Flatten TagHelperOutput when writing

This change 'flattens' a TagHelperOutput when writing it out to an
HtmlTextWriter. This is a beneficial perf change because more often than
not the thing being written to is a ViewBuffer in Razor, which uses pooled
memory. This allows us to 'join' islands of pooled ViewBuffer memory back
into the main buffer instead of keeping them wrapped up in a
TagHelperOutput or TagHelperContent.

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

@@ -283,14 +283,14 @@ namespace Microsoft.AspNetCore.Razor.TagHelpers
 
             if (!isTagNameNullOrWhitespace)
             {
-                writer.Write('<');
+                writer.Write("<");
                 writer.Write(TagName);
 
                 // Perf: Avoid allocating enumerator
                 for (var i = 0; i < Attributes.Count; i++)
                 {
                     var attribute = Attributes[i];
-                    writer.Write(' ');
+                    writer.Write(" ");
                     writer.Write(attribute.Name);
 
                     if (attribute.Minimized)
@@ -309,10 +309,9 @@ namespace Microsoft.AspNetCore.Razor.TagHelpers
                         using (var stringWriter = new StringWriter())
                         {
                             htmlContent.WriteTo(stringWriter, encoder);
+                            stringWriter.GetStringBuilder().Replace("\"", "&quot;");
 
                             var stringValue = stringWriter.ToString();
-                            stringValue = stringValue.Replace("\"", "&quot;");
-
                             writer.Write(stringValue);
                         }
                     }
@@ -321,7 +320,7 @@ namespace Microsoft.AspNetCore.Razor.TagHelpers
                         encoder.Encode(writer, value.ToString());
                     }
 
-                    writer.Write('"');
+                    writer.Write("\"");
                 }
 
                 if (TagMode == TagMode.SelfClosing)
@@ -329,7 +328,7 @@ namespace Microsoft.AspNetCore.Razor.TagHelpers
                     writer.Write(" /");
                 }
 
-                writer.Write('>');
+                writer.Write(">");
             }
 
             if (isTagNameNullOrWhitespace || TagMode == TagMode.StartTagAndEndTag)

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

@@ -4,8 +4,10 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
+using System.Text;
 using System.Text.Encodings.Web;
 using System.Threading.Tasks;
+using Microsoft.AspNetCore.Html;
 using Microsoft.AspNetCore.Razor.Runtime.TagHelpers;
 using Microsoft.Extensions.WebEncoders.Testing;
 using Xunit;
@@ -990,6 +992,34 @@ namespace Microsoft.AspNetCore.Razor.TagHelpers
             Assert.Equal(expected, writer.ToString(), StringComparer.Ordinal);
         }
 
+        // This tests a separate code path that's used by THO when the writer is an HtmlTextWriter.
+        // The output should be the same, but we do some specific perf optimizations on this path.
+        [Theory]
+        [MemberData(nameof(WriteTagHelper_InputData))]
+        public void WriteTo_WritesFormattedTagHelper_HtmlTextWriter(TagHelperOutput output, string expected)
+        {
+            // Arrange
+            var inner = new StringWriter();
+            var testEncoder = new HtmlTestEncoder();
+            var writer = new MockHtmlTextWriter(inner, testEncoder);
+
+            var tagHelperExecutionContext = new TagHelperExecutionContext(
+                tagName: output.TagName,
+                tagMode: output.TagMode,
+                items: new Dictionary<object, object>(),
+                uniqueId: string.Empty,
+                executeChildContentAsync: () => Task.FromResult(result: true),
+                startTagHelperWritingScope: _ => { },
+                endTagHelperWritingScope: () => new DefaultTagHelperContent());
+            tagHelperExecutionContext.Output = output;
+
+            // Act
+            output.WriteTo(writer, testEncoder);
+
+            // Assert
+            Assert.Equal(expected, inner.ToString(), StringComparer.Ordinal);
+        }
+
         private static TagHelperOutput GetTagHelperOutput(
             string tagName,
             TagHelperAttributeList attributes,
@@ -1036,5 +1066,29 @@ namespace Microsoft.AspNetCore.Razor.TagHelpers
 
             return output;
         }
+
+        private class MockHtmlTextWriter : HtmlTextWriter
+        {
+            private readonly HtmlEncoder _encoder;
+            private readonly TextWriter _inner;
+
+            public MockHtmlTextWriter(TextWriter inner, HtmlEncoder encoder)
+            {
+                _inner = inner;
+                _encoder = encoder;
+            }
+
+            public override Encoding Encoding => _inner.Encoding;
+
+            public override void Write(IHtmlContent value)
+            {
+                value.WriteTo(_inner, _encoder);
+            }
+
+            public override void Write(char value)
+            {
+                _inner.Write(value);
+            }
+        }
     }
 }