributeList attributes,
-            TagMode tagMode,
-            string preElement,
-            string preContent,
-            string content,
-            string postContent,
-            string postElement)
-        {
-            var output = new TagHelperOutput(
-                tagName,
-                attributes,
-                getChildContentAsync: (_) => Task.FromResult<TagHelperContent>(new DefaultTagHelperContent()))
-            {
-                TagMode = tagMode
-            };
-
-            if (preElement != null)
-            {
-                output.PreElement.AppendHtml(preElement);
-            }
-
-            if (preContent != null)
-            {
-                output.PreContent.AppendHtml(preContent);
-            }
-
-            if (content != null)
-            {
-                output.Content.AppendHtml(content);
-            }
-
-            if (postContent != null)
-            {
-                output.PostContent.AppendHtml(postContent);
-            }
-
-            if (postElement != null)
-            {
-                output.PostElement.AppendHtml(postElement);
-            }
-
-            return output;
-        }
-    }
-}

test/Microsoft.AspNet.Razor.Runtime.Test/Runtime/TagHelpers/TagHelperScopeManagerTest.cs

@@ -167,7 +167,7 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
             Assert.Equal(tagMode, executionContext.TagMode);
         }
 
-    [Fact]
+        [Fact]
         public void End_ReturnsParentExecutionContext()
         {
             // Arrange
@@ -228,7 +228,7 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                 tagMode,
                 uniqueId: string.Empty,
                 executeChildContentAsync: async () => await Task.FromResult(result: true),
-                startTagHelperWritingScope: () => { },
+                startTagHelperWritingScope: _ => { },
                 endTagHelperWritingScope: () => new DefaultTagHelperContent());
         }
     }

test/Microsoft.AspNet.Razor.Runtime.Test/TagHelpers/NullHtmlEncoderTest.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.IO;
+using Xunit;
+
+namespace Microsoft.AspNet.Razor.TagHelpers
+{
+    public class NullHtmlEncoderTest
+    {
+        [Fact]
+        public void MaxOutputCharactersPerInputCharacter_Returns1()
+        {
+            // Arrange
+            var encoder = NullHtmlEncoder.Default;
+
+            // Act
+            var result = encoder.MaxOutputCharactersPerInputCharacter;
+
+            // Assert
+            Assert.Equal(1, result);
+        }
+
+        [Theory]
+        [InlineData("")]
+        [InlineData("abcd")]
+        [InlineData("<<''\"\">>")]
+        public void Encode_String_DoesNotEncode(string value)
+        {
+            // Arrange
+            var encoder = NullHtmlEncoder.Default;
+
+            // Act
+            var result = encoder.Encode(value);
+
+            // Assert
+            Assert.Equal(value, result);
+        }
+
+        [Theory]
+        [InlineData("")]
+        [InlineData("abcd")]
+        [InlineData("<<''\"\">>")]
+        public void Encode_StringToTextWriter_DoesNotEncode(string value)
+        {
+            // Arrange
+            var encoder = NullHtmlEncoder.Default;
+
+            // Act
+            string result;
+            using (var writer = new StringWriter())
+            {
+                encoder.Encode(writer, value);
+                result = writer.ToString();
+            }
+
+            // Assert
+            Assert.Equal(value, result);
+        }
+
+        [Theory]
+        [InlineData("", 0, 0, "")]
+        [InlineData("abcd", 0, 0, "")]
+        [InlineData("<<''\"\">>", 0, 0, "")]
+        [InlineData("abcd", 0, 1, "a")]
+        [InlineData("<<''\"\">>", 0, 1, "<")]
+        [InlineData("abcd", 0, 4, "abcd")]
+        [InlineData("<<''\"\">>", 0, 8, "<<''\"\">>")]
+        [InlineData("abcd", 2, 0, "")]
+        [InlineData("<<''\"\">>", 2, 0, "")]
+        [InlineData("abcd", 2, 2, "cd")]
+        [InlineData("<<''\"\">>", 2, 2, "''")]
+        [InlineData("abcd", 3, 0, "")]
+        [InlineData("<<''\"\">>", 7, 0, "")]
+        [InlineData("abcd", 3, 1, "d")]
+        [InlineData("<<''\"\">>", 7, 1, ">")]
+        public void Encode_StringToTextWriter_DoesNotEncodeSubstring(
+            string value,
+            int startIndex,
+            int characterCount,
+            string expectedResult)
+        {
+            // Arrange
+            var encoder = NullHtmlEncoder.Default;
+
+            // Act
+            string result;
+            using (var writer = new StringWriter())
+            {
+                encoder.Encode(writer, value, startIndex, characterCount);
+                result = writer.ToString();
+            }
+
+            // Assert
+            Assert.Equal(expectedResult, result);
+        }
+
+        [Theory]
+        [InlineData("", 0, 0, "")]
+        [InlineData("abcd", 0, 0, "")]
+        [InlineData("<<''\"\">>", 0, 0, "")]
+        [InlineData("abcd", 0, 1, "a")]
+        [InlineData("<<''\"\">>", 0, 1, "<")]
+        [InlineData("abcd", 0, 4, "abcd")]
+        [InlineData("<<''\"\">>", 0, 8, "<<''\"\">>")]
+        [InlineData("abcd", 2, 0, "")]
+        [InlineData("<<''\"\">>", 2, 0, "")]
+        [InlineData("abcd", 2, 2, "cd")]
+        [InlineData("<<''\"\">>", 2, 2, "''")]
+        [InlineData("abcd", 3, 0, "")]
+        [InlineData("<<''\"\">>", 7, 0, "")]
+        [InlineData("abcd", 3, 1, "d")]
+        [InlineData("<<''\"\">>", 7, 1, ">")]
+        public void Encode_CharsToTextWriter_DoesNotEncodeSubstring(
+            string value,
+            int startIndex,
+            int characterCount,
+            string expectedResult)
+        {
+            // Arrange
+            var encoder = NullHtmlEncoder.Default;
+            var chars = value.ToCharArray();
+
+            // Act
+            string result;
+            using (var writer = new StringWriter())
+            {
+                encoder.Encode(writer, chars, startIndex, characterCount);
+                result = writer.ToString();
+            }
+
+            // Assert
+            Assert.Equal(expectedResult, result);
+        }
+
+        [Theory]
+        [InlineData(int.MaxValue)]
+        [InlineData(int.MinValue)]
+        [InlineData(0)]
+        [InlineData((int)'\n')]
+        [InlineData((int)'\r')]
+        [InlineData((int)'<')]
+        [InlineData((int)'>')]
+        [InlineData((int)'\'')]
+        [InlineData((int)'"')]
+        public void WillEncode_ReturnsFalse(int unicodeScalar)
+        {
+            // Arrange
+            var encoder = NullHtmlEncoder.Default;
+
+            // Act
+            var result = encoder.WillEncode(unicodeScalar);
+
+            // Assert
+            Assert.False(result);
+        }
+    }
+}

test/Microsoft.AspNet.Razor.Runtime.Test/TagHelpers/TagHelperOutputTest.cs

@@ -1,7 +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.Text.Encodings.Web;
 using System.Threading.Tasks;
+using Microsoft.AspNet.Razor.Runtime.TagHelpers;
 using Microsoft.Extensions.WebEncoders.Testing;
 using Xunit;
 
@@ -9,27 +14,148 @@ namespace Microsoft.AspNet.Razor.TagHelpers
 {
     public class TagHelperOutputTest
     {
-        [Theory]
-        [InlineData(true)]
-        [InlineData(false)]
-        public async Task GetChildContentAsync_PassesUseCachedResultAsExpected(bool expectedUseCachedResultValue)
+        [Fact]
+        public async Task GetChildContentAsync_CallsGetChildContentAsync()
         {
             // Arrange
-            bool? useCachedResultValue = null;
+            bool? passedUseCacheResult = null;
+            HtmlEncoder passedEncoder = null;
+            var content = new DefaultTagHelperContent();
             var output = new TagHelperOutput(
-                tagName: "p",
+                tagName: "tag",
                 attributes: new TagHelperAttributeList(),
-                getChildContentAsync: useCachedResult =>
+                getChildContentAsync: (useCachedResult, encoder) =>
                 {
-                    useCachedResultValue = useCachedResult;
-                    return Task.FromResult<TagHelperContent>(new DefaultTagHelperContent());
+                    passedUseCacheResult = useCachedResult;
+                    passedEncoder = encoder;
+                    return Task.FromResult<TagHelperContent>(content);
                 });
 
             // Act
-            await output.GetChildContentAsync(expectedUseCachedResultValue);
+            var result = await output.GetChildContentAsync();
 
             // Assert
-            Assert.Equal(expectedUseCachedResultValue, useCachedResultValue);
+            Assert.True(passedUseCacheResult.HasValue);
+            Assert.True(passedUseCacheResult.Value);
+            Assert.Null(passedEncoder);
+            Assert.Same(content, result);
+        }
+
+        public static TheoryData<HtmlEncoder> HtmlEncoderData
+        {
+            get
+            {
+                return new TheoryData<HtmlEncoder>
+                {
+                    null,
+                    HtmlEncoder.Default,
+                    NullHtmlEncoder.Default,
+                    new HtmlTestEncoder(),
+                };
+            }
+        }
+
+        [Theory]
+        [MemberData(nameof(HtmlEncoderData))]
+        public async Task GetChildContentAsync_CallsGetChildContentAsync(HtmlEncoder encoder)
+        {
+            // Arrange
+            bool? passedUseCacheResult = null;
+            HtmlEncoder passedEncoder = null;
+            var content = new DefaultTagHelperContent();
+            var output = new TagHelperOutput(
+                tagName: "tag",
+                attributes: new TagHelperAttributeList(),
+                getChildContentAsync: (useCachedResult, encoderArgument) =>
+                {
+                    passedUseCacheResult = useCachedResult;
+                    passedEncoder = encoderArgument;
+                    return Task.FromResult<TagHelperContent>(content);
+                });
+
+            // Act
+            var result = await output.GetChildContentAsync(encoder);
+
+            // Assert
+            Assert.True(passedUseCacheResult.HasValue);
+            Assert.True(passedUseCacheResult.Value);
+            Assert.Same(encoder, passedEncoder);
+            Assert.Same(content, result);
+        }
+
+        [Theory]
+        [InlineData(false)]
+        [InlineData(true)]
+        public async Task GetChildContentAsync_CallsGetChildContentAsync(bool useCachedResult)
+        {
+            // Arrange
+            bool? passedUseCacheResult = null;
+            HtmlEncoder passedEncoder = null;
+            var content = new DefaultTagHelperContent();
+            var output = new TagHelperOutput(
+                tagName: "tag",
+                attributes: new TagHelperAttributeList(),
+                getChildContentAsync: (useCachedResultArgument, encoder) =>
+                {
+                    passedUseCacheResult = useCachedResultArgument;
+                    passedEncoder = encoder;
+                    return Task.FromResult<TagHelperContent>(content);
+                });
+
+            // Act
+            var result = await output.GetChildContentAsync(useCachedResult);
+
+            // Assert
+            Assert.True(passedUseCacheResult.HasValue);
+            Assert.Equal(useCachedResult, passedUseCacheResult.Value);
+            Assert.Null(passedEncoder);
+            Assert.Same(content, result);
+        }
+
+        public static TheoryData<bool, HtmlEncoder> UseCachedResultAndHtmlEncoderData
+        {
+            get
+            {
+                var data = new TheoryData<bool, HtmlEncoder>();
+                foreach (var useCachedResult in new[] { false, true })
+                {
+                    foreach (var encoderEntry in HtmlEncoderData)
+                    {
+                        var encoder = (HtmlEncoder)(encoderEntry[0]);
+                        data.Add(useCachedResult, encoder);
+                    }
+                }
+
+                return data;
+            }
+        }
+
+        [Theory]
+        [MemberData(nameof(UseCachedResultAndHtmlEncoderData))]
+        public async Task GetChildContentAsync_CallsGetChildContentAsync(bool useCachedResult, HtmlEncoder encoder)
+        {
+            // Arrange
+            bool? passedUseCacheResult = null;
+            HtmlEncoder passedEncoder = null;
+            var content = new DefaultTagHelperContent();
+            var output = new TagHelperOutput(
+                tagName: "tag",
+                attributes: new TagHelperAttributeList(),
+                getChildContentAsync: (useCachedResultArgument, encoderArgument) =>
+                {
+                    passedUseCacheResult = useCachedResultArgument;
+                    passedEncoder = encoderArgument;
+                    return Task.FromResult<TagHelperContent>(content);
+                });
+
+            // Act
+            var result = await output.GetChildContentAsync(useCachedResult, encoder);
+
+            // Assert
+            Assert.True(passedUseCacheResult.HasValue);
+            Assert.Equal(useCachedResult, passedUseCacheResult.Value);
+            Assert.Same(encoder, passedEncoder);
+            Assert.Same(content, result);
         }
 
         [Fact]
@@ -175,13 +301,14 @@ namespace Microsoft.AspNet.Razor.TagHelpers
         public void SuppressOutput_PreventsTagOutput()
         {
             // Arrange
-            var tagHelperOutput = new TagHelperOutput("p",
+            var tagHelperOutput = new TagHelperOutput(
+                "p",
                 new TagHelperAttributeList
                 {
                     { "class", "btn" },
                     { "something", "   spaced    " }
                 },
-                (cachedResult) => Task.FromResult<TagHelperContent>(new DefaultTagHelperContent()));
+                (cachedResult, encoder) => Task.FromResult<TagHelperContent>(new DefaultTagHelperContent()));
             tagHelperOutput.PreContent.Append("Pre Content");
             tagHelperOutput.Content.Append("Content");
             tagHelperOutput.PostContent.Append("Post Content");
@@ -209,12 +336,13 @@ namespace Microsoft.AspNet.Razor.TagHelpers
         public void Attributes_IgnoresCase(string originalName, string updateName)
         {
             // Arrange
-            var tagHelperOutput = new TagHelperOutput("p",
+            var tagHelperOutput = new TagHelperOutput(
+                "p",
                 new TagHelperAttributeList
                 {
                     { originalName, "btn" },
                 },
-                (cachedResult) => Task.FromResult<TagHelperContent>(new DefaultTagHelperContent()));
+                (cachedResult, encoder) => Task.FromResult<TagHelperContent>(new DefaultTagHelperContent()));
 
             // Act
             tagHelperOutput.Attributes[updateName] = "super button";
@@ -226,5 +354,687 @@ namespace Microsoft.AspNet.Razor.TagHelpers
                 attribute,
                 CaseSensitiveTagHelperAttributeComparer.Default);
         }
+
+        public static TheoryData<TagHelperOutput, string> WriteTagHelper_InputData
+        {
+            get
+            {
+                // parameters: TagHelperOutput, expectedOutput
+                return new TheoryData<TagHelperOutput, string>
+                {
+                    {
+                        // parameters: TagName, Attributes, SelfClosing, PreContent, Content, PostContent
+                        GetTagHelperOutput(
+                            tagName:     "div",
+                            attributes:  new TagHelperAttributeList(),
+                            tagMode: TagMode.StartTagAndEndTag,
+                            preElement:  null,
+                            preContent:  null,
+                            content:     "Hello World!",
+                            postContent: null,
+                            postElement: null),
+                        "<div>Hello World!</div>"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     string.Empty,
+                            attributes:  new TagHelperAttributeList(),
+                            tagMode: TagMode.StartTagAndEndTag,
+                            preElement:  null,
+                            preContent:  null,
+                            content:     "Hello World!",
+                            postContent: null,
+                            postElement: null),
+                        "Hello World!"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "  ",
+                            attributes:  new TagHelperAttributeList(),
+                            tagMode: TagMode.StartTagAndEndTag,
+                            preElement:  null,
+                            preContent:  null,
+                            content:     "Hello World!",
+                            postContent: null,
+                            postElement: null),
+                        "Hello World!"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "p",
+                            attributes:  new TagHelperAttributeList() { { "test", "testVal" } },
+                            tagMode: TagMode.StartTagAndEndTag,
+                            preElement:  null,
+                            preContent:  null,
+                            content:     "Hello World!",
+                            postContent: null,
+                            postElement: null),
+                        "<p test=\"HtmlEncode[[testVal]]\">Hello World!</p>"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "p",
+                            attributes:  new TagHelperAttributeList()
+                            {
+                                { "test", "testVal" },
+                                { "something", "  spaced  " }
+                            },
+                            tagMode: TagMode.StartTagAndEndTag,
+                            preElement:  null,
+                            preContent:  null,
+                            content:     "Hello World!",
+                            postContent: null,
+                            postElement: null),
+                        "<p test=\"HtmlEncode[[testVal]]\" something=\"HtmlEncode[[  spaced  ]]\">Hello World!</p>"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "p",
+                            attributes:  new TagHelperAttributeList()
+                            {
+                                ["test"] = new TagHelperAttribute
+                                {
+                                    Name = "test",
+                                    Minimized = true
+                                },
+                            },
+                            tagMode: TagMode.StartTagAndEndTag,
+                            preElement:  null,
+                            preContent:  null,
+                            content:     "Hello World!",
+                            postContent: null,
+                            postElement: null),
+                        "<p test>Hello World!</p>"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "p",
+                            attributes:  new TagHelperAttributeList()
+                            {
+                                ["test"] = new TagHelperAttribute
+                                {
+                                    Name = "test",
+                                    Minimized = true
+                                },
+                                ["test2"] = new TagHelperAttribute
+                                {
+                                    Name = "test2",
+                                    Minimized = true
+                                },
+                            },
+                            tagMode: TagMode.StartTagAndEndTag,
+                            preElement:  null,
+                            preContent:  null,
+                            content:     "Hello World!",
+                            postContent: null,
+                            postElement: null),
+                        "<p test test2>Hello World!</p>"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "p",
+                            attributes:  new TagHelperAttributeList()
+                            {
+                                ["first"] = "unminimized",
+                                ["test"] = new TagHelperAttribute
+                                {
+                                    Name = "test",
+                                    Minimized = true
+                                },
+                            },
+                            tagMode: TagMode.StartTagAndEndTag,
+                            preElement:  null,
+                            preContent:  null,
+                            content:     "Hello World!",
+                            postContent: null,
+                            postElement: null),
+                        "<p first=\"HtmlEncode[[unminimized]]\" test>Hello World!</p>"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "p",
+                            attributes:  new TagHelperAttributeList()
+                            {
+                                ["test"] = new TagHelperAttribute
+                                {
+                                    Name = "test",
+                                    Minimized = true
+                                },
+                                ["last"] = "unminimized",
+                            },
+                            tagMode: TagMode.StartTagAndEndTag,
+                            preElement:  null,
+                            preContent:  null,
+                            content:     "Hello World!",
+                            postContent: null,
+                            postElement: null),
+                        "<p test last=\"HtmlEncode[[unminimized]]\">Hello World!</p>"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "p",
+                            attributes:  new TagHelperAttributeList() { { "test", "testVal" } },
+                            tagMode: TagMode.SelfClosing,
+                            preElement:  null,
+                            preContent:  null,
+                            content:     "Hello World!",
+                            postContent: null,
+                            postElement: null),
+                        "<p test=\"HtmlEncode[[testVal]]\" />"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "p",
+                            attributes:  new TagHelperAttributeList()
+                            {
+                                { "test", "testVal" },
+                                { "something", "  spaced  " }
+                            },
+                            tagMode: TagMode.SelfClosing,
+                            preElement:  null,
+                            preContent:  null,
+                            content:     "Hello World!",
+                            postContent: null,
+                            postElement: null),
+                        "<p test=\"HtmlEncode[[testVal]]\" something=\"HtmlEncode[[  spaced  ]]\" />"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "p",
+                            attributes:  new TagHelperAttributeList() { { "test", "testVal" } },
+                            tagMode: TagMode.StartTagOnly,
+                            preElement:  null,
+                            preContent:  null,
+                            content:     "Hello World!",
+                            postContent: null,
+                            postElement: null),
+                        "<p test=\"HtmlEncode[[testVal]]\">"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "p",
+                            attributes:  new TagHelperAttributeList()
+                            {
+                                { "test", "testVal" },
+                                { "something", "  spaced  " }
+                            },
+                            tagMode: TagMode.StartTagOnly,
+                            preElement:  null,
+                            preContent:  null,
+                            content:     "Hello World!",
+                            postContent: null,
+                            postElement: null),
+                        "<p test=\"HtmlEncode[[testVal]]\" something=\"HtmlEncode[[  spaced  ]]\">"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "p",
+                            attributes:  new TagHelperAttributeList(),
+                            tagMode: TagMode.StartTagAndEndTag,
+                            preElement:  null,
+                            preContent:  "Hello World!",
+                            content:     null,
+                            postContent: null,
+                            postElement: null),
+                        "<p>Hello World!</p>"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "p",
+                            attributes:  new TagHelperAttributeList(),
+                            tagMode: TagMode.StartTagAndEndTag,
+                            preElement:  null,
+                            preContent:  null,
+                            content:     "Hello World!",
+                            postContent: null,
+                            postElement: null),
+                        "<p>Hello World!</p>"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "p",
+                            attributes:  new TagHelperAttributeList(),
+                            tagMode: TagMode.StartTagAndEndTag,
+                            preElement:  null,
+                            preContent:  null,
+                            content:     null,
+                            postContent: "Hello World!",
+                            postElement: null),
+                        "<p>Hello World!</p>"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "p",
+                            attributes:  new TagHelperAttributeList(),
+                            tagMode: TagMode.StartTagAndEndTag,
+                            preElement:  null,
+                            preContent:  "Hello",
+                            content:     "Test",
+                            postContent: "World!",
+                            postElement: null),
+                        "<p>HelloTestWorld!</p>"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "p",
+                            attributes:  new TagHelperAttributeList(),
+                            tagMode: TagMode.SelfClosing,
+                            preElement:  null,
+                            preContent:  "Hello",
+                            content:     "Test",
+                            postContent: "World!",
+                            postElement: null),
+                        "<p />"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "p",
+                            attributes:  new TagHelperAttributeList(),
+                            tagMode: TagMode.StartTagOnly,
+                            preElement:  null,
+                            preContent:  "Hello",
+                            content:     "Test",
+                            postContent: "World!",
+                            postElement: null),
+                        "<p>"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "custom",
+                            attributes:  new TagHelperAttributeList(),
+                            tagMode: TagMode.StartTagAndEndTag,
+                            preElement:  null,
+                            preContent:  "Hello",
+                            content:     "Test",
+                            postContent: "World!",
+                            postElement: null),
+                        "<custom>HelloTestWorld!</custom>"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "random",
+                            attributes:  new TagHelperAttributeList(),
+                            tagMode: TagMode.SelfClosing,
+                            preElement:  null,
+                            preContent:  "Hello",
+                            content:     "Test",
+                            postContent: "World!",
+                            postElement: null),
+                        "<random />"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "random",
+                            attributes:  new TagHelperAttributeList(),
+                            tagMode: TagMode.StartTagOnly,
+                            preElement:  null,
+                            preContent:  "Hello",
+                            content:     "Test",
+                            postContent: "World!",
+                            postElement: null),
+                        "<random>"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "custom",
+                            attributes:  new TagHelperAttributeList(),
+                            tagMode: TagMode.StartTagAndEndTag,
+                            preElement:  "Before",
+                            preContent:  null,
+                            content:     null,
+                            postContent: null,
+                            postElement: null),
+                        "Before<custom></custom>"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     string.Empty,
+                            attributes:  new TagHelperAttributeList(),
+                            tagMode: TagMode.StartTagAndEndTag,
+                            preElement:  "Before",
+                            preContent:  null,
+                            content:     null,
+                            postContent: null,
+                            postElement: null),
+                        "Before"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     string.Empty,
+                            attributes:  new TagHelperAttributeList { { "test", "testVal" } },
+                            tagMode: TagMode.SelfClosing,
+                            preElement:  "Before",
+                            preContent:  null,
+                            content:     null,
+                            postContent: null,
+                            postElement: null),
+                        "Before"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "custom",
+                            attributes:  new TagHelperAttributeList { { "test", "testVal" } },
+                            tagMode: TagMode.SelfClosing,
+                            preElement:  "Before",
+                            preContent:  null,
+                            content:     null,
+                            postContent: null,
+                            postElement: null),
+                        "Before<custom test=\"HtmlEncode[[testVal]]\" />"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "custom",
+                            attributes:  new TagHelperAttributeList(),
+                            tagMode: TagMode.SelfClosing,
+                            preElement:  "Before",
+                            preContent:  null,
+                            content:     null,
+                            postContent: null,
+                            postElement: null),
+                        "Before<custom />"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     string.Empty,
+                            attributes:  new TagHelperAttributeList { { "test", "testVal" } },
+                            tagMode: TagMode.StartTagOnly,
+                            preElement:  "Before",
+                            preContent:  null,
+                            content:     null,
+                            postContent: null,
+                            postElement: null),
+                        "Before"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "custom",
+                            attributes:  new TagHelperAttributeList { { "test", "testVal" } },
+                            tagMode: TagMode.StartTagOnly,
+                            preElement:  "Before",
+                            preContent:  null,
+                            content:     null,
+                            postContent: null,
+                            postElement: null),
+                        "Before<custom test=\"HtmlEncode[[testVal]]\">"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "custom",
+                            attributes:  new TagHelperAttributeList(),
+                            tagMode: TagMode.StartTagOnly,
+                            preElement:  "Before",
+                            preContent:  null,
+                            content:     null,
+                            postContent: null,
+                            postElement: null),
+                        "Before<custom>"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "custom",
+                            attributes:  new TagHelperAttributeList(),
+                            tagMode: TagMode.StartTagAndEndTag,
+                            preElement:  null,
+                            preContent:  null,
+                            content:     null,
+                            postContent: null,
+                            postElement: "After"),
+                        "<custom></custom>After"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     string.Empty,
+                            attributes:  new TagHelperAttributeList(),
+                            tagMode: TagMode.StartTagAndEndTag,
+                            preElement:  null,
+                            preContent:  null,
+                            content:     null,
+                            postContent: null,
+                            postElement: "After"),
+                        "After"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     string.Empty,
+                            attributes:  new TagHelperAttributeList { { "test", "testVal" } },
+                            tagMode: TagMode.SelfClosing,
+                            preElement:  null,
+                            preContent:  null,
+                            content:     null,
+                            postContent: null,
+                            postElement: "After"),
+                        "After"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "custom",
+                            attributes:  new TagHelperAttributeList { { "test", "testVal" } },
+                            tagMode: TagMode.SelfClosing,
+                            preElement:  null,
+                            preContent:  null,
+                            content:     null,
+                            postContent: null,
+                            postElement: "After"),
+                        "<custom test=\"HtmlEncode[[testVal]]\" />After"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "custom",
+                            attributes:  new TagHelperAttributeList(),
+                            tagMode: TagMode.SelfClosing,
+                            preElement:  null,
+                            preContent:  null,
+                            content:     null,
+                            postContent: null,
+                            postElement: "After"),
+                        "<custom />After"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     string.Empty,
+                            attributes:  new TagHelperAttributeList { { "test", "testVal" } },
+                            tagMode: TagMode.StartTagOnly,
+                            preElement:  null,
+                            preContent:  null,
+                            content:     null,
+                            postContent: null,
+                            postElement: "After"),
+                        "After"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "custom",
+                            attributes:  new TagHelperAttributeList { { "test", "testVal" } },
+                            tagMode: TagMode.StartTagOnly,
+                            preElement:  null,
+                            preContent:  null,
+                            content:     null,
+                            postContent: null,
+                            postElement: "After"),
+                        "<custom test=\"HtmlEncode[[testVal]]\">After"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "custom",
+                            attributes:  new TagHelperAttributeList(),
+                            tagMode: TagMode.StartTagOnly,
+                            preElement:  null,
+                            preContent:  null,
+                            content:     null,
+                            postContent: null,
+                            postElement: "After"),
+                        "<custom>After"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "custom",
+                            attributes:  new TagHelperAttributeList(),
+                            tagMode: TagMode.StartTagAndEndTag,
+                            preElement:  "Before",
+                            preContent:  "Hello",
+                            content:     "Test",
+                            postContent: "World!",
+                            postElement: "After"),
+                        "Before<custom>HelloTestWorld!</custom>After"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "custom",
+                            attributes:  new TagHelperAttributeList { { "test", "testVal" } },
+                            tagMode: TagMode.StartTagAndEndTag,
+                            preElement:  "Before",
+                            preContent:  "Hello",
+                            content:     "Test",
+                            postContent: "World!",
+                            postElement: "After"),
+                        "Before<custom test=\"HtmlEncode[[testVal]]\">HelloTestWorld!</custom>After"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "custom",
+                            attributes:  new TagHelperAttributeList(),
+                            tagMode: TagMode.SelfClosing,
+                            preElement:  "Before",
+                            preContent:  "Hello",
+                            content:     "Test",
+                            postContent: "World!",
+                            postElement: "After"),
+                        "Before<custom />After"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     string.Empty,
+                            attributes:  new TagHelperAttributeList(),
+                            tagMode: TagMode.SelfClosing,
+                            preElement:  "Before",
+                            preContent:  "Hello",
+                            content:     "Test",
+                            postContent: "World!",
+                            postElement: "After"),
+                        "BeforeHelloTestWorld!After"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     "custom",
+                            attributes:  new TagHelperAttributeList(),
+                            tagMode: TagMode.StartTagOnly,
+                            preElement:  "Before",
+                            preContent:  "Hello",
+                            content:     "Test",
+                            postContent: "World!",
+                            postElement: "After"),
+                        "Before<custom>After"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     string.Empty,
+                            attributes:  new TagHelperAttributeList(),
+                            tagMode: TagMode.StartTagOnly,
+                            preElement:  "Before",
+                            preContent:  "Hello",
+                            content:     "Test",
+                            postContent: "World!",
+                            postElement: "After"),
+                        "BeforeHelloTestWorld!After"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     string.Empty,
+                            attributes:  new TagHelperAttributeList(),
+                            tagMode: TagMode.StartTagAndEndTag,
+                            preElement:  "Before",
+                            preContent:  "Hello",
+                            content:     "Test",
+                            postContent: "World!",
+                            postElement: "After"),
+                        "BeforeHelloTestWorld!After"
+                    },
+                    {
+                        GetTagHelperOutput(
+                            tagName:     string.Empty,
+                            attributes:  new TagHelperAttributeList { { "test", "testVal" } },
+                            tagMode: TagMode.StartTagAndEndTag,
+                            preElement:  "Before",
+                            preContent:  "Hello",
+                            content:     "Test",
+                            postContent: "World!",
+                            postElement: "After"),
+                        "BeforeHelloTestWorld!After"
+                    },
+                };
+            }
+        }
+
+        [Theory]
+        [MemberData(nameof(WriteTagHelper_InputData))]
+        public void WriteTo_WritesFormattedTagHelper(TagHelperOutput output, string expected)
+        {
+            // Arrange
+            var writer = new StringWriter();
+            var tagHelperExecutionContext = new TagHelperExecutionContext(
+                tagName: output.TagName,
+                tagMode: output.TagMode,
+                items: new Dictionary<object, object>(),
+                uniqueId: string.Empty,
+                executeChildContentAsync: () => Task.FromResult(result: true),
+                startTagHelperWritingScope: _ => { },
+                endTagHelperWritingScope: () => new DefaultTagHelperContent());
+            tagHelperExecutionContext.Output = output;
+            var testEncoder = new HtmlTestEncoder();
+
+            // Act
+            output.WriteTo(writer, testEncoder);
+
+            // Assert
+            Assert.Equal(expected, writer.ToString(), StringComparer.Ordinal);
+        }
+
+        private static TagHelperOutput GetTagHelperOutput(
+            string tagName,
+            TagHelperAttributeList attributes,
+            TagMode tagMode,
+            string preElement,
+            string preContent,
+            string content,
+            string postContent,
+            string postElement)
+        {
+            var output = new TagHelperOutput(
+                tagName,
+                attributes,
+                getChildContentAsync: (useCachedContent, encoder) => Task.FromResult<TagHelperContent>(
+                    new DefaultTagHelperContent()))
+            {
+                TagMode = tagMode
+            };
+
+            if (preElement != null)
+            {
+                output.PreElement.AppendHtml(preElement);
+            }
+
+            if (preContent != null)
+            {
+                output.PreContent.AppendHtml(preContent);
+            }
+
+            if (content != null)
+            {
+                output.Content.AppendHtml(content);
+            }
+
+            if (postContent != null)
+            {
+                output.PostContent.AppendHtml(postContent);
+            }
+
+            if (postElement != null)
+            {
+                output.PostElement.AppendHtml(postElement);
+            }
+
+            return output;
+        }
     }
 }

test/Microsoft.AspNet.Razor.Test/Microsoft.AspNet.Razor.Test.xproj

@@ -16,5 +16,8 @@
   <ItemGroup>
     <Service Include="{82a7f48d-3b50-4b1e-b82e-3ada8210c358}" />
   </ItemGroup>
+  <ItemGroup>
+    <DnxInvisibleFolder Include=".testPublish\" />
+  </ItemGroup>
   <Import Project="$(VSToolsPath)\DNX\Microsoft.DNX.targets" Condition="'$(VSToolsPath)' != ''" />
 </Project>

test/Microsoft.AspNet.Razor.Test/TestFiles/CodeGenerator/Output/BasicTagHelpers.CustomAttributeCodeGenerator.cs

@@ -51,7 +51,7 @@ namespace TestOutput
                 __tagHelperExecutionContext.Add(__TestNamespace_InputTagHelper);
                 __TestNamespace_InputTagHelper2 = CreateTagHelper<global::TestNamespace.InputTagHelper2>();
                 __tagHelperExecutionContext.Add(__TestNamespace_InputTagHelper2);
-                StartTagHelperWritingScope();
+                StartTagHelperWritingScope(null);
                 WriteLiteral("2000 + ");
 #line 6 "BasicTagHelpers.cshtml"
                                 Write(ViewBag.DefaultInterval);

test/Microsoft.AspNet.Razor.Test/TestFiles/CodeGenerator/Output/BasicTagHelpers.cs

@@ -52,7 +52,7 @@ namespace TestOutput
                 __tagHelperExecutionContext.Add(__TestNamespace_InputTagHelper);
                 __TestNamespace_InputTagHelper2 = CreateTagHelper<global::TestNamespace.InputTagHelper2>();
                 __tagHelperExecutionContext.Add(__TestNamespace_InputTagHelper2);
-                StartTagHelperWritingScope();
+                StartTagHelperWritingScope(null);
                 WriteLiteral("2000 + ");
 #line 6 "BasicTagHelpers.cshtml"
                                 Write(ViewBag.DefaultInterval);

test/Microsoft.AspNet.Razor.Test/TestFiles/CodeGenerator/Output/ComplexTagHelpers.cs

@@ -119,7 +119,7 @@ namespace TestOutput
                     __tagHelperExecutionContext.Add(__TestNamespace_InputTagHelper);
                     __TestNamespace_InputTagHelper2 = CreateTagHelper<global::TestNamespace.InputTagHelper2>();
                     __tagHelperExecutionContext.Add(__TestNamespace_InputTagHelper2);
-                    StartTagHelperWritingScope();
+                    StartTagHelperWritingScope(null);
 #line 16 "ComplexTagHelpers.cshtml"
                                  WriteLiteral(checkbox);
 
@@ -163,7 +163,7 @@ namespace TestOutput
                 __tagHelperExecutionContext.Add(__TestNamespace_InputTagHelper);
                 __TestNamespace_InputTagHelper2 = CreateTagHelper<global::TestNamespace.InputTagHelper2>();
                 __tagHelperExecutionContext.Add(__TestNamespace_InputTagHelper2);
-                StartTagHelperWritingScope();
+                StartTagHelperWritingScope(null);
 #line 17 "ComplexTagHelpers.cshtml"
                   WriteLiteral(true ? "checkbox" : "anything");
 
@@ -188,7 +188,7 @@ namespace TestOutput
                 __tagHelperExecutionContext.Add(__TestNamespace_InputTagHelper);
                 __TestNamespace_InputTagHelper2 = CreateTagHelper<global::TestNamespace.InputTagHelper2>();
                 __tagHelperExecutionContext.Add(__TestNamespace_InputTagHelper2);
-                StartTagHelperWritingScope();
+                StartTagHelperWritingScope(null);
 #line 18 "ComplexTagHelpers.cshtml"
                               if(true) {
 

test/Microsoft.AspNet.Razor.Test/TestFiles/CodeGenerator/Output/DynamicAttributeTagHelpers.cs

@@ -103,7 +103,7 @@ WriteTo(__razor_attribute_value_writer, string.Empty);
             , StartTagHelperWritingScope, EndTagHelperWritingScope);
             __TestNamespace_InputTagHelper = CreateTagHelper<global::TestNamespace.InputTagHelper>();
             __tagHelperExecutionContext.Add(__TestNamespace_InputTagHelper);
-            StartTagHelperWritingScope();
+            StartTagHelperWritingScope(null);
             WriteLiteral("prefix ");
 #line 7 "DynamicAttributeTagHelpers.cshtml"
          WriteLiteral(DateTime.Now);
@@ -136,7 +136,7 @@ AddHtmlAttributeValue(" ", 210, DateTime.Now, 211, 14, false);
             , StartTagHelperWritingScope, EndTagHelperWritingScope);
             __TestNamespace_InputTagHelper = CreateTagHelper<global::TestNamespace.InputTagHelper>();
             __tagHelperExecutionContext.Add(__TestNamespace_InputTagHelper);
-            StartTagHelperWritingScope();
+            StartTagHelperWritingScope(null);
 #line 9 "DynamicAttributeTagHelpers.cshtml"
   WriteLiteral(long.MinValue);
 

test/Microsoft.AspNet.Razor.Test/TestFiles/CodeGenerator/Output/EscapedTagHelpers.cs

@@ -44,7 +44,7 @@ namespace TestOutput
             __tagHelperExecutionContext.Add(__TestNamespace_InputTagHelper);
             __TestNamespace_InputTagHelper2 = CreateTagHelper<global::TestNamespace.InputTagHelper2>();
             __tagHelperExecutionContext.Add(__TestNamespace_InputTagHelper2);
-            StartTagHelperWritingScope();
+            StartTagHelperWritingScope(null);
 #line 6 "EscapedTagHelpers.cshtml"
                                       WriteLiteral(DateTime.Now);
 

test/Microsoft.AspNet.Razor.Test/TestFiles/CodeGenerator/Output/NestedScriptTagTagHelpers.cs

@@ -54,7 +54,7 @@ namespace TestOutput
                 __tagHelperExecutionContext.Add(__TestNamespace_InputTagHelper);
                 __TestNamespace_InputTagHelper2 = CreateTagHelper<global::TestNamespace.InputTagHelper2>();
                 __tagHelperExecutionContext.Add(__TestNamespace_InputTagHelper2);
-                StartTagHelperWritingScope();
+                StartTagHelperWritingScope(null);
                 WriteLiteral("2000 + ");
 #line 8 "NestedScriptTagTagHelpers.cshtml"
                                             Write(ViewBag.DefaultInterval);

test/Microsoft.AspNet.Razor.Test/TestFiles/CodeGenerator/Output/PrefixedAttributeTagHelpers.Reversed.cs

@@ -172,7 +172,7 @@ __TestNamespace_InputTagHelper2.IntDictionaryProperty["salt"] = 37;
             __TestNamespace_InputTagHelper2.StringDictionaryProperty["paprika"] = "another string";
             __tagHelperExecutionContext.AddTagHelperAttribute("string-prefix-paprika", __TestNamespace_InputTagHelper2.StringDictionaryProperty["paprika"]);
             __TestNamespace_InputTagHelper1.StringDictionaryProperty["paprika"] = __TestNamespace_InputTagHelper2.StringDictionaryProperty["paprika"];
-            StartTagHelperWritingScope();
+            StartTagHelperWritingScope(null);
             WriteLiteral("literate ");
 #line 21 "PrefixedAttributeTagHelpers.cshtml"
                              WriteLiteral(literate);

test/Microsoft.AspNet.Razor.Test/TestFiles/CodeGenerator/Output/PrefixedAttributeTagHelpers.cs

@@ -172,7 +172,7 @@ __TestNamespace_InputTagHelper1.IntDictionaryProperty["salt"] = 37;
             __TestNamespace_InputTagHelper1.StringDictionaryProperty["paprika"] = "another string";
             __tagHelperExecutionContext.AddTagHelperAttribute("string-prefix-paprika", __TestNamespace_InputTagHelper1.StringDictionaryProperty["paprika"]);
             __TestNamespace_InputTagHelper2.StringDictionaryProperty["paprika"] = __TestNamespace_InputTagHelper1.StringDictionaryProperty["paprika"];
-            StartTagHelperWritingScope();
+            StartTagHelperWritingScope(null);
             WriteLiteral("literate ");
 #line 21 "PrefixedAttributeTagHelpers.cshtml"
                              WriteLiteral(literate);

test/Microsoft.AspNet.Razor.Test/TestFiles/CodeGenerator/Output/TagHelpersInSection.cs

@@ -76,7 +76,7 @@ namespace TestOutput
                 , StartTagHelperWritingScope, EndTagHelperWritingScope);
                 __TestNamespace_MyTagHelper = CreateTagHelper<global::TestNamespace.MyTagHelper>();
                 __tagHelperExecutionContext.Add(__TestNamespace_MyTagHelper);
-                StartTagHelperWritingScope();
+                StartTagHelperWritingScope(null);
                 WriteLiteral("Current Time: ");
 #line 9 "TagHelpersInSection.cshtml"
                                       WriteLiteral(DateTime.Now);

test/Microsoft.AspNet.Razor.Test/TestFiles/CodeGenerator/Output/TagHelpersWithWeirdlySpacedAttributes.cs

@@ -43,7 +43,7 @@ __TestNamespace_PTagHelper.Age = 1337;
 #line default
 #line hidden
             __tagHelperExecutionContext.AddTagHelperAttribute("age", __TestNamespace_PTagHelper.Age);
-            StartTagHelperWritingScope();
+            StartTagHelperWritingScope(null);
 #line 7 "TagHelpersWithWeirdlySpacedAttributes.cshtml"
              Write(true);