Implement MoveTo semantics in Mvc

- Simplify things that used to rely on HtmlTextWriter. This behavior is
  now the default.

- Simplify a whole mess of Razor TextWriter code.

- Integration test for merging of TagHelper buffers back into the 'main'
  buffer.

global.json

@@ -1,3 +1,3 @@
 {
-    "projects": ["src", "test/WebSites", "samples"]
+    "projects": ["src", "test/WebSites", "samples", "d:\\k\\HtmlAbstractions\\src", "d:\\k\\Razor\\src"]
 }

src/Microsoft.AspNetCore.Mvc.Localization/LocalizedHtmlString.cs

@@ -100,15 +100,7 @@ namespace Microsoft.AspNetCore.Mvc.Localization
                 throw new ArgumentNullException(nameof(encoder));
             }
 
-            var htmlTextWriter = writer as HtmlTextWriter;
+            FormatValue(writer, encoder, Value, _arguments);
-            if (htmlTextWriter == null)
-            {
-                FormatValue(writer, encoder, Value, _arguments);
-            }
-            else
-            {
-                htmlTextWriter.Write(this);
-            }
         }
 
         private static void FormatValue(

src/Microsoft.AspNetCore.Mvc.Razor/RazorPage.cs

@@ -37,7 +37,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor
         private bool _renderedBody;
         private AttributeInfo _attributeInfo;
         private TagHelperAttributeInfo _tagHelperAttributeInfo;
-        private HtmlContentWrapperTextWriter _valueBuffer;
+        private StringWriter _valueBuffer;
         private IViewBufferScope _bufferScope;
         private bool _ignoreBody;
         private HashSet<string> _ignoredSections;
@@ -214,7 +214,8 @@ namespace Microsoft.AspNetCore.Mvc.Razor
         /// </remarks>
         public void StartTagHelperWritingScope(HtmlEncoder encoder)
         {
-            _tagHelperScopes.Push(new TagHelperScopeInfo(HtmlEncoder, ViewContext.Writer));
+            var buffer = new ViewBuffer(BufferScope, Path, ViewBuffer.TagHelperPageSize);
+            _tagHelperScopes.Push(new TagHelperScopeInfo(buffer, HtmlEncoder, ViewContext.Writer));
 
             // If passed an HtmlEncoder, override the property.
             if (encoder != null)
@@ -224,9 +225,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor
 
             // We need to replace the ViewContext's Writer to ensure that all content (including content written
             // from HTML helpers) is redirected.
-            var buffer = new ViewBuffer(BufferScope, Path);
+            ViewContext.Writer = new ViewBufferTextWriter(buffer, ViewContext.Writer.Encoding);
-            var writer = new HtmlContentWrapperTextWriter(buffer, ViewContext.Writer.Encoding);
-            ViewContext.Writer = writer;
         }
 
         /// <summary>
@@ -240,13 +239,13 @@ namespace Microsoft.AspNetCore.Mvc.Razor
                 throw new InvalidOperationException(Resources.RazorPage_ThereIsNoActiveWritingScopeToEnd);
             }
 
+            var scopeInfo = _tagHelperScopes.Pop();
+
             // Get the content written during the current scope.
-            var writer = ViewContext.Writer as HtmlContentWrapperTextWriter;
             var tagHelperContent = new DefaultTagHelperContent();
-            tagHelperContent.AppendHtml(writer?.ContentBuilder);
+            tagHelperContent.AppendHtml(scopeInfo.Buffer);
 
             // Restore previous scope.
-            var scopeInfo = _tagHelperScopes.Pop();
             HtmlEncoder = scopeInfo.Encoder;
             ViewContext.Writer = scopeInfo.Writer;
 
@@ -317,16 +316,25 @@ namespace Microsoft.AspNetCore.Mvc.Razor
             var htmlContent = value as IHtmlContent;
             if (htmlContent != null)
             {
-                var htmlTextWriter = writer as HtmlTextWriter;
+                var bufferedWriter = writer as ViewBufferTextWriter;
-                if (htmlTextWriter == null)
+                if (bufferedWriter == null || !bufferedWriter.IsBuffering)
                 {
                     htmlContent.WriteTo(writer, encoder);
                 }
                 else
                 {
-                    // This special case allows us to keep buffering as IHtmlContent until we get to the 'final'
+                    var htmlContentContainer = value as IHtmlContentContainer;
-                    // TextWriter.
+                    if (htmlContentContainer != null)
-                    htmlTextWriter.Write(htmlContent);
+                    {
+                        // This is likely another ViewBuffer.
+                        htmlContentContainer.MoveTo(bufferedWriter.Buffer);
+                    }
+                    else
+                    {
+                        // Perf: This is the common case for IHtmlContent, ViewBufferTextWriter is inefficient
+                        // for writing character by character.
+                        bufferedWriter.Buffer.AppendHtml(htmlContent);
+                    }
                 }
 
                 return;
@@ -354,7 +362,10 @@ namespace Microsoft.AspNetCore.Mvc.Razor
         {
             if (!string.IsNullOrEmpty(value))
             {
-                encoder.Encode(writer, value);
+                // Perf: Encode right away instead of writing it character-by-character.
+                // character-by-character isn't efficient when using a writer backed by a ViewBuffer.
+                var encoded = encoder.Encode(value);
+                writer.Write(encoded);
             }
         }
 
@@ -529,7 +540,6 @@ namespace Microsoft.AspNetCore.Mvc.Razor
             int attributeValuesCount)
         {
             _tagHelperAttributeInfo = new TagHelperAttributeInfo(executionContext, attributeName, attributeValuesCount);
-            _valueBuffer = null;
         }
 
         public void AddHtmlAttributeValue(
@@ -567,10 +577,11 @@ namespace Microsoft.AspNetCore.Mvc.Razor
 
             if (value != null)
             {
+                // Perf: We'll use this buffer for all of the attribute values and then clear it to
+                // reduce allocations.
                 if (_valueBuffer == null)
                 {
-                    var buffer = new ViewBuffer(BufferScope, Path);
+                    _valueBuffer = new StringWriter();
-                    _valueBuffer = new HtmlContentWrapperTextWriter(buffer, Output.Encoding);
                 }
 
                 if (!string.IsNullOrEmpty(prefix))
@@ -586,10 +597,12 @@ namespace Microsoft.AspNetCore.Mvc.Razor
         {
             if (!_tagHelperAttributeInfo.Suppressed)
             {
-                executionContext.AddHtmlAttribute(
+                // Perf: _valueBuffer might be null if nothing was written. If it is set, clear it so
-                    _tagHelperAttributeInfo.Name,
+                // it is reset for the next value.
-                    (IHtmlContent)_valueBuffer?.ContentBuilder ?? HtmlString.Empty);
+                var content = _valueBuffer == null ? HtmlString.Empty : new HtmlString(_valueBuffer.ToString());
-                _valueBuffer = null;
+                _valueBuffer?.GetStringBuilder().Clear();
+
+                executionContext.AddHtmlAttribute(_tagHelperAttributeInfo.Name, content);
             }
         }
 
@@ -1053,12 +1066,15 @@ namespace Microsoft.AspNetCore.Mvc.Razor
 
         private struct TagHelperScopeInfo
         {
-            public TagHelperScopeInfo(HtmlEncoder encoder, TextWriter writer)
+            public TagHelperScopeInfo(ViewBuffer buffer, HtmlEncoder encoder, TextWriter writer)
             {
+                Buffer = buffer;
                 Encoder = encoder;
                 Writer = writer;
             }
 
+            public ViewBuffer Buffer { get; }
+
             public HtmlEncoder Encoder { get; }
 
             public TextWriter Writer { get; }

src/Microsoft.AspNetCore.Mvc.Razor/RazorTextWriter.cs

@@ -1,209 +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.IO;
-using System.Text;
-using System.Text.Encodings.Web;
-using System.Threading.Tasks;
-using Microsoft.AspNetCore.Html;
-using Microsoft.AspNetCore.Mvc.ViewFeatures.Internal;
-
-namespace Microsoft.AspNetCore.Mvc.Razor
-{
-    /// <summary>
-    /// An <see cref="HtmlTextWriter"/> that is backed by a unbuffered writer (over the Response stream) and a buffered
-    /// <see cref="IHtmlContentBuilder"/>. When <c>Flush</c> or <c>FlushAsync</c> is invoked, the writer
-    /// copies all content from the buffered writer to the unbuffered one and switches to writing to the unbuffered
-    /// writer for all further write operations.
-    /// </summary>
-    public class RazorTextWriter : HtmlTextWriter
-    {
-        /// <summary>
-        /// Creates a new instance of <see cref="RazorTextWriter"/>.
-        /// </summary>
-        /// <param name="unbufferedWriter">The <see cref="TextWriter"/> to write output to when this instance
-        /// is no longer buffering.</param>
-        /// <param name="buffer">The <see cref="ViewBuffer"/> to buffer output to.</param>
-        /// <param name="encoder">The HTML encoder.</param>
-        public RazorTextWriter(TextWriter unbufferedWriter, ViewBuffer buffer, HtmlEncoder encoder)
-        {
-            UnbufferedWriter = unbufferedWriter;
-            Buffer = buffer;
-            HtmlEncoder = encoder;
-
-            BufferedWriter = new HtmlContentWrapperTextWriter(buffer, unbufferedWriter.Encoding);
-            TargetWriter = BufferedWriter;
-        }
-
-        /// <inheritdoc />
-        public override Encoding Encoding
-        {
-            get { return BufferedWriter.Encoding; }
-        }
-
-        /// <inheritdoc />
-        public bool IsBuffering { get; private set; } = true;
-
-        /// <summary>
-        /// Gets the buffered content.
-        /// </summary>
-        public ViewBuffer Buffer { get; }
-
-        // Internal for unit testing
-        internal HtmlContentWrapperTextWriter BufferedWriter { get; }
-
-        private TextWriter UnbufferedWriter { get; }
-
-        private TextWriter TargetWriter { get; set; }
-
-        private HtmlEncoder HtmlEncoder { get; }
-
-        /// <inheritdoc />
-        public override void Write(char value)
-        {
-            TargetWriter.Write(value);
-        }
-
-        /// <inheritdoc />
-        public override void Write(char[] buffer, int index, int count)
-        {
-            if (buffer == null)
-            {
-                throw new ArgumentNullException(nameof(buffer));
-            }
-
-            if (index < 0)
-            {
-                throw new ArgumentOutOfRangeException(nameof(index));
-            }
-
-            if (count < 0 || (index + count > buffer.Length))
-            {
-                throw new ArgumentOutOfRangeException(nameof(count));
-            }
-
-            TargetWriter.Write(buffer, index, count);
-        }
-
-        /// <inheritdoc />
-        public override void Write(string value)
-        {
-            if (!string.IsNullOrEmpty(value))
-            {
-                TargetWriter.Write(value);
-            }
-        }
-
-        /// <inheritdoc />
-        public override void Write(IHtmlContent value)
-        {
-            // Perf: We don't special case 'TargetWriter is HtmlTextWriter' here, because want to delegate to the 
-            // IHtmlContent if it wants to 'flatten' itself or not. This is an important optimization for TagHelpers.
-            value.WriteTo(TargetWriter, HtmlEncoder);
-        }
-
-        /// <inheritdoc />
-        public override Task WriteAsync(char value)
-        {
-            return TargetWriter.WriteAsync(value);
-        }
-
-        /// <inheritdoc />
-        public override Task WriteAsync(char[] buffer, int index, int count)
-        {
-            if (buffer == null)
-            {
-                throw new ArgumentNullException(nameof(buffer));
-            }
-
-            if (index < 0)
-            {
-                throw new ArgumentOutOfRangeException(nameof(index));
-            }
-            if (count < 0 || (buffer.Length - index < count))
-            {
-                throw new ArgumentOutOfRangeException(nameof(count));
-            }
-
-            return TargetWriter.WriteAsync(buffer, index, count);
-        }
-
-        /// <inheritdoc />
-        public override Task WriteAsync(string value)
-        {
-            return TargetWriter.WriteAsync(value);
-        }
-
-        /// <inheritdoc />
-        public override void WriteLine()
-        {
-            TargetWriter.WriteLine();
-        }
-
-        /// <inheritdoc />
-        public override void WriteLine(string value)
-        {
-            TargetWriter.WriteLine(value);
-        }
-
-        /// <inheritdoc />
-        public override Task WriteLineAsync(char value)
-        {
-            return TargetWriter.WriteLineAsync(value);
-        }
-
-        /// <inheritdoc />
-        public override Task WriteLineAsync(char[] value, int start, int offset)
-        {
-            return TargetWriter.WriteLineAsync(value, start, offset);
-        }
-
-        /// <inheritdoc />
-        public override Task WriteLineAsync(string value)
-        {
-            return TargetWriter.WriteLineAsync(value);
-        }
-
-        /// <inheritdoc />
-        public override Task WriteLineAsync()
-        {
-            return TargetWriter.WriteLineAsync();
-        }
-
-        /// <summary>
-        /// Copies the buffered content to the unbuffered writer and invokes flush on it.
-        /// Additionally causes this instance to no longer buffer and direct all write operations
-        /// to the unbuffered writer.
-        /// </summary>
-        public override void Flush()
-        {
-            if (IsBuffering)
-            {
-                IsBuffering = false;
-                TargetWriter = UnbufferedWriter;
-                Buffer.WriteTo(UnbufferedWriter, HtmlEncoder);
-            }
-
-            UnbufferedWriter.Flush();
-        }
-
-        /// <summary>
-        /// Copies the buffered content to the unbuffered writer and invokes flush on it.
-        /// Additionally causes this instance to no longer buffer and direct all write operations
-        /// to the unbuffered writer.
-        /// </summary>
-        /// <returns>A <see cref="Task"/> that represents the asynchronous copy and flush operations.</returns>
-        public override Task FlushAsync()
-        {
-            if (IsBuffering)
-            {
-                IsBuffering = false;
-                TargetWriter = UnbufferedWriter;
-                Buffer.WriteTo(UnbufferedWriter, HtmlEncoder);
-            }
-
-            return UnbufferedWriter.FlushAsync();
-        }
-    }
-}

src/Microsoft.AspNetCore.Mvc.Razor/RazorView.cs

@@ -2,7 +2,6 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
-using System.Buffers;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
@@ -103,20 +102,35 @@ namespace Microsoft.AspNetCore.Mvc.Razor
             await RenderLayoutAsync(context, bodyWriter);
         }
 
-        private async Task<RazorTextWriter> RenderPageAsync(
+        private async Task<ViewBufferTextWriter> RenderPageAsync(
             IRazorPage page,
             ViewContext context,
             bool invokeViewStarts)
         {
-            Debug.Assert(_bufferScope != null);
+            var writer = context.Writer as ViewBufferTextWriter;
-            var buffer = new ViewBuffer(_bufferScope, page.Path);
+            if (writer == null)
-            var razorTextWriter = new RazorTextWriter(context.Writer, buffer, _htmlEncoder);
+            {
+                Debug.Assert(_bufferScope != null);
+
+                // If we get here, this is likely the top-level page (not a partial) - this means
+                // that context.Writer is wrapping the output stream. We need to buffer, so create a buffered writer.
+                var buffer = new ViewBuffer(_bufferScope, page.Path, ViewBuffer.ViewPageSize);
+                writer = new ViewBufferTextWriter(buffer, context.Writer.Encoding, _htmlEncoder, context.Writer);
+            }
+            else
+            {
+                // This means we're writing something like a partial, where the output needs to be buffered.
+                // Create a new buffer, but without the ability to flush.
+                var buffer = new ViewBuffer(_bufferScope, page.Path, ViewBuffer.ViewPageSize);
+                writer = new ViewBufferTextWriter(buffer, context.Writer.Encoding);
+            }
 
             // The writer for the body is passed through the ViewContext, allowing things like HtmlHelpers
             // and ViewComponents to reference it.
             var oldWriter = context.Writer;
             var oldFilePath = context.ExecutingFilePath;
-            context.Writer = razorTextWriter;
+
+            context.Writer = writer;
             context.ExecutingFilePath = page.Path;
 
             try
@@ -128,13 +142,12 @@ namespace Microsoft.AspNetCore.Mvc.Razor
                 }
 
                 await RenderPageCoreAsync(page, context);
-                return razorTextWriter;
+                return writer;
             }
             finally
             {
                 context.Writer = oldWriter;
                 context.ExecutingFilePath = oldFilePath;
-                razorTextWriter.Dispose();
             }
         }
 
@@ -184,12 +197,16 @@ namespace Microsoft.AspNetCore.Mvc.Razor
 
         private async Task RenderLayoutAsync(
             ViewContext context,
-            RazorTextWriter bodyWriter)
+            ViewBufferTextWriter bodyWriter)
         {
             // A layout page can specify another layout page. We'll need to continue
             // looking for layout pages until they're no longer specified.
             var previousPage = RazorPage;
             var renderedLayouts = new List<IRazorPage>();
+
+            // This loop will execute Layout pages from the inside to the outside. With each
+            // iteration, bodyWriter is replaced with the aggregate of all the "body" content
+            // (including the layout page we just rendered).
             while (!string.IsNullOrEmpty(previousPage.Layout))
             {
                 if (!bodyWriter.IsBuffering)
@@ -225,6 +242,8 @@ namespace Microsoft.AspNetCore.Mvc.Razor
                 previousPage = layoutPage;
             }
 
+            // Now we've reached and rendered the outer-most layout page. Nothing left to execute.
+
             // Ensure all defined sections were rendered or RenderBody was invoked for page without defined sections.
             foreach (var layoutPage in renderedLayouts)
             {
@@ -233,10 +252,24 @@ namespace Microsoft.AspNetCore.Mvc.Razor
 
             if (bodyWriter.IsBuffering)
             {
-                // Only copy buffered content to the Output if we're currently buffering.
+                // If IsBuffering - then we've got a bunch of content in the view buffer. How to best deal with it
-                using (var writer = _bufferScope.CreateWriter(context.Writer))
+                // really depends on whether or not we're writing directly to the output or if we're writing to
+                // another buffer.
+                var viewBufferTextWriter = context.Writer as ViewBufferTextWriter;
+                if (viewBufferTextWriter == null || !viewBufferTextWriter.IsBuffering)
                 {
-                    await bodyWriter.Buffer.WriteToAsync(writer, _htmlEncoder);
+                    // This means we're writing to a 'real' writer, probably to the actual output stream.
+                    // We're using PagedBufferedTextWriter here to 'smooth' synchronous writes of IHtmlContent values.
+                    using (var writer = _bufferScope.CreateWriter(context.Writer))
+                    {
+                        await bodyWriter.Buffer.WriteToAsync(writer, _htmlEncoder);
+                    }
+                }
+                else
+                {
+                    // This means we're writing to another buffer. Use MoveTo to combine them.
+                    bodyWriter.Buffer.MoveTo(viewBufferTextWriter.Buffer);
+                    return;
                 }
             }
         }

src/Microsoft.AspNetCore.Mvc.Razor/TagHelpers/UrlResolutionTagHelper.cs

@@ -354,16 +354,8 @@ namespace Microsoft.AspNetCore.Mvc.Razor.TagHelpers
 
             public void WriteTo(TextWriter writer, HtmlEncoder encoder)
             {
-                var htmlTextWriter = writer as HtmlTextWriter;
+                encoder.Encode(writer, _firstSegment, 0, _firstSegmentLength);
-                if (htmlTextWriter != null)
+                writer.Write(_secondSegment);
-                {
-                    htmlTextWriter.Write(this);
-                }
-                else
-                {
-                    encoder.Encode(writer, _firstSegment, 0, _firstSegmentLength);
-                    writer.Write(_secondSegment);
-                }
             }
         }
     }

src/Microsoft.AspNetCore.Mvc.TagHelpers/CacheTagHelper.cs

@@ -400,13 +400,6 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
 
             public void WriteTo(TextWriter writer, HtmlEncoder encoder)
             {
-                var htmlTextWriter = writer as HtmlTextWriter;
-                if (htmlTextWriter != null)
-                {
-                    htmlTextWriter.Write(this);
-                    return;
-                }
-
                 for (var i = 0; i < _builder.Length; i++)
                 {
                     writer.Write(_builder[i]);

src/Microsoft.AspNetCore.Mvc.ViewFeatures/Internal/HtmlContentWrapperTextWriter.cs

@@ -1,176 +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.Text;
-using System.Threading.Tasks;
-using Microsoft.AspNetCore.Html;
-using Microsoft.AspNetCore.Mvc.Internal;
-
-namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
-{
-    /// <summary>
-    /// <see cref="HtmlTextWriter"/> implementation which writes to an <see cref="IHtmlContentBuilder"/> instance.
-    /// </summary>
-    public class HtmlContentWrapperTextWriter : HtmlTextWriter
-    {
-        private const int MaxCharToStringLength = 1024;
-
-        /// <summary>
-        /// Initializes a new instance of the <see cref="HtmlContentWrapperTextWriter"/> class.
-        /// </summary>
-        /// <param name="contentBuilder">The <see cref="IHtmlContentBuilder"/> to write to.</param>
-        /// <param name="encoding">The <see cref="System.Text.Encoding"/> in which output is written.</param>
-        public HtmlContentWrapperTextWriter(IHtmlContentBuilder contentBuilder, Encoding encoding)
-        {
-            if (contentBuilder == null)
-            {
-                throw new ArgumentNullException(nameof(contentBuilder));
-            }
-
-            if (encoding == null)
-            {
-                throw new ArgumentNullException(nameof(encoding));
-            }
-
-            ContentBuilder = contentBuilder;
-            Encoding = encoding;
-        }
-
-        /// <summary>
-        /// The <see cref="IHtmlContentBuilder"/> this <see cref="HtmlContentWrapperTextWriter"/> writes to.
-        /// </summary>
-        public IHtmlContentBuilder ContentBuilder { get; }
-
-        /// <inheritdoc />
-        public override Encoding Encoding { get; }
-
-        /// <inheritdoc />
-        public override void Write(char value)
-        {
-            Write(value.ToString());
-        }
-
-        /// <inheritdoc />
-        public override void Write(char[] buffer, int index, int count)
-        {
-            if (buffer == null)
-            {
-                throw new ArgumentNullException(nameof(buffer));
-            }
-
-            if (index < 0)
-            {
-                throw new ArgumentOutOfRangeException(nameof(index));
-            }
-
-            if (count < 0 || (index + count > buffer.Length))
-            {
-                throw new ArgumentOutOfRangeException(nameof(count));
-            }
-
-            while (count > 0)
-            {
-                // Split large char arrays into 1KB strings.
-                var currentCount = count;
-                if (MaxCharToStringLength < currentCount)
-                {
-                    currentCount = MaxCharToStringLength;
-                }
-
-                Write(new string(buffer, index, currentCount));
-                index += currentCount;
-                count -= currentCount;
-            }
-        }
-
-        /// <inheritdoc />
-        public override void Write(string value)
-        {
-            if (string.IsNullOrEmpty(value))
-            {
-                return;
-            }
-
-            ContentBuilder.Append(value);
-        }
-
-        /// <inheritdoc />
-        public override void Write(IHtmlContent value)
-        {
-            if (value == null)
-            {
-                throw new ArgumentNullException(nameof(value));
-            }
-
-            ContentBuilder.AppendHtml(value);
-        }
-
-        /// <inheritdoc />
-        public override Task WriteAsync(char value)
-        {
-            Write(value.ToString());
-            return TaskCache.CompletedTask;
-        }
-
-        /// <inheritdoc />
-        public override Task WriteAsync(char[] buffer, int index, int count)
-        {
-            if (buffer == null)
-            {
-                throw new ArgumentNullException(nameof(buffer));
-            }
-
-            Write(buffer, index, count);
-            return TaskCache.CompletedTask;
-        }
-
-        /// <inheritdoc />
-        public override Task WriteAsync(string value)
-        {
-            Write(value);
-            return TaskCache.CompletedTask;
-        }
-
-        /// <inheritdoc />
-        public override void WriteLine()
-        {
-            Write(Environment.NewLine);
-        }
-
-        /// <inheritdoc />
-        public override void WriteLine(string value)
-        {
-            Write(value);
-            WriteLine();
-        }
-
-        /// <inheritdoc />
-        public override Task WriteLineAsync(char value)
-        {
-            WriteLine(value);
-            return TaskCache.CompletedTask;
-        }
-
-        /// <inheritdoc />
-        public override Task WriteLineAsync(char[] value, int start, int offset)
-        {
-            WriteLine(value, start, offset);
-            return TaskCache.CompletedTask;
-        }
-
-        /// <inheritdoc />
-        public override Task WriteLineAsync(string value)
-        {
-            WriteLine(value);
-            return TaskCache.CompletedTask;
-        }
-
-        /// <inheritdoc />
-        public override Task WriteLineAsync()
-        {
-            WriteLine();
-            return TaskCache.CompletedTask;
-        }
-    }
-}

src/Microsoft.AspNetCore.Mvc.ViewFeatures/Internal/IViewBufferScope.cs

@@ -13,8 +13,9 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
         /// <summary>
         /// Gets a <see cref="T:ViewBufferValue[]"/>.
         /// </summary>
+        /// <param name="pageSize">The minimum size of the segment.</param>
         /// <returns>The <see cref="T:ViewBufferValue[]"/>.</returns>
-        ViewBufferValue[] GetSegment();
+        ViewBufferValue[] GetPage(int pageSize);
 
         /// <summary>
         /// Returns a <see cref="T:ViewBufferValue[]"/> that can be reused.
@@ -23,11 +24,11 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
         void ReturnSegment(ViewBufferValue[] segment);
 
         /// <summary>
-        /// Creates a <see cref="ViewBufferTextWriter"/> that will delegate to the provided
+        /// Creates a <see cref="PagedBufferedTextWriter"/> that will delegate to the provided
         /// <paramref name="writer"/>.
         /// </summary>
         /// <param name="writer">The <see cref="TextWriter"/>.</param>
-        /// <returns>A <see cref="ViewBufferTextWriter"/>.</returns>
+        /// <returns>A <see cref="PagedBufferedTextWriter"/>.</returns>
-        ViewBufferTextWriter CreateWriter(TextWriter writer);
+        PagedBufferedTextWriter CreateWriter(TextWriter writer);
     }
 }

src/Microsoft.AspNetCore.Mvc.ViewFeatures/Internal/MemoryPoolViewBufferScope.cs

@@ -13,7 +13,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
     /// </summary>
     public class MemoryPoolViewBufferScope : IViewBufferScope, IDisposable
     {
-        public static readonly int SegmentSize = 512;
+        public static readonly int MinimumSize = 16;
         private readonly ArrayPool<ViewBufferValue> _viewBufferPool;
         private readonly ArrayPool<char> _charPool;
         private List<ViewBufferValue[]> _available;
@@ -27,7 +27,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
         /// The <see cref="ArrayPool{ViewBufferValue}"/> for creating <see cref="ViewBufferValue"/> instances.
         /// </param>
         /// <param name="charPool">
-        /// The <see cref="ArrayPool{Char}"/> for creating <see cref="ViewBufferTextWriter"/> instances.
+        /// The <see cref="ArrayPool{Char}"/> for creating <see cref="PagedBufferedTextWriter"/> instances.
         /// </param>
         public MemoryPoolViewBufferScope(ArrayPool<ViewBufferValue> viewBufferPool, ArrayPool<char> charPool)
         {
@@ -36,8 +36,13 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
         }
 
         /// <inheritdoc />
-        public ViewBufferValue[] GetSegment()
+        public ViewBufferValue[] GetPage(int pageSize)
         {
+            if (pageSize <= 0)
+            {
+                throw new ArgumentOutOfRangeException(nameof(pageSize));
+            }
+
             if (_disposed)
             {
                 throw new ObjectDisposedException(typeof(MemoryPoolViewBufferScope).FullName);
@@ -60,7 +65,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
 
             try
             {
-                segment = _viewBufferPool.Rent(SegmentSize);
+                segment = _viewBufferPool.Rent(Math.Max(pageSize, MinimumSize));
                 _leased.Add(segment);
             }
             catch when (segment != null)
@@ -91,14 +96,14 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
         }
 
         /// <inheritdoc />
-        public ViewBufferTextWriter CreateWriter(TextWriter writer)
+        public PagedBufferedTextWriter CreateWriter(TextWriter writer)
         {
             if (writer == null)
             {
                 throw new ArgumentNullException(nameof(writer));
             }
 
-            return new ViewBufferTextWriter(_charPool, writer);
+            return new PagedBufferedTextWriter(_charPool, writer);
         }
 
         /// <inheritdoc />

src/Microsoft.AspNetCore.Mvc.ViewFeatures/Internal/PagedBufferedTextWriter.cs

@@ -0,0 +1,211 @@
+// 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.Buffers;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.IO;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
+{
+    public class PagedBufferedTextWriter : TextWriter
+    {
+        public const int PageSize = 1024;
+
+        private readonly TextWriter _inner;
+        private readonly List<char[]> _pages;
+        private readonly ArrayPool<char> _pool;
+
+        private int _currentPage;
+        private int _currentIndex; // The next 'free' character
+
+        public PagedBufferedTextWriter(ArrayPool<char> pool, TextWriter inner)
+        {
+            _pool = pool;
+            _inner = inner;
+            _pages = new List<char[]>();
+        }
+
+        public override Encoding Encoding => _inner.Encoding;
+
+        public override void Flush()
+        {
+            // Don't do anything. We'll call FlushAsync.
+        }
+
+        public override async Task FlushAsync()
+        {
+            if (_pages.Count == 0)
+            {
+                return;
+            }
+
+            for (var i = 0; i <= _currentPage; i++)
+            {
+                var page = _pages[i];
+
+                var count = i == _currentPage ? _currentIndex : page.Length;
+                if (count > 0)
+                {
+                    await _inner.WriteAsync(page, 0, count);
+                }
+            }
+
+            // Return all but one of the pages. This way if someone writes a large chunk of
+            // content, we can return those buffers and avoid holding them for the whole
+            // page's lifetime.
+            for (var i = _pages.Count - 1; i > 0; i--)
+            {
+                var page = _pages[i];
+
+                try
+                {
+                    _pages.RemoveAt(i);
+                }
+                finally
+                {
+                    _pool.Return(page);
+                }
+            }
+
+            _currentPage = 0;
+            _currentIndex = 0;
+        }
+
+        public override void Write(char value)
+        {
+            var page = GetCurrentPage();
+            page[_currentIndex++] = value;
+        }
+
+        public override void Write(char[] buffer)
+        {
+            Write(buffer, 0, buffer.Length);
+        }
+
+        public override void Write(char[] buffer, int index, int count)
+        {
+            while (count > 0)
+            {
+                var page = GetCurrentPage();
+                var copyLength = Math.Min(count, page.Length - _currentIndex);
+                Debug.Assert(copyLength > 0);
+
+                Array.Copy(
+                    buffer,
+                    index,
+                    page,
+                    _currentIndex,
+                    copyLength);
+
+                _currentIndex += copyLength;
+                index += copyLength;
+
+                count -= copyLength;
+            }
+        }
+
+        public override void Write(string value)
+        {
+            var index = 0;
+            var count = value.Length;
+
+            while (count > 0)
+            {
+                var page = GetCurrentPage();
+                var copyLength = Math.Min(count, page.Length - _currentIndex);
+                Debug.Assert(copyLength > 0);
+
+                value.CopyTo(
+                    index,
+                    page,
+                    _currentIndex,
+                    copyLength);
+
+                _currentIndex += copyLength;
+                index += copyLength;
+
+                count -= copyLength;
+            }
+        }
+
+        public override Task WriteAsync(char value)
+        {
+            return _inner.WriteAsync(value);
+        }
+
+        public override Task WriteAsync(char[] buffer, int index, int count)
+        {
+            return _inner.WriteAsync(buffer, index, count);
+        }
+
+        public override Task WriteAsync(string value)
+        {
+            return _inner.WriteAsync(value);
+        }
+
+        private char[] GetCurrentPage()
+        {
+            char[] page = null;
+            if (_pages.Count == 0)
+            {
+                Debug.Assert(_currentPage == 0);
+                Debug.Assert(_currentIndex == 0);
+
+                try
+                {
+                    page = _pool.Rent(PageSize);
+                    _pages.Add(page);
+                }
+                catch when (page != null)
+                {
+                    _pool.Return(page);
+                    throw;
+                }
+
+                return page;
+            }
+
+            Debug.Assert(_pages.Count > _currentPage);
+            page = _pages[_currentPage];
+
+            if (_currentIndex == page.Length)
+            {
+                // Current page is full.
+                _currentPage++;
+                _currentIndex = 0;
+
+                if (_pages.Count == _currentPage)
+                {
+                    try
+                    {
+                        page = _pool.Rent(PageSize);
+                        _pages.Add(page);
+                    }
+                    catch when (page != null)
+                    {
+                        _pool.Return(page);
+                        throw;
+                    }
+                }
+            }
+
+            return page;
+        }
+
+        protected override void Dispose(bool disposing)
+        {
+            base.Dispose(disposing);
+
+            for (var i = 0; i < _pages.Count; i++)
+            {
+                _pool.Return(_pages[i]);
+            }
+
+            _pages.Clear();
+        }
+    }
+}

src/Microsoft.AspNetCore.Mvc.ViewFeatures/Internal/ViewBuffer.cs

@@ -2,15 +2,12 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
-using System.Buffers;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.IO;
-using System.Text;
 using System.Text.Encodings.Web;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Html;
-using Microsoft.AspNetCore.Mvc.Rendering;
 
 namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
 {
@@ -20,23 +17,36 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
     [DebuggerDisplay("{DebuggerToString()}")]
     public class ViewBuffer : IHtmlContentBuilder
     {
+        public static readonly int PartialViewPageSize = 32;
+        public static readonly int TagHelperPageSize = 32;
+        public static readonly int ViewComponentPageSize = 32;
+        public static readonly int ViewPageSize = 256;
+
         private readonly IViewBufferScope _bufferScope;
         private readonly string _name;
+        private readonly int _pageSize;
 
         /// <summary>
         /// Initializes a new instance of <see cref="ViewBuffer"/>.
         /// </summary>
         /// <param name="bufferScope">The <see cref="IViewBufferScope"/>.</param>
         /// <param name="name">A name to identify this instance.</param>
-        public ViewBuffer(IViewBufferScope bufferScope, string name)
+        /// <param name="pageSize">The size of buffer pages.</param>
+        public ViewBuffer(IViewBufferScope bufferScope, string name, int pageSize)
         {
             if (bufferScope == null)
             {
                 throw new ArgumentNullException(nameof(bufferScope));
             }
 
+            if (pageSize <= 0)
+            {
+                throw new ArgumentOutOfRangeException(nameof(pageSize));
+            }
+
             _bufferScope = bufferScope;
             _name = name;
+            _pageSize = pageSize;
 
             Pages = new List<ViewBufferPage>();
         }
@@ -54,7 +64,9 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
                 return this;
             }
 
-            AppendValue(new ViewBufferValue(unencoded));
+            // Text that needs encoding is the uncommon case in views, which is why it
+            // creates a wrapper and pre-encoded text does not.
+            AppendValue(new ViewBufferValue(new EncodingWrapper(unencoded)));
             return this;
         }
 
@@ -66,48 +78,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
                 return this;
             }
 
-            // Perf: special case ViewBuffers so we can 'combine' them.
+            AppendValue(new ViewBufferValue(content));
-            var otherBuffer = content as ViewBuffer;
-            if (otherBuffer == null)
-            {
-                AppendValue(new ViewBufferValue(content));
-                return this;
-            }
-
-            for (var i = 0; i < otherBuffer.Pages.Count; i++)
-            {
-                var otherPage = otherBuffer.Pages[i];
-                var currentPage = Pages.Count == 0 ? null : Pages[Pages.Count - 1];
-
-                // If the other page is less or equal to than half full, let's copy it's to the current page if
-                // possible.
-                var isLessThanHalfFull = 2 * otherPage.Count <= otherPage.Capacity;
-                if (isLessThanHalfFull &&
-                    currentPage != null &&
-                    currentPage.Capacity - currentPage.Count >= otherPage.Count)
-                {
-                    // We have room, let's copy the items.
-                    Array.Copy(
-                        sourceArray: otherPage.Buffer,
-                        sourceIndex: 0,
-                        destinationArray: currentPage.Buffer,
-                        destinationIndex: currentPage.Count,
-                        length: otherPage.Count);
-
-                    currentPage.Count += otherPage.Count;
-
-                    // Now we can return this page, and it can be reused in the scope of this request.
-                    _bufferScope.ReturnSegment(otherPage.Buffer);
-                }
-                else
-                {
-                    // Otherwise, let's just take the the page from the other buffer.
-                    Pages.Add(otherPage);
-                }
-
-            }
-
-            otherBuffer.Clear();
             return this;
         }
 
@@ -119,8 +90,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
                 return this;
             }
             
-            var value = new HtmlString(encoded);
+            AppendValue(new ViewBufferValue(encoded));
-            AppendValue(new ViewBufferValue(value));
             return this;
         }
 
@@ -135,7 +105,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
             ViewBufferPage page;
             if (Pages.Count == 0)
             {
-                page = new ViewBufferPage(_bufferScope.GetSegment());
+                page = new ViewBufferPage(_bufferScope.GetPage(_pageSize));
                 Pages.Add(page);
             }
             else
@@ -143,7 +113,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
                 page = Pages[Pages.Count - 1];
                 if (page.IsFull)
                 {
-                    page = new ViewBufferPage(_bufferScope.GetSegment());
+                    page = new ViewBufferPage(_bufferScope.GetPage(_pageSize));
                     Pages.Add(page);
                 }
             }
@@ -165,15 +135,18 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
         /// <inheritdoc />
         public void WriteTo(TextWriter writer, HtmlEncoder encoder)
         {
-            if (Pages == null)
+            if (writer == null)
             {
-                return;
+                throw new ArgumentNullException(nameof(writer));
             }
 
-            var htmlTextWriter = writer as HtmlTextWriter;
+            if (encoder == null)
-            if (htmlTextWriter != null)
+            {
+                throw new ArgumentNullException(nameof(encoder));
+            }
+
+            if (Pages == null)
             {
-                htmlTextWriter.Write(this);
                 return;
             }
 
@@ -209,15 +182,18 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
         /// <returns>A <see cref="Task"/> which will complete once content has been written.</returns>
         public async Task WriteToAsync(TextWriter writer, HtmlEncoder encoder)
         {
-            if (Pages == null)
+            if (writer == null)
             {
-                return;
+                throw new ArgumentNullException(nameof(writer));
             }
 
-            var htmlTextWriter = writer as HtmlTextWriter;
+            if (encoder == null)
-            if (htmlTextWriter != null)
+            {
+                throw new ArgumentNullException(nameof(encoder));
+            }
+
+            if (Pages == null)
             {
-                htmlTextWriter.Write(this);
                 return;
             }
 
@@ -254,5 +230,153 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
         }
 
         private string DebuggerToString() => _name;
+
+        public void CopyTo(IHtmlContentBuilder destination)
+        {
+            if (destination == null)
+            {
+                throw new ArgumentNullException(nameof(destination));
+            }
+
+            if (Pages == null)
+            {
+                return;
+            }
+
+            for (var i = 0; i < Pages.Count; i++)
+            {
+                var page = Pages[i];
+                for (var j = 0; j < page.Count; j++)
+                {
+                    var value = page.Buffer[j];
+
+                    string valueAsString;
+                    IHtmlContentContainer valueAsContainer;
+                    if ((valueAsString = value.Value as string) != null)
+                    {
+                        destination.AppendHtml(valueAsString);
+                    }
+                    else if ((valueAsContainer = value.Value as IHtmlContentContainer) != null)
+                    {
+                        valueAsContainer.CopyTo(destination);
+                    }
+                    else
+                    {
+                        destination.AppendHtml((IHtmlContent)value.Value);
+                    }
+                }
+            }
+        }
+
+        public void MoveTo(IHtmlContentBuilder destination)
+        {
+            if (destination == null)
+            {
+                throw new ArgumentNullException(nameof(destination));
+            }
+
+            if (Pages == null)
+            {
+                return;
+            }
+
+            // Perf: We have an efficient implementation when the destination is another view buffer,
+            // we can just insert our pages as-is.
+            var other = destination as ViewBuffer;
+            if (other != null)
+            {
+                MoveTo(other);
+                return;
+            }
+
+            for (var i = 0; i < Pages.Count; i++)
+            {
+                var page = Pages[i];
+                for (var j = 0; j < page.Count; j++)
+                {
+                    var value = page.Buffer[j];
+
+                    string valueAsString;
+                    IHtmlContentContainer valueAsContainer;
+                    if ((valueAsString = value.Value as string) != null)
+                    {
+                        destination.AppendHtml(valueAsString);
+                    }
+                    else if ((valueAsContainer = value.Value as IHtmlContentContainer) != null)
+                    {
+                        valueAsContainer.MoveTo(destination);
+                    }
+                    else
+                    {
+                        destination.AppendHtml((IHtmlContent)value.Value);
+                    }
+                }
+            }
+
+            for (var i = 0; i < Pages.Count; i++)
+            {
+                var page = Pages[i];
+                Array.Clear(page.Buffer, 0, page.Count);
+                _bufferScope.ReturnSegment(page.Buffer);
+            }
+
+            Pages.Clear();
+        }
+
+        private void MoveTo(ViewBuffer destination)
+        {
+            for (var i = 0; i < Pages.Count; i++)
+            {
+                var page = Pages[i];
+
+                var destinationPage = destination.Pages.Count == 0 ? null : destination.Pages[destination.Pages.Count - 1];
+
+                // If the source page is less or equal to than half full, let's copy it's content to the destination
+                // page if possible.
+                var isLessThanHalfFull = 2 * page.Count <= page.Capacity;
+                if (isLessThanHalfFull &&
+                    destinationPage != null &&
+                    destinationPage.Capacity - destinationPage.Count >= page.Count)
+                {
+                    // We have room, let's copy the items.
+                    Array.Copy(
+                        sourceArray: page.Buffer,
+                        sourceIndex: 0,
+                        destinationArray: destinationPage.Buffer,
+                        destinationIndex: destinationPage.Count,
+                        length: page.Count);
+
+                    destinationPage.Count += page.Count;
+
+                    // Now we can return the source page, and it can be reused in the scope of this request.
+                    Array.Clear(page.Buffer, 0, page.Count);
+                    _bufferScope.ReturnSegment(page.Buffer);
+                    
+                }
+                else
+                {
+                    // Otherwise, let's just add the source page to the other buffer.
+                    destination.Pages.Add(page);
+                }
+
+            }
+
+            Pages.Clear();
+        }
+
+        private class EncodingWrapper : IHtmlContent
+        {
+            private readonly string _unencoded;
+
+            public EncodingWrapper(string unencoded)
+            {
+                _unencoded = unencoded;
+            }
+
+            public void WriteTo(TextWriter writer, HtmlEncoder encoder)
+            {
+                encoder.Encode(writer, _unencoded);
+            }
+        }
     }
 }

src/Microsoft.AspNetCore.Mvc.ViewFeatures/Internal/ViewBufferTextWriter.cs

@@ -1,211 +1,437 @@
-// Copyright (c) .NET Foundation. All rights reserved.
+// 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.Buffers;
-using System.Collections.Generic;
-using System.Diagnostics;
 using System.IO;
 using System.Text;
+using System.Text.Encodings.Web;
 using System.Threading.Tasks;
+using Microsoft.AspNetCore.Html;
+using Microsoft.AspNetCore.Mvc.Internal;
 
 namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
 {
+    /// <summary>
+    /// <para>
+    /// A <see cref="TextWriter"/> that is backed by a unbuffered writer (over the Response stream) and/or a 
+    /// <see cref="ViewBuffer"/>
+    /// </para>
+    /// <para>
+    /// When <c>Flush</c> or <c>FlushAsync</c> is invoked, the writer copies all content from the buffer to
+    /// the writer and switches to writing to the unbuffered writer for all further write operations.
+    /// </para>
+    /// </summary>
     public class ViewBufferTextWriter : TextWriter
     {
-        public const int PageSize = 1024;
-
         private readonly TextWriter _inner;
-        private readonly List<char[]> _pages;
+        private readonly HtmlEncoder _htmlEncoder;
-        private readonly ArrayPool<char> _pool;
 
-        private int _currentPage;
+        /// <summary>
-        private int _currentIndex; // The next 'free' character
+        /// Creates a new instance of <see cref="ViewBufferTextWriter"/>.
-
+        /// </summary>
-        public ViewBufferTextWriter(ArrayPool<char> pool, TextWriter inner)
+        /// <param name="buffer">The <see cref="ViewBuffer"/> for buffered output.</param>
+        /// <param name="encoding">The <see cref="System.Text.Encoding"/>.</param>
+        public ViewBufferTextWriter(ViewBuffer buffer, Encoding encoding)
         {
-            _pool = pool;
+            if (buffer == null)
+            {
+                throw new ArgumentNullException(nameof(buffer));
+            }
+
+            if (encoding == null)
+            {
+                throw new ArgumentNullException(nameof(encoding));
+            }
+
+            Buffer = buffer;
+            Encoding = encoding;
+        }
+
+        /// <summary>
+        /// Creates a new instance of <see cref="ViewBufferTextWriter"/>.
+        /// </summary>
+        /// <param name="buffer">The <see cref="ViewBuffer"/> for buffered output.</param>
+        /// <param name="encoding">The <see cref="System.Text.Encoding"/>.</param>
+        /// <param name="htmlEncoder">The HTML encoder.</param>
+        /// <param name="inner">
+        /// The inner <see cref="TextWriter"/> to write output to when this instance is no longer buffering.
+        /// </param>
+        public ViewBufferTextWriter(ViewBuffer buffer, Encoding encoding, HtmlEncoder htmlEncoder, TextWriter inner)
+        {
+            if (buffer == null)
+            {
+                throw new ArgumentNullException(nameof(buffer));
+            }
+
+            if (encoding == null)
+            {
+                throw new ArgumentNullException(nameof(encoding));
+            }
+
+            if (htmlEncoder == null)
+            {
+                throw new ArgumentNullException(nameof(htmlEncoder));
+            }
+
+            if (inner == null)
+            {
+                throw new ArgumentNullException(nameof(inner));
+            }
+
+            Buffer = buffer;
+            Encoding = encoding;
+            _htmlEncoder = htmlEncoder;
             _inner = inner;
-            _pages = new List<char[]>();
         }
 
-        public override Encoding Encoding => _inner.Encoding;
+        /// <inheritdoc />
+        public override Encoding Encoding { get; }
 
-        public override void Flush()
+        /// <inheritdoc />
+        public bool IsBuffering { get; private set; } = true;
+
+        /// <summary>
+        /// Gets the <see cref="ViewBuffer"/>.
+        /// </summary>
+        public ViewBuffer Buffer { get; }
+
+        /// <inheritdoc />
+        public override void Write(char value)
         {
-            // Don't do anything. We'll call FlushAsync.
+            if (IsBuffering)
+            {
+                Buffer.AppendHtml(value.ToString());
+            }
+            else
+            {
+                _inner.Write(value);
+            }
         }
 
-        public override async Task FlushAsync()
+        /// <inheritdoc />
+        public override void Write(char[] buffer, int index, int count)
         {
-            if (_pages.Count == 0)
+            if (buffer == null)
+            {
+                throw new ArgumentNullException(nameof(buffer));
+            }
+
+            if (index < 0 || index >= buffer.Length)
+            {
+                throw new ArgumentOutOfRangeException(nameof(index));
+            }
+
+            if (count < 0 || (buffer.Length - index < count))
+            {
+                throw new ArgumentOutOfRangeException(nameof(count));
+            }
+
+            if (IsBuffering)
+            {
+                Buffer.AppendHtml(new string(buffer, index, count));
+            }
+            else
+            {
+                _inner.Write(buffer, index, count);
+            }
+        }
+
+        /// <inheritdoc />
+        public override void Write(string value)
+        {
+            if (string.IsNullOrEmpty(value))
             {
                 return;
             }
 
-            for (var i = 0; i <= _currentPage; i++)
+            if (IsBuffering)
             {
-                var page = _pages[i];
+                Buffer.AppendHtml(value);
-
-                var count = i == _currentPage ? _currentIndex : page.Length;
-                if (count > 0)
-                {
-                    await _inner.WriteAsync(page, 0, count);
-                }
             }
-
+            else
-            // Return all but one of the pages. This way if someone writes a large chunk of
-            // content, we can return those buffers and avoid holding them for the whole
-            // page's lifetime.
-            for (var i = _pages.Count - 1; i > 0; i--)
             {
-                var page = _pages[i];
+                _inner.Write(value);
-
-                try
-                {
-                    _pages.RemoveAt(i);
-                }
-                finally
-                {
-                    _pool.Return(page);
-                }
-            }
-
-            _currentPage = 0;
-            _currentIndex = 0;
-        }
-
-        public override void Write(char value)
-        {
-            var page = GetCurrentPage();
-            page[_currentIndex++] = value;
-        }
-
-        public override void Write(char[] buffer)
-        {
-            Write(buffer, 0, buffer.Length);
-        }
-
-        public override void Write(char[] buffer, int index, int count)
-        {
-            while (count > 0)
-            {
-                var page = GetCurrentPage();
-                var copyLength = Math.Min(count, page.Length - _currentIndex);
-                Debug.Assert(copyLength > 0);
-
-                Array.Copy(
-                    buffer,
-                    index,
-                    page,
-                    _currentIndex,
-                    copyLength);
-
-                _currentIndex += copyLength;
-                index += copyLength;
-
-                count -= copyLength;
             }
         }
 
-        public override void Write(string value)
+        /// <inheritdoc />
+        public override void Write(object value)
         {
-            var index = 0;
+            if (value == null)
-            var count = value.Length;
-
-            while (count > 0)
             {
-                var page = GetCurrentPage();
+                return;
-                var copyLength = Math.Min(count, page.Length - _currentIndex);
+            }
-                Debug.Assert(copyLength > 0);
 
-                value.CopyTo(
+            IHtmlContentContainer container;
-                    index,
+            IHtmlContent content;
-                    page,
+            if ((container = value as IHtmlContentContainer) != null)
-                    _currentIndex,
+            {
-                    copyLength);
+                Write(container);
-
+            }
-                _currentIndex += copyLength;
+            else if ((content = value as IHtmlContent) != null)
-                index += copyLength;
+            {
-
+                Write(content);
-                count -= copyLength;
+            }
+            else
+            {
+                Write(value.ToString());
             }
         }
 
+        /// <summary>
+        /// Writes an <see cref="IHtmlContent"/> value.
+        /// </summary>
+        /// <param name="value">The <see cref="IHtmlContent"/> value.</param>
+        public void Write(IHtmlContent value)
+        {
+            if (value == null)
+            {
+                return;
+            }
+
+            if (IsBuffering)
+            {
+                Buffer.AppendHtml(value);
+            }
+            else
+            {
+                value.WriteTo(_inner, _htmlEncoder);
+            }
+        }
+
+        /// <summary>
+        /// Writes an <see cref="IHtmlContentContainer"/> value.
+        /// </summary>
+        /// <param name="value">The <see cref="IHtmlContentContainer"/> value.</param>
+        public void Write(IHtmlContentContainer value)
+        {
+            if (value == null)
+            {
+                return;
+            }
+
+            if (IsBuffering)
+            {
+                value.MoveTo(Buffer);
+            }
+            else
+            {
+                value.WriteTo(_inner, _htmlEncoder);
+            }
+        }
+
+        /// <inheritdoc />
+        public override void WriteLine(object value)
+        {
+            if (value == null)
+            {
+                return;
+            }
+
+            IHtmlContentContainer container;
+            IHtmlContent content;
+            if ((container = value as IHtmlContentContainer) != null)
+            {
+                Write(container);
+                Write(NewLine);
+            }
+            else if ((content = value as IHtmlContent) != null)
+            {
+                Write(content);
+                Write(NewLine);
+            }
+            else
+            {
+                Write(value.ToString());
+                Write(NewLine);
+            }
+        }
+
+        /// <inheritdoc />
         public override Task WriteAsync(char value)
         {
-            return _inner.WriteAsync(value);
+            if (IsBuffering)
+            {
+                Buffer.AppendHtml(value.ToString());
+                return TaskCache.CompletedTask;
+            }
+            else
+            {
+                return _inner.WriteAsync(value);
+            }
         }
 
+        /// <inheritdoc />
         public override Task WriteAsync(char[] buffer, int index, int count)
         {
-            return _inner.WriteAsync(buffer, index, count);
+            if (buffer == null)
+            {
+                throw new ArgumentNullException(nameof(buffer));
+            }
+
+            if (index < 0)
+            {
+                throw new ArgumentOutOfRangeException(nameof(index));
+            }
+            if (count < 0 || (buffer.Length - index < count))
+            {
+                throw new ArgumentOutOfRangeException(nameof(count));
+            }
+
+            if (IsBuffering)
+            {
+                Buffer.AppendHtml(new string(buffer, index, count));
+                return TaskCache.CompletedTask;
+            }
+            else
+            {
+                return _inner.WriteAsync(buffer, index, count);
+            }
         }
 
+        /// <inheritdoc />
         public override Task WriteAsync(string value)
         {
-            return _inner.WriteAsync(value);
+            if (IsBuffering)
+            {
+                Buffer.AppendHtml(value);
+                return TaskCache.CompletedTask;
+            }
+            else
+            {
+                return _inner.WriteAsync(value);
+            }
         }
 
-        private char[] GetCurrentPage()
+        /// <inheritdoc />
+        public override void WriteLine()
         {
-            char[] page = null;
+            if (IsBuffering)
-            if (_pages.Count == 0)
             {
-                Debug.Assert(_currentPage == 0);
+                Buffer.AppendHtml(NewLine);
-                Debug.Assert(_currentIndex == 0);
-
-                try
-                {
-                    page = _pool.Rent(PageSize);
-                    _pages.Add(page);
-                }
-                catch when (page != null)
-                {
-                    _pool.Return(page);
-                    throw;
-                }
-
-                return page;
             }
-
+            else
-            Debug.Assert(_pages.Count > _currentPage);
-            page = _pages[_currentPage];
-
-            if (_currentIndex == page.Length)
             {
-                // Current page is full.
+                _inner.WriteLine();
-                _currentPage++;
-                _currentIndex = 0;
-
-                if (_pages.Count == _currentPage)
-                {
-                    try
-                    {
-                        page = _pool.Rent(PageSize);
-                        _pages.Add(page);
-                    }
-                    catch when (page != null)
-                    {
-                        _pool.Return(page);
-                        throw;
-                    }
-                }
             }
-
-            return page;
         }
 
-        protected override void Dispose(bool disposing)
+        /// <inheritdoc />
+        public override void WriteLine(string value)
         {
-            base.Dispose(disposing);
+            if (IsBuffering)
-
-            for (var i = 0; i < _pages.Count; i++)
             {
-                _pool.Return(_pages[i]);
+                Buffer.AppendHtml(value);
+                Buffer.AppendHtml(NewLine);
+            }
+            else
+            {
+                _inner.WriteLine(value);
+            }
+        }
+
+        /// <inheritdoc />
+        public override Task WriteLineAsync(char value)
+        {
+            if (IsBuffering)
+            {
+                Buffer.AppendHtml(value.ToString());
+                Buffer.AppendHtml(NewLine);
+                return TaskCache.CompletedTask;
+            }
+            else
+            {
+                return _inner.WriteLineAsync(value);
+            }
+        }
+
+        /// <inheritdoc />
+        public override Task WriteLineAsync(char[] value, int start, int offset)
+        {
+            if (IsBuffering)
+            {
+                Buffer.AppendHtml(new string(value, start, offset));
+                Buffer.AppendHtml(NewLine);
+                return TaskCache.CompletedTask;
+            }
+            else
+            {
+                return _inner.WriteLineAsync(value, start, offset);
+            }
+        }
+
+        /// <inheritdoc />
+        public override Task WriteLineAsync(string value)
+        {
+            if (IsBuffering)
+            {
+                Buffer.AppendHtml(value);
+                Buffer.AppendHtml(NewLine);
+                return TaskCache.CompletedTask;
+            }
+            else
+            {
+                return _inner.WriteLineAsync(value);
+            }
+        }
+
+        /// <inheritdoc />
+        public override Task WriteLineAsync()
+        {
+            if (IsBuffering)
+            {
+                Buffer.AppendHtml(NewLine);
+                return TaskCache.CompletedTask;
+            }
+            else
+            {
+                return _inner.WriteLineAsync();
+            }
+        }
+
+        /// <summary>
+        /// Copies the buffered content to the unbuffered writer and invokes flush on it.
+        /// Additionally causes this instance to no longer buffer and direct all write operations
+        /// to the unbuffered writer.
+        /// </summary>
+        public override void Flush()
+        {
+            if (_inner == null || _inner is ViewBufferTextWriter)
+            {
+                return;
             }
 
-            _pages.Clear();
+            if (IsBuffering)
+            {
+                IsBuffering = false;
+                Buffer.WriteTo(_inner, _htmlEncoder);
+                Buffer.Clear();
+            }
+
+            _inner.Flush();
+        }
+
+        /// <summary>
+        /// Copies the buffered content to the unbuffered writer and invokes flush on it.
+        /// Additionally causes this instance to no longer buffer and direct all write operations
+        /// to the unbuffered writer.
+        /// </summary>
+        /// <returns>A <see cref="Task"/> that represents the asynchronous copy and flush operations.</returns>
+        public override async Task FlushAsync()
+        {
+            if (_inner == null || _inner is ViewBufferTextWriter)
+            {
+                return;
+            }
+
+            if (IsBuffering)
+            {
+                IsBuffering = false;
+                await Buffer.WriteToAsync(_inner, _htmlEncoder);
+                Buffer.Clear();
+            }
+
+            await _inner.FlushAsync();
         }
     }
 }

src/Microsoft.AspNetCore.Mvc.ViewFeatures/Rendering/MvcForm.cs

@@ -3,9 +3,8 @@
 
 using System;
 using System.Text.Encodings.Web;
-using Microsoft.AspNetCore.Html;
 using Microsoft.AspNetCore.Mvc.ViewFeatures;
-using Microsoft.Extensions.DependencyInjection;
+using Microsoft.AspNetCore.Mvc.ViewFeatures.Internal;
 
 namespace Microsoft.AspNetCore.Mvc.Rendering
 {
@@ -15,20 +14,29 @@ namespace Microsoft.AspNetCore.Mvc.Rendering
     public class MvcForm : IDisposable
     {
         private readonly ViewContext _viewContext;
+        private readonly HtmlEncoder _htmlEncoder;
+
         private bool _disposed;
 
         /// <summary>
         /// Initializes a new instance of <see cref="MvcForm"/>.
         /// </summary>
         /// <param name="viewContext">The <see cref="ViewContext"/>.</param>
-        public MvcForm(ViewContext viewContext)
+        /// <param name="htmlEncoder">The <see cref="HtmlEncoder"/>.</param>
+        public MvcForm(ViewContext viewContext, HtmlEncoder htmlEncoder)
         {
             if (viewContext == null)
             {
                 throw new ArgumentNullException(nameof(viewContext));
             }
 
+            if (htmlEncoder == null)
+            {
+                throw new ArgumentNullException(nameof(htmlEncoder));
+            }
+
             _viewContext = viewContext;
+            _htmlEncoder = htmlEncoder;
         }
 
         /// <inheritdoc />
@@ -63,27 +71,24 @@ namespace Microsoft.AspNetCore.Mvc.Rendering
         private void RenderEndOfFormContent()
         {
             var formContext = _viewContext.FormContext;
-            if (formContext.HasEndOfFormContent)
+            if (!formContext.HasEndOfFormContent)
             {
-                var writer = _viewContext.Writer;
+                return;
-                var htmlWriter = writer as HtmlTextWriter;
+            }
-
-                HtmlEncoder htmlEncoder = null;
-                if (htmlWriter == null)
-                {
-                    htmlEncoder = _viewContext.HttpContext.RequestServices.GetRequiredService<HtmlEncoder>();
-                }
 
+            var viewBufferWriter = _viewContext.Writer as ViewBufferTextWriter;
+            if (viewBufferWriter == null)
+            {
                 foreach (var content in formContext.EndOfFormContent)
                 {
-                    if (htmlWriter == null)
+                    content.WriteTo(_viewContext.Writer, _htmlEncoder);
-                    {
+                }
-                        content.WriteTo(writer, htmlEncoder);
+            }
-                    }
+            else
-                    else
+            {
-                    {
+                foreach (var content in formContext.EndOfFormContent)
-                        htmlWriter.Write(content);
+                {
-                    }
+                    viewBufferWriter.Write(content);
                 }
             }
         }

src/Microsoft.AspNetCore.Mvc.ViewFeatures/Rendering/TagBuilder.cs

@@ -251,13 +251,14 @@ namespace Microsoft.AspNetCore.Mvc.Rendering
         /// <inheritdoc />
         public void WriteTo(TextWriter writer, HtmlEncoder encoder)
         {
-            var htmlTextWriter = writer as HtmlTextWriter;
+            if (writer == null)
-            if (htmlTextWriter != null)
             {
-                // As a perf optimization, we can buffer this output rather than writing it
+                throw new ArgumentNullException(nameof(writer));
-                // out character by character.
+            }
-                htmlTextWriter.Write(this);
+
-                return;
+            if (encoder == null)
+            {
+                throw new ArgumentNullException(nameof(encoder));
             }
 
             switch (TagRenderMode)

src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewComponents/DefaultViewComponentHelper.cs

@@ -134,8 +134,8 @@ namespace Microsoft.AspNetCore.Mvc.ViewComponents
             ViewComponentDescriptor descriptor,
             object arguments)
         {
-            var viewBuffer = new ViewBuffer(_viewBufferScope, descriptor.FullName);
+            var viewBuffer = new ViewBuffer(_viewBufferScope, descriptor.FullName, ViewBuffer.ViewComponentPageSize);
-            using (var writer = new HtmlContentWrapperTextWriter(viewBuffer, _viewContext.Writer.Encoding))
+            using (var writer = new ViewBufferTextWriter(viewBuffer, _viewContext.Writer.Encoding))
             {
                 var context = new ViewComponentContext(
                     descriptor,
@@ -152,7 +152,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewComponents
                 }
 
                 await invoker.InvokeAsync(context);
-                return writer.ContentBuilder;
+                return viewBuffer;
             }
         }
     }

src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewComponents/HtmlContentViewComponentResult.cs

@@ -46,15 +46,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewComponents
                 throw new ArgumentNullException(nameof(context));
             }
 
-            var htmlWriter = context.Writer as HtmlTextWriter;
+            context.Writer.Write(EncodedContent);
-            if (htmlWriter == null)
-            {
-                EncodedContent.WriteTo(context.Writer, context.HtmlEncoder);
-            }
-            else
-            {
-                htmlWriter.Write(EncodedContent);
-            }
         }
 
         /// <summary>

src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewFeatures/AntiforgeryExtensions.cs

@@ -57,14 +57,6 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
             // _fieldName containing almost any character.
             public void WriteTo(TextWriter writer, HtmlEncoder encoder)
             {
-                var htmlTextWriter = writer as HtmlTextWriter;
-                if (htmlTextWriter != null)
-                {
-                    // If possible, defer encoding until we're writing to the response.
-                    htmlTextWriter.Write(this);
-                    return;
-                }
-
                 writer.Write("<input name=\"");
                 encoder.Encode(writer, _fieldName);
                 writer.Write("\" type=\"hidden\" value=\"");

src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewFeatures/HtmlHelper.cs

@@ -505,11 +505,11 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
                 throw new ArgumentNullException(nameof(partialViewName));
             }
 
-            var viewBuffer = new ViewBuffer(_bufferScope, partialViewName);
+            var viewBuffer = new ViewBuffer(_bufferScope, partialViewName, ViewBuffer.PartialViewPageSize);
-            using (var writer = new HtmlContentWrapperTextWriter(viewBuffer, Encoding.UTF8))
+            using (var writer = new ViewBufferTextWriter(viewBuffer, Encoding.UTF8))
             {
                 await RenderPartialCoreAsync(partialViewName, model, viewData, writer);
-                return writer.ContentBuilder;
+                return viewBuffer;
             }
         }
 
@@ -742,7 +742,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
         /// <returns>A new <see cref="MvcForm"/> instance.</returns>
         protected virtual MvcForm CreateForm()
         {
-            return new MvcForm(ViewContext);
+            return new MvcForm(ViewContext, _htmlEncoder);
         }
 
         protected virtual IHtmlContent GenerateCheckBox(

src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewFeatures/StringHtmlContent.cs

@@ -39,17 +39,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
                 throw new ArgumentNullException(nameof(encoder));
             }
 
-            var htmlTextWriter = writer as HtmlTextWriter;
+            encoder.Encode(writer, _input);
-            if (htmlTextWriter != null)
-            {
-                // As a perf optimization, we can buffer this output rather than writing it
-                // out character by character.
-                htmlTextWriter.Write(this);
-            }
-            else
-            {
-                encoder.Encode(writer, _input);
-            }
         }
 
         private string DebuggerToString()

src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewFeatures/TemplateRenderer.cs

@@ -126,8 +126,8 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
 
                 if (viewEngineResult.Success)
                 {
-                    var viewBuffer = new ViewBuffer(_bufferScope, viewName);
+                    var viewBuffer = new ViewBuffer(_bufferScope, viewName, ViewBuffer.PartialViewPageSize);
-                    using (var writer = new HtmlContentWrapperTextWriter(viewBuffer, _viewContext.Writer.Encoding))
+                    using (var writer = new ViewBufferTextWriter(viewBuffer, _viewContext.Writer.Encoding))
                     {
                         // Forcing synchronous behavior so users don't have to await templates.
                         var view = viewEngineResult.View;
@@ -136,7 +136,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
                             var viewContext = new ViewContext(_viewContext, viewEngineResult.View, _viewData, writer);
                             var renderTask = viewEngineResult.View.RenderAsync(viewContext);
                             renderTask.GetAwaiter().GetResult();
-                            return writer.ContentBuilder;
+                            return viewBuffer;
                         }
                     }
                 }

test/Microsoft.AspNetCore.Mvc.Razor.Test/RazorPageTest.cs

@@ -5,6 +5,7 @@ using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.IO;
+using System.Text;
 using System.Text.Encodings.Web;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Html;
@@ -52,7 +53,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor
 
             // Act
             await page.ExecuteAsync();
-            var pageOutput = page.Output.ToString();
+            var pageOutput = page.RenderedContent;
 
             // Assert
             Assert.Equal(
@@ -79,7 +80,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor
 
             // Act
             await page.ExecuteAsync();
-            var pageOutput = page.Output.ToString();
+            var pageOutput = page.RenderedContent;
 
             // Assert
             Assert.Equal("HtmlEncode[[Hello Prefix]]HtmlEncode[[From Scope: ]]HtmlEncode[[Hello In Scope]]", pageOutput);
@@ -113,7 +114,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor
             await page.ExecuteAsync();
 
             // Assert
-            var pageOutput = page.Output.ToString();
+            var pageOutput = page.RenderedContent;
             Assert.Equal(
                 "HtmlEncode[[Hello Prefix]]HtmlEncode[[From Scopes: ]]HtmlEncode[[Hello In Scope Pre Nest]]" +
                 "HtmlEncode[[Hello In Scope Post Nest]]HtmlEncode[[Hello In Nested Scope]]",
@@ -224,6 +225,138 @@ namespace Microsoft.AspNetCore.Mvc.Razor
             await page.ExecuteAsync();
         }
 
+        // This is an integration test for ensuring that ViewBuffer segments used by
+        // TagHelpers can be merged back into the 'main' segment where possible.
+        [Fact]
+        public async Task TagHelperScopes_ViewBuffersCanCombine()
+        {
+            // Arrange
+            var bufferScope = new TestViewBufferScope();
+            var viewContext = CreateViewContext(bufferScope: bufferScope);
+
+            var page = CreatePage(async v =>
+            {
+                Assert.Equal(0, bufferScope.CreatedBuffers.Count);
+                v.Write("Level:0"); // Creates a 'top-level' buffer.
+                Assert.Equal(1, bufferScope.CreatedBuffers.Count);
+
+                // Run a TagHelper
+                {
+                    v.StartTagHelperWritingScope(encoder: null);
+
+                    Assert.Equal(1, bufferScope.CreatedBuffers.Count);
+                    Assert.Equal(0, bufferScope.ReturnedBuffers.Count);
+                    v.Write("Level:1-A"); // Creates a new buffer for the taghelper.
+                    Assert.Equal(2, bufferScope.CreatedBuffers.Count);
+                    Assert.Equal(0, bufferScope.ReturnedBuffers.Count);
+
+                    TagHelperContent innerContentLevel1 = null;
+                    var outputLevel1 = new TagHelperOutput("t1", new TagHelperAttributeList(), (_, encoder) =>
+                    {
+                        return Task.FromResult(innerContentLevel1);
+                    });
+
+                    innerContentLevel1 = v.EndTagHelperWritingScope();
+                    outputLevel1.Content = await outputLevel1.GetChildContentAsync();
+
+                    Assert.Equal(2, bufferScope.CreatedBuffers.Count);
+                    Assert.Equal(0, bufferScope.ReturnedBuffers.Count);
+                    v.Write(outputLevel1); // Writing the taghelper to output returns a buffer.
+                    Assert.Equal(2, bufferScope.CreatedBuffers.Count);
+                    Assert.Equal(1, bufferScope.ReturnedBuffers.Count);
+                }
+
+                Assert.Equal(2, bufferScope.CreatedBuffers.Count);
+                Assert.Equal(1, bufferScope.ReturnedBuffers.Count);
+                v.Write("Level:0"); // Already have a buffer for this scope.
+                Assert.Equal(2, bufferScope.CreatedBuffers.Count);
+                Assert.Equal(1, bufferScope.ReturnedBuffers.Count);
+
+                // Run another TagHelper
+                {
+                    v.StartTagHelperWritingScope(encoder: null);
+
+                    Assert.Equal(2, bufferScope.CreatedBuffers.Count);
+                    Assert.Equal(1, bufferScope.ReturnedBuffers.Count);
+                    v.Write("Level:1-B"); // Creates a new buffer for the taghelper.
+                    Assert.Equal(3, bufferScope.CreatedBuffers.Count);
+                    Assert.Equal(1, bufferScope.ReturnedBuffers.Count);
+
+                    TagHelperContent innerContentLevel1 = null;
+                    var outputLevel1 = new TagHelperOutput("t2", new TagHelperAttributeList(), (_, encoder) =>
+                    {
+                        return Task.FromResult(innerContentLevel1);
+                    });
+
+                    // Run a nested TagHelper
+                    {
+                        v.StartTagHelperWritingScope(encoder: null);
+
+                        Assert.Equal(3, bufferScope.CreatedBuffers.Count);
+                        Assert.Equal(1, bufferScope.ReturnedBuffers.Count);
+                        v.Write("Level:2"); // Creates a new buffer for the taghelper.
+                        Assert.Equal(4, bufferScope.CreatedBuffers.Count);
+                        Assert.Equal(1, bufferScope.ReturnedBuffers.Count);
+
+                        TagHelperContent innerContentLevel2 = null;
+                        var outputLevel2 = new TagHelperOutput("t3", new TagHelperAttributeList(), (_, encoder) =>
+                        {
+                            return Task.FromResult(innerContentLevel2);
+                        });
+
+                        innerContentLevel2 = v.EndTagHelperWritingScope();
+                        outputLevel2.Content = await outputLevel2.GetChildContentAsync();
+
+                        Assert.Equal(4, bufferScope.CreatedBuffers.Count);
+                        Assert.Equal(1, bufferScope.ReturnedBuffers.Count);
+                        v.Write(outputLevel2); // Writing the taghelper to output returns a buffer.
+                        Assert.Equal(4, bufferScope.CreatedBuffers.Count);
+                        Assert.Equal(2, bufferScope.ReturnedBuffers.Count);
+                    }
+
+                    Assert.Equal(4, bufferScope.CreatedBuffers.Count);
+                    Assert.Equal(2, bufferScope.ReturnedBuffers.Count);
+                    v.Write("Level:1-B"); // Already have a buffer for this scope.
+                    Assert.Equal(4, bufferScope.CreatedBuffers.Count);
+                    Assert.Equal(2, bufferScope.ReturnedBuffers.Count);
+
+                    innerContentLevel1 = v.EndTagHelperWritingScope();
+                    outputLevel1.Content = await outputLevel1.GetChildContentAsync();
+
+                    Assert.Equal(4, bufferScope.CreatedBuffers.Count);
+                    Assert.Equal(2, bufferScope.ReturnedBuffers.Count);
+                    v.Write(outputLevel1); // Writing the taghelper to output returns a buffer.
+                    Assert.Equal(4, bufferScope.CreatedBuffers.Count);
+                    Assert.Equal(3, bufferScope.ReturnedBuffers.Count);
+                }
+
+                Assert.Equal(4, bufferScope.CreatedBuffers.Count);
+                Assert.Equal(3, bufferScope.ReturnedBuffers.Count);
+                v.Write("Level:0"); // Already have a buffer for this scope.
+                Assert.Equal(4, bufferScope.CreatedBuffers.Count);
+                Assert.Equal(3, bufferScope.ReturnedBuffers.Count);
+
+            }, viewContext);
+
+            // Act & Assert
+            await page.ExecuteAsync();
+            Assert.Equal(
+                "HtmlEncode[[Level:0]]" +
+                "<t1>" +
+                    "HtmlEncode[[Level:1-A]]" + 
+                "</t1>" +
+                "HtmlEncode[[Level:0]]" +
+                "<t2>" +
+                    "HtmlEncode[[Level:1-B]]" +
+                    "<t3>" +
+                        "HtmlEncode[[Level:2]]" +
+                    "</t3>" +
+                    "HtmlEncode[[Level:1-B]]" +
+                "</t2>" +
+                "HtmlEncode[[Level:0]]", 
+                page.RenderedContent);
+        }
+
         [Fact]
         public async Task DefineSection_ThrowsIfSectionIsAlreadyDefined()
         {
@@ -1237,8 +1370,8 @@ namespace Microsoft.AspNetCore.Mvc.Razor
         public async Task Write_WithHtmlString_WritesValueWithoutEncoding()
         {
             // Arrange
-            var buffer = new ViewBuffer(new TestViewBufferScope(), string.Empty);
+            var buffer = new ViewBuffer(new TestViewBufferScope(), string.Empty, pageSize: 32);
-            var writer = new RazorTextWriter(TextWriter.Null, buffer, new HtmlTestEncoder());
+            var writer = new ViewBufferTextWriter(buffer, Encoding.UTF8);
 
             var page = CreatePage(p =>
             {
@@ -1250,7 +1383,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor
             await page.ExecuteAsync();
 
             // Assert
-            Assert.Equal("Hello world", HtmlContentUtilities.HtmlContentToString(writer.Buffer));
+            Assert.Equal("Hello world", HtmlContentUtilities.HtmlContentToString(buffer));
         }
 
         private static TestableRazorPage CreatePage(
@@ -1284,12 +1417,18 @@ namespace Microsoft.AspNetCore.Mvc.Razor
             return view.Object;
         }
 
-        private static ViewContext CreateViewContext(TextWriter writer = null, string viewPath = null)
+        private static ViewContext CreateViewContext(
+            TextWriter writer = null,
+            IViewBufferScope bufferScope = null,
+            string viewPath = null)
         {
-            writer = writer ?? new StringWriter();
+            bufferScope = bufferScope ?? new TestViewBufferScope();
+            var buffer = new ViewBuffer(bufferScope, viewPath ?? "TEST", 32);
+            writer = writer ?? new ViewBufferTextWriter(buffer, Encoding.UTF8);
+
             var httpContext = new DefaultHttpContext();
             var serviceProvider = new ServiceCollection()
-                .AddSingleton<IViewBufferScope, TestViewBufferScope>()
+                .AddSingleton<IViewBufferScope>(bufferScope)
                 .BuildServiceProvider();
             httpContext.RequestServices = serviceProvider;
             var actionContext = new ActionContext(
@@ -1321,8 +1460,12 @@ namespace Microsoft.AspNetCore.Mvc.Razor
             {
                 get
                 {
-                    var writer = Assert.IsType<StringWriter>(Output);
+                    var bufferedWriter = Assert.IsType<ViewBufferTextWriter>(Output);
-                    return writer.ToString();
+                    using (var stringWriter = new StringWriter())
+                    {
+                        bufferedWriter.Buffer.WriteTo(stringWriter, HtmlEncoder);
+                        return stringWriter.ToString();
+                    }
                 }
             }
 

test/Microsoft.AspNetCore.Mvc.Razor.Test/RazorViewTest.cs

@@ -54,7 +54,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor
 
             // Assert
             Assert.NotSame(expected, actual);
-            Assert.IsType<RazorTextWriter>(actual);
+            Assert.IsType<ViewBufferTextWriter>(actual);
             Assert.Equal("HtmlEncode[[Hello world]]", viewContext.Writer.ToString());
         }
 
@@ -248,7 +248,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor
             await view.RenderAsync(viewContext);
 
             // Assert
-            Assert.IsType<RazorTextWriter>(actual);
+            Assert.IsType<ViewBufferTextWriter>(actual);
             Assert.NotSame(original, actual);
         }
 

test/Microsoft.AspNetCore.Mvc.Razor.Test/SpanFactory.cs

@@ -8,7 +8,6 @@ using System.Linq;
 using Microsoft.AspNetCore.Razor;
 using Microsoft.AspNetCore.Razor.Chunks.Generators;
 using Microsoft.AspNetCore.Razor.Editor;
-using Microsoft.AspNetCore.Razor.Generator;
 using Microsoft.AspNetCore.Razor.Parser.SyntaxTree;
 using Microsoft.AspNetCore.Razor.Text;
 using Microsoft.AspNetCore.Razor.Tokenizer;

test/Microsoft.AspNetCore.Mvc.ViewFeatures.Test/Internal/HtmlContentWrapperTextWriterTest.cs

@@ -1,186 +0,0 @@
-// Copyright (c) .NET Foundation. All rights reserved.
-// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
-
-using System;
-using System.Collections.Generic;
-using System.IO;
-using System.Linq;
-using System.Text;
-using System.Text.Encodings.Web;
-using System.Threading.Tasks;
-using Microsoft.AspNetCore.Html;
-using Microsoft.AspNetCore.Mvc.Rendering;
-using Xunit;
-
-namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
-{
-    public class HtmlContentWrapperTextWriterTest
-    {
-        [Fact]
-        public async Task Write_WritesCharBuffer()
-        {
-            // Arrange
-            var input1 = new ArraySegment<char>(new char[] { 'a', 'b', 'c', 'd' }, 1, 3);
-            var input2 = new ArraySegment<char>(new char[] { 'e', 'f' }, 0, 2);
-            var input3 = new ArraySegment<char>(new char[] { 'g', 'h', 'i', 'j' }, 3, 1);
-            var buffer = new TestHtmlContentBuilder();
-            var writer = new HtmlContentWrapperTextWriter(buffer, Encoding.UTF8);
-
-            // Act
-            writer.Write(input1.Array, input1.Offset, input1.Count);
-            await writer.WriteAsync(input2.Array, input2.Offset, input2.Count);
-            await writer.WriteLineAsync(input3.Array, input3.Offset, input3.Count);
-
-            // Assert
-            Assert.Collection(buffer.Values,
-                value => Assert.Equal("bcd", value),
-                value => Assert.Equal("ef", value),
-                value => Assert.Equal("j", value),
-                value => Assert.Equal(Environment.NewLine, value));
-        }
-
-        [Fact]
-        public void Write_SplitsCharBuffer_Into1kbStrings()
-        {
-            // Arrange
-            var charArray = Enumerable.Range(0, 2050).Select(_ => 'a').ToArray();
-            var buffer = new TestHtmlContentBuilder();
-            var writer = new HtmlContentWrapperTextWriter(buffer, Encoding.UTF8);
-
-            // Act
-            writer.Write(charArray);
-
-            // Assert
-            Assert.Collection(
-                buffer.Values,
-                value => Assert.Equal(new string('a', 1024), value),
-                value => Assert.Equal(new string('a', 1024), value),
-                value => Assert.Equal("aa", value));
-        }
-
-        [Fact]
-        public void Write_HtmlContent_AddsToEntries()
-        {
-            // Arrange
-            var buffer = new TestHtmlContentBuilder();
-            var writer = new HtmlContentWrapperTextWriter(buffer, Encoding.UTF8);
-            var content = new HtmlString("Hello, world!");
-
-            // Act
-            writer.Write(content);
-
-            // Assert
-            Assert.Collection(
-                buffer.Values,
-                item => Assert.Same(content, item));
-        }
-
-        [Fact]
-        public void Write_Object_HtmlContent_AddsToEntries()
-        {
-            // Arrange
-            var buffer = new TestHtmlContentBuilder();
-            var writer = new HtmlContentWrapperTextWriter(buffer, Encoding.UTF8);
-            var content = new HtmlString("Hello, world!");
-
-            // Act
-            writer.Write((object)content);
-
-            // Assert
-            Assert.Collection(
-                buffer.Values,
-                item => Assert.Same(content, item));
-        }
-
-        [Fact]
-        public void WriteLine_Object_HtmlContent_AddsToEntries()
-        {
-            // Arrange
-            var buffer = new TestHtmlContentBuilder();
-            var writer = new HtmlContentWrapperTextWriter(buffer, Encoding.UTF8);
-            var content = new HtmlString("Hello, world!");
-
-            // Act
-            writer.WriteLine(content);
-
-            // Assert
-            Assert.Collection(
-                buffer.Values,
-                item => Assert.Same(content, item),
-                item => Assert.Equal(Environment.NewLine, item));
-        }
-
-        [Fact]
-        public async Task Write_WritesStringBuffer()
-        {
-            // Arrange
-            var newLine = Environment.NewLine;
-            var input1 = "Hello";
-            var input2 = "from";
-            var input3 = "ASP";
-            var input4 = ".Net";
-            var buffer = new TestHtmlContentBuilder();
-            var writer = new HtmlContentWrapperTextWriter(buffer, Encoding.UTF8);
-
-            // Act
-            writer.Write(input1);
-            writer.WriteLine(input2);
-            await writer.WriteAsync(input3);
-            await writer.WriteLineAsync(input4);
-
-            // Assert
-            Assert.Equal(new[] { input1, input2, newLine, input3, input4, newLine }, buffer.Values);
-        }
-
-        [Fact]
-        public void Write_HtmlContent_WritesToBuffer()
-        {
-            // Arrange
-            var buffer = new TestHtmlContentBuilder();
-            var writer = new HtmlContentWrapperTextWriter(buffer, Encoding.UTF8);
-            var content = new HtmlString("Hello, world!");
-
-            // Act
-            writer.Write(content);
-
-            // Assert
-            Assert.Collection(
-                buffer.Values,
-                item => Assert.Same(content, item));
-        }
-
-        private class TestHtmlContentBuilder : IHtmlContentBuilder
-        {
-            public List<object> Values { get; } = new List<object>();
-
-            public IHtmlContentBuilder Append(string unencoded)
-            {
-                Values.Add(unencoded);
-                return this;
-            }
-
-            public IHtmlContentBuilder AppendHtml(IHtmlContent content)
-            {
-                Values.Add(content);
-                return this;
-            }
-
-            public IHtmlContentBuilder AppendHtml(string encoded)
-            {
-                Values.Add(new HtmlString(encoded));
-                return this;
-            }
-
-            public IHtmlContentBuilder Clear()
-            {
-                Values.Clear();
-                return this;
-            }
-
-            public void WriteTo(TextWriter writer, HtmlEncoder encoder)
-            {
-                throw new NotSupportedException();
-            }
-        }
-    }
-}

test/Microsoft.AspNetCore.Mvc.ViewFeatures.Test/Internal/PagedBufferedStringWriterTest.cs

@@ -9,13 +9,13 @@ using Xunit;
 
 namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
 {
-    public class ViewBufferTextWriterTest
+    public class PagedBufferedStringWriterTest
     {
         private static readonly char[] Content;
 
-        static ViewBufferTextWriterTest()
+        static PagedBufferedStringWriterTest()
         {
-            Content = new char[4 * ViewBufferTextWriter.PageSize];
+            Content = new char[4 * PagedBufferedTextWriter.PageSize];
             for (var i = 0; i < Content.Length; i++)
             {
                 Content[i] = (char)((i % 26) + 'A');
@@ -29,7 +29,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
             var pool = new TestArrayPool();
             var inner = new StringWriter();
 
-            var writer = new ViewBufferTextWriter(pool, inner);
+            var writer = new PagedBufferedTextWriter(pool, inner);
 
             // Act
             for (var i = 0; i < Content.Length; i++)
@@ -50,7 +50,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
             var pool = new TestArrayPool();
             var inner = new StringWriter();
 
-            var writer = new ViewBufferTextWriter(pool, inner);
+            var writer = new PagedBufferedTextWriter(pool, inner);
 
             // These numbers chosen to hit boundary conditions in buffer lengths
             Assert.Equal(4096, Content.Length); // Update these numbers if this changes.
@@ -89,7 +89,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
             var pool = new TestArrayPool();
             var inner = new StringWriter();
 
-            var writer = new ViewBufferTextWriter(pool, inner);
+            var writer = new PagedBufferedTextWriter(pool, inner);
 
             // These numbers chosen to hit boundary conditions in buffer lengths
             Assert.Equal(4096, Content.Length); // Update these numbers if this changes.
@@ -116,7 +116,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
             var pool = new TestArrayPool();
             var inner = new StringWriter();
 
-            var writer = new ViewBufferTextWriter(pool, inner);
+            var writer = new PagedBufferedTextWriter(pool, inner);
 
             // These numbers chosen to hit boundary conditions in buffer lengths
             Assert.Equal(4096, Content.Length); // Update these numbers if this changes.
@@ -144,7 +144,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
             var pool = new TestArrayPool();
             var inner = new StringWriter();
 
-            var writer = new ViewBufferTextWriter(pool, inner);
+            var writer = new PagedBufferedTextWriter(pool, inner);
 
             for (var i = 0; i < Content.Length; i++)
             {

test/Microsoft.AspNetCore.Mvc.ViewFeatures.Test/Internal/TestViewBufferScope.cs

@@ -9,26 +9,25 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
 {
     public class TestViewBufferScope : IViewBufferScope
     {
-        public const int DefaultBufferSize = 128;
+        public IList<ViewBufferValue[]> CreatedBuffers { get; } = new List<ViewBufferValue[]>();
-        private readonly int _bufferSize;
-
-        public TestViewBufferScope(int bufferSize = DefaultBufferSize)
-        {
-            _bufferSize = bufferSize;
-        }
 
         public IList<ViewBufferValue[]> ReturnedBuffers { get; } = new List<ViewBufferValue[]>();
 
-        public ViewBufferValue[] GetSegment() => new ViewBufferValue[_bufferSize];
+        public ViewBufferValue[] GetPage(int size)
+        {
+            var buffer = new ViewBufferValue[size];
+            CreatedBuffers.Add(buffer);
+            return buffer;
+        }
 
         public void ReturnSegment(ViewBufferValue[] segment)
         {
             ReturnedBuffers.Add(segment);
         }
 
-        public ViewBufferTextWriter CreateWriter(TextWriter writer)
+        public PagedBufferedTextWriter CreateWriter(TextWriter writer)
         {
-            return new ViewBufferTextWriter(ArrayPool<char>.Shared, writer);
+            return new PagedBufferedTextWriter(ArrayPool<char>.Shared, writer);
         }
     }
 }

test/Microsoft.AspNetCore.Mvc.ViewFeatures.Test/Internal/ViewBufferTest.cs

@@ -16,10 +16,10 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
     public class ViewBufferTest
     {
         [Fact]
-        public void Append_AddsStringRazorValue()
+        public void Append_AddsEncodingWrapper()
         {
             // Arrange
-            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name");
+            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name", pageSize: 32);
 
             // Act
             buffer.Append("Hello world");
@@ -27,14 +27,14 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
             // Assert
             var page = Assert.Single(buffer.Pages);
             Assert.Equal(1, page.Count);
-            Assert.Equal("Hello world", page.Buffer[0].Value);
+            Assert.IsAssignableFrom<IHtmlContent>(page.Buffer[0].Value);
         }
 
         [Fact]
-        public void Append_AddsHtmlContentRazorValue()
+        public void AppendHtml_AddsHtmlContentRazorValue()
         {
             // Arrange
-            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name");
+            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name", pageSize: 32);
             var content = new HtmlString("hello-world");
 
             // Act
@@ -47,10 +47,10 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
         }
 
         [Fact]
-        public void AppendHtml_AddsHtmlStringValues()
+        public void AppendHtml_AddsString()
         {
             // Arrange
-            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name");
+            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name", pageSize: 32);
             var value = "Hello world";
 
             // Act
@@ -59,24 +59,23 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
             // Assert
             var page = Assert.Single(buffer.Pages);
             Assert.Equal(1, page.Count);
-            var htmlString = Assert.IsType<HtmlString>(page.Buffer[0].Value);
+            Assert.Equal("Hello world", Assert.IsType<string>(page.Buffer[0].Value));
-            Assert.Equal("Hello world", htmlString.ToString());
         }
 
         [Fact]
         public void Append_CreatesNewPages_WhenCurrentPageIsFull()
         {
             // Arrange
-            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name");
+            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name", pageSize: 32);
-            var expected = Enumerable.Range(0, TestViewBufferScope.DefaultBufferSize).Select(i => i.ToString());
+            var expected = Enumerable.Range(0, 32).Select(i => i.ToString());
 
             // Act
             foreach (var item in expected)
             {
-                buffer.Append(item);
+                buffer.AppendHtml(item);
             }
-            buffer.Append("Hello");
+            buffer.AppendHtml("Hello");
-            buffer.Append("world");
+            buffer.AppendHtml("world");
 
             // Assert
             Assert.Equal(2, buffer.Pages.Count);
@@ -92,19 +91,19 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
 
         [Theory]
         [InlineData(1)]
-        [InlineData(TestViewBufferScope.DefaultBufferSize + 3)]
+        [InlineData(35)]
         public void Clear_ResetsBackingBufferAndIndex(int valuesToWrite)
         {
             // Arrange
-            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name");
+            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name", pageSize: 32);
 
             // Act
             for (var i = 0; i < valuesToWrite; i++)
             {
-                buffer.Append("Hello");
+                buffer.AppendHtml("Hello");
             }
             buffer.Clear();
-            buffer.Append("world");
+            buffer.AppendHtml("world");
 
             // Assert
             var page = Assert.Single(buffer.Pages);
@@ -112,27 +111,11 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
             Assert.Equal("world", page.Buffer[0].Value);
         }
 
-        [Fact]
-        public void WriteTo_WritesSelf_WhenWriterIsHtmlTextWriter()
-        {
-            // Arrange
-            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name");
-            var htmlWriter = new Mock<HtmlTextWriter>();
-            htmlWriter.Setup(w => w.Write(buffer)).Verifiable();
-
-            // Act
-            buffer.Append("Hello world");
-            buffer.WriteTo(htmlWriter.Object, new HtmlTestEncoder());
-
-            // Assert
-            htmlWriter.Verify();
-        }
-
         [Fact]
         public void WriteTo_WritesRazorValues_ToTextWriter()
         {
             // Arrange
-            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name");
+            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name", pageSize: 32);
             var writer = new StringWriter();
 
             // Act
@@ -142,7 +125,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
             buffer.WriteTo(writer, new HtmlTestEncoder());
 
             // Assert
-            Assert.Equal("Hello world 123", writer.ToString());
+            Assert.Equal("HtmlEncode[[Hello]] world 123", writer.ToString());
         }
 
         [Theory]
@@ -153,7 +136,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
         public void WriteTo_WritesRazorValuesFromAllBuffers(int valuesToWrite)
         {
             // Arrange
-            var buffer = new ViewBuffer(new TestViewBufferScope(4), "some-name");
+            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name", pageSize: 4);
             var writer = new StringWriter();
             var expected = string.Join("", Enumerable.Range(0, valuesToWrite).Select(_ => "abc"));
 
@@ -168,27 +151,11 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
             Assert.Equal(expected, writer.ToString());
         }
 
-        [Fact]
-        public async Task WriteToAsync_WritesSelf_WhenWriterIsHtmlTextWriter()
-        {
-            // Arrange
-            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name");
-            var htmlWriter = new Mock<HtmlTextWriter>();
-            htmlWriter.Setup(w => w.Write(buffer)).Verifiable();
-
-            // Act
-            buffer.Append("Hello world");
-            await buffer.WriteToAsync(htmlWriter.Object, new HtmlTestEncoder());
-
-            // Assert
-            htmlWriter.Verify();
-        }
-
         [Fact]
         public async Task WriteToAsync_WritesRazorValues_ToTextWriter()
         {
             // Arrange
-            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name");
+            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name", pageSize: 128);
             var writer = new StringWriter();
 
             // Act
@@ -199,7 +166,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
             await buffer.WriteToAsync(writer, new HtmlTestEncoder());
 
             // Assert
-            Assert.Equal("Hello world 123", writer.ToString());
+            Assert.Equal("HtmlEncode[[Hello]] world 123", writer.ToString());
         }
 
         [Theory]
@@ -210,7 +177,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
         public async Task WriteToAsync_WritesRazorValuesFromAllBuffers(int valuesToWrite)
         {
             // Arrange
-            var buffer = new ViewBuffer(new TestViewBufferScope(4), "some-name");
+            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name", pageSize: 4);
             var writer = new StringWriter();
             var expected = string.Join("", Enumerable.Range(0, valuesToWrite).Select(_ => "abc"));
 
@@ -227,20 +194,66 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
         }
 
         [Fact]
-        public void AppendHtml_ViewBuffer_TakesPage_IfOriginalIsEmpty()
+        public void CopyTo_Flattens()
         {
             // Arrange
-            var scope = new TestViewBufferScope(4);
+            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name", pageSize: 4);
 
-            var original = new ViewBuffer(scope, "original");
+            var nestedItems = new List<object>();
-            var other = new ViewBuffer(scope, "other");
+            var nested = new HtmlContentBuilder(nestedItems);
+            nested.AppendHtml("Hello");
+            buffer.AppendHtml(nested);
 
-            other.Append("Hi");
+            var destinationItems = new List<object>();
+            var destination = new HtmlContentBuilder(destinationItems);
+
+            // Act
+            buffer.CopyTo(destination);
+
+            // Assert
+            Assert.Same(nested, buffer.Pages[0].Buffer[0].Value);
+            Assert.Equal("Hello", Assert.IsType<HtmlEncodedString>(nestedItems[0]).Value);
+            Assert.Equal("Hello", Assert.IsType<HtmlEncodedString>(destinationItems[0]).Value);
+        }
+
+        [Fact]
+        public void MoveTo_FlattensAndClears()
+        {
+            // Arrange
+            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name", pageSize: 4);
+
+            var nestedItems = new List<object>();
+            var nested = new HtmlContentBuilder(nestedItems);
+            nested.AppendHtml("Hello");
+            buffer.AppendHtml(nested);
+
+            var destinationItems = new List<object>();
+            var destination = new HtmlContentBuilder(destinationItems);
+
+            // Act
+            buffer.MoveTo(destination);
+
+            // Assert
+            Assert.Empty(nestedItems);
+            Assert.Empty(buffer.Pages);
+            Assert.Equal("Hello", Assert.IsType<HtmlEncodedString>(destinationItems[0]).Value);
+        }
+
+        [Fact]
+        public void MoveTo_ViewBuffer_TakesPage_IfOriginalIsEmpty()
+        {
+            // Arrange
+            var scope = new TestViewBufferScope();
+
+            var original = new ViewBuffer(scope, "original", pageSize: 4);
+            var other = new ViewBuffer(scope, "other", pageSize: 4);
+
+            other.AppendHtml("Hi");
 
             var page = other.Pages[0];
 
             // Act
-            original.AppendHtml(other);
+            other.MoveTo(original);
 
             // Assert
             Assert.Empty(other.Pages); // Page was taken
@@ -248,25 +261,25 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
         }
 
         [Fact]
-        public void AppendHtml_ViewBuffer_TakesPage_IfCurrentPageInOriginalIsFull()
+        public void MoveTo_ViewBuffer_TakesPage_IfCurrentPageInOriginalIsFull()
         {
             // Arrange
-            var scope = new TestViewBufferScope(4);
+            var scope = new TestViewBufferScope();
 
-            var original = new ViewBuffer(scope, "original");
+            var original = new ViewBuffer(scope, "original", pageSize: 4);
-            var other = new ViewBuffer(scope, "other");
+            var other = new ViewBuffer(scope, "other", pageSize: 4);
 
             for (var i = 0; i < 4; i++)
             {
-                original.Append($"original-{i}");
+                original.AppendHtml($"original-{i}");
             }
 
-            other.Append("Hi");
+            other.AppendHtml("Hi");
 
             var page = other.Pages[0];
 
             // Act
-            original.AppendHtml(other);
+            other.MoveTo(original);
 
             // Assert
             Assert.Empty(other.Pages); // Page was taken
@@ -275,30 +288,30 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
         }
 
         [Fact]
-        public void AppendHtml_ViewBuffer_TakesPage_IfCurrentPageDoesNotHaveCapacity()
+        public void MoveTo_ViewBuffer_TakesPage_IfCurrentPageDoesNotHaveCapacity()
         {
             // Arrange
-            var scope = new TestViewBufferScope(4);
+            var scope = new TestViewBufferScope();
 
-            var original = new ViewBuffer(scope, "original");
+            var original = new ViewBuffer(scope, "original", pageSize: 4);
-            var other = new ViewBuffer(scope, "other");
+            var other = new ViewBuffer(scope, "other", pageSize: 4);
 
             for (var i = 0; i < 3; i++)
             {
-                original.Append($"original-{i}");
+                original.AppendHtml($"original-{i}");
             }
 
             // With two items, we'd try to copy the items, but there's no room in the current page.
             // So we just take over the page.
             for (var i = 0; i < 2; i++)
             {
-                other.Append($"other-{i}");
+                other.AppendHtml($"other-{i}");
             }
 
             var page = other.Pages[0];
 
             // Act
-            original.AppendHtml(other);
+            other.MoveTo(original);
 
             // Assert
             Assert.Empty(other.Pages); // Page was taken
@@ -307,29 +320,29 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
         }
 
         [Fact]
-        public void AppendHtml_ViewBuffer_CopiesItems_IfCurrentPageHasRoom()
+        public void MoveTo_ViewBuffer_CopiesItems_IfCurrentPageHasRoom()
         {
             // Arrange
-            var scope = new TestViewBufferScope(4);
+            var scope = new TestViewBufferScope();
 
-            var original = new ViewBuffer(scope, "original");
+            var original = new ViewBuffer(scope, "original", pageSize: 4);
-            var other = new ViewBuffer(scope, "other");
+            var other = new ViewBuffer(scope, "other", pageSize: 4);
 
             for (var i = 0; i < 2; i++)
             {
-                original.Append($"original-{i}");
+                original.AppendHtml($"original-{i}");
             }
 
             // With two items, this is half full so we try to copy the items.
             for (var i = 0; i < 2; i++)
             {
-                other.Append($"other-{i}");
+                other.AppendHtml($"other-{i}");
             }
 
             var page = other.Pages[0];
 
             // Act
-            original.AppendHtml(other);
+            other.MoveTo(original);
 
             // Assert
             Assert.Empty(other.Pages); // Other is cleared
@@ -344,30 +357,30 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
         }
 
         [Fact]
-        public void AppendHtml_ViewBuffer_CanAddToTakenPage()
+        public void MoveTo_ViewBuffer_CanAddToTakenPage()
         {
             // Arrange
-            var scope = new TestViewBufferScope(4);
+            var scope = new TestViewBufferScope();
 
-            var original = new ViewBuffer(scope, "original");
+            var original = new ViewBuffer(scope, "original", pageSize: 4);
-            var other = new ViewBuffer(scope, "other");
+            var other = new ViewBuffer(scope, "other", pageSize: 4);
 
             for (var i = 0; i < 3; i++)
             {
-                original.Append($"original-{i}");
+                original.AppendHtml($"original-{i}");
             }
 
             // More than half full, so we take the page
             for (var i = 0; i < 3; i++)
             {
-                other.Append($"other-{i}");
+                other.AppendHtml($"other-{i}");
             }
 
             var page = other.Pages[0];
-            original.AppendHtml(other);
+            other.MoveTo(original);
 
             // Act
-            original.Append("after-merge");
+            original.AppendHtml("after-merge");
 
             // Assert
             Assert.Empty(other.Pages); // Other is cleared
@@ -388,28 +401,28 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
         }
 
         [Fact]
-        public void AppendHtml_ViewBuffer_MultiplePages()
+        public void MoveTo_ViewBuffer_MultiplePages()
         {
             // Arrange
-            var scope = new TestViewBufferScope(4);
+            var scope = new TestViewBufferScope();
 
-            var original = new ViewBuffer(scope, "original");
+            var original = new ViewBuffer(scope, "original", pageSize: 4);
-            var other = new ViewBuffer(scope, "other");
+            var other = new ViewBuffer(scope, "other", pageSize: 4);
 
             for (var i = 0; i < 2; i++)
             {
-                original.Append($"original-{i}");
+                original.AppendHtml($"original-{i}");
             }
             
             for (var i = 0; i < 9; i++)
             {
-                other.Append($"other-{i}");
+                other.AppendHtml($"other-{i}");
             }
 
             var pages = new List<ViewBufferPage>(other.Pages);
 
             // Act
-            original.AppendHtml(other);
+            other.MoveTo(original);
 
             // Assert
             Assert.Empty(other.Pages); // Other is cleared

test/Microsoft.AspNetCore.Mvc.ViewFeatures.Test/Internal/ViewBufferTextWriterTest.cs

@@ -8,15 +8,14 @@ using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Mvc.Rendering;
-using Microsoft.AspNetCore.Mvc.ViewFeatures.Internal;
 using Microsoft.AspNetCore.Testing;
 using Microsoft.Extensions.WebEncoders.Testing;
 using Moq;
 using Xunit;
 
-namespace Microsoft.AspNetCore.Mvc.Razor.Test
+namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
 {
-    public class RazorTextWriterTest
+    public class ViewBufferTextWriterTest
     {
         [Fact]
         [ReplaceCulture]
@@ -24,8 +23,8 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Test
         {
             // Arrange
             var expected = new object[] { "True", "3", "18446744073709551615", "Hello world", "3.14", "2.718", "m" };
-            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name");
+            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name", pageSize: 4);
-            var writer = new RazorTextWriter(TextWriter.Null, buffer, new HtmlTestEncoder());
+            var writer = new ViewBufferTextWriter(buffer, Encoding.UTF8);
 
             // Act
             writer.Write(true);
@@ -46,10 +45,9 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Test
         {
             // Arrange
             var expected = new[] { "True", "3", "18446744073709551615", "Hello world", "3.14", "2.718" };
-            var unbufferedWriter = new Mock<TextWriter>();
+            var inner = new Mock<TextWriter>();
-            unbufferedWriter.SetupGet(w => w.Encoding).Returns(Encoding.UTF8);
+            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name", pageSize: 4);
-            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name");
+            var writer = new ViewBufferTextWriter(buffer, Encoding.UTF8, new HtmlTestEncoder(), inner.Object);
-            var writer = new RazorTextWriter(unbufferedWriter.Object, buffer, new HtmlTestEncoder());
             var testClass = new TestClass();
 
             // Act
@@ -65,7 +63,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Test
             Assert.Empty(buffer.Pages);
             foreach (var item in expected)
             {
-                unbufferedWriter.Verify(v => v.Write(item), Times.Once());
+                inner.Verify(v => v.Write(item), Times.Once());
             }
         }
 
@@ -74,10 +72,9 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Test
         public async Task Write_WritesCharValues_ToUnderlyingStream_WhenNotBuffering()
         {
             // Arrange
-            var unbufferedWriter = new Mock<TextWriter> { CallBase = true };
+            var inner = new Mock<TextWriter> { CallBase = true };
-            unbufferedWriter.SetupGet(w => w.Encoding).Returns(Encoding.UTF8);
+            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name", pageSize: 4);
-            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name");
+            var writer = new ViewBufferTextWriter(buffer, Encoding.UTF8, new HtmlTestEncoder(), inner.Object);
-            var writer = new RazorTextWriter(unbufferedWriter.Object, buffer, new HtmlTestEncoder());
             var buffer1 = new[] { 'a', 'b', 'c', 'd' };
             var buffer2 = new[] { 'd', 'e', 'f' };
 
@@ -90,12 +87,12 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Test
             await writer.WriteLineAsync(buffer1);
 
             // Assert
-            unbufferedWriter.Verify(v => v.Write('x'), Times.Once());
+            inner.Verify(v => v.Write('x'), Times.Once());
-            unbufferedWriter.Verify(v => v.Write(buffer1, 1, 2), Times.Once());
+            inner.Verify(v => v.Write(buffer1, 1, 2), Times.Once());
-            unbufferedWriter.Verify(v => v.Write(buffer1, 0, 4), Times.Once());
+            inner.Verify(v => v.Write(buffer1, 0, 4), Times.Once());
-            unbufferedWriter.Verify(v => v.Write(buffer2, 0, 3), Times.Once());
+            inner.Verify(v => v.Write(buffer2, 0, 3), Times.Once());
-            unbufferedWriter.Verify(v => v.WriteAsync(buffer2, 1, 1), Times.Once());
+            inner.Verify(v => v.WriteAsync(buffer2, 1, 1), Times.Once());
-            unbufferedWriter.Verify(v => v.WriteLine(), Times.Once());
+            inner.Verify(v => v.WriteLine(), Times.Once());
         }
 
         [Fact]
@@ -103,10 +100,9 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Test
         public async Task Write_WritesStringValues_ToUnbufferedStream_WhenNotBuffering()
         {
             // Arrange
-            var unbufferedWriter = new Mock<TextWriter>();
+            var inner = new Mock<TextWriter>();
-            unbufferedWriter.SetupGet(w => w.Encoding).Returns(Encoding.UTF8);
+            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name", pageSize: 4);
-            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name");
+            var writer = new ViewBufferTextWriter(buffer, Encoding.UTF8, new HtmlTestEncoder(), inner.Object);
-            var writer = new RazorTextWriter(unbufferedWriter.Object, buffer, new HtmlTestEncoder());
 
             // Act
             await writer.FlushAsync();
@@ -116,10 +112,10 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Test
             await writer.WriteLineAsync("gh");
 
             // Assert
-            unbufferedWriter.Verify(v => v.Write("a"), Times.Once());
+            inner.Verify(v => v.Write("a"), Times.Once());
-            unbufferedWriter.Verify(v => v.WriteLine("ab"), Times.Once());
+            inner.Verify(v => v.WriteLine("ab"), Times.Once());
-            unbufferedWriter.Verify(v => v.WriteAsync("ef"), Times.Once());
+            inner.Verify(v => v.WriteAsync("ef"), Times.Once());
-            unbufferedWriter.Verify(v => v.WriteLineAsync("gh"), Times.Once());
+            inner.Verify(v => v.WriteLineAsync("gh"), Times.Once());
         }
 
         [Fact]
@@ -129,8 +125,8 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Test
             // Arrange
             var newLine = Environment.NewLine;
             var expected = new List<object> { "False", newLine, "1.1", newLine, "3", newLine };
-            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name");
+            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name", pageSize: 4);
-            var writer = new RazorTextWriter(TextWriter.Null, buffer, new HtmlTestEncoder());
+            var writer = new ViewBufferTextWriter(buffer, Encoding.UTF8);
 
             // Act
             writer.WriteLine(false);
@@ -146,10 +142,9 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Test
         public void WriteLine_WritesDataTypes_ToUnbufferedStream_WhenNotBuffering()
         {
             // Arrange
-            var unbufferedWriter = new Mock<TextWriter>();
+            var inner = new Mock<TextWriter>();
-            unbufferedWriter.SetupGet(w => w.Encoding).Returns(Encoding.UTF8);
+            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name", pageSize: 4);
-            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name");
+            var writer = new ViewBufferTextWriter(buffer, Encoding.UTF8, new HtmlTestEncoder(), inner.Object);
-            var writer = new RazorTextWriter(unbufferedWriter.Object, buffer, new HtmlTestEncoder());
 
             // Act
             writer.Flush();
@@ -158,10 +153,10 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Test
             writer.WriteLine(3L);
 
             // Assert
-            unbufferedWriter.Verify(v => v.Write("False"), Times.Once());
+            inner.Verify(v => v.Write("False"), Times.Once());
-            unbufferedWriter.Verify(v => v.Write("1.1"), Times.Once());
+            inner.Verify(v => v.Write("1.1"), Times.Once());
-            unbufferedWriter.Verify(v => v.Write("3"), Times.Once());
+            inner.Verify(v => v.Write("3"), Times.Once());
-            unbufferedWriter.Verify(v => v.WriteLine(), Times.Exactly(3));
+            inner.Verify(v => v.WriteLine(), Times.Exactly(3));
         }
 
         [Fact]
@@ -169,8 +164,8 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Test
         {
             // Arrange
             var newLine = Environment.NewLine;
-            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name");
+            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name", pageSize: 4);
-            var writer = new RazorTextWriter(TextWriter.Null, buffer, new HtmlTestEncoder());
+            var writer = new ViewBufferTextWriter(buffer, Encoding.UTF8);
 
             // Act
             writer.WriteLine();
@@ -190,8 +185,8 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Test
             var input2 = "from";
             var input3 = "ASP";
             var input4 = ".Net";
-            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name");
+            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name", pageSize: 4);
-            var writer = new RazorTextWriter(TextWriter.Null, buffer, new HtmlTestEncoder());
+            var writer = new ViewBufferTextWriter(buffer, Encoding.UTF8);
 
             // Act
             writer.Write(input1);
@@ -208,9 +203,10 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Test
         public void Write_HtmlContent_AfterFlush_GoesToStream()
         {
             // Arrange
-            var stringWriter = new StringWriter();
+            var inner = new StringWriter();
-            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name");
+            var buffer = new ViewBuffer(new TestViewBufferScope(), "some-name", pageSize: 4);
-            var writer = new RazorTextWriter(stringWriter, buffer, new HtmlTestEncoder());
+            var writer = new ViewBufferTextWriter(buffer, Encoding.UTF8, new HtmlTestEncoder(), inner);
+
             writer.Flush();
 
             var content = new HtmlString("Hello, world!");
@@ -219,7 +215,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Test
             writer.Write(content);
 
             // Assert
-            Assert.Equal("Hello, world!", stringWriter.ToString());
+            Assert.Equal("Hello, world!", inner.ToString());
         }
 
         private static object[] GetValues(ViewBuffer buffer)

test/Microsoft.AspNetCore.Mvc.ViewFeatures.Test/Rendering/ViewContextTests.cs

@@ -60,7 +60,7 @@ namespace Microsoft.AspNetCore.Mvc.Rendering
                 htmlHelperOptions: new HtmlHelperOptions());
             var view = Mock.Of<IView>();
             var viewData = new ViewDataDictionary(originalContext.ViewData);
-            var writer = new HtmlContentWrapperTextWriter(new HtmlContentBuilder(), Encoding.UTF8);
+            var writer = new StringWriter();
 
             // Act
             var context = new ViewContext(originalContext, view, viewData, writer);