[Perf] Avoid ViewBuffers for writing bound TagHelper attribute values. Fixes aspnet/Razor#717

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

@@ -106,6 +106,8 @@ namespace Microsoft.AspNetCore.Mvc.Razor
                     FormatInvalidIndexerAssignmentMethodName = "InvalidTagHelperIndexerAssignment",
                     StartTagHelperWritingScopeMethodName = "StartTagHelperWritingScope",
                     EndTagHelperWritingScopeMethodName = "EndTagHelperWritingScope",
+                    BeginWriteTagHelperAttributeMethodName = "BeginWriteTagHelperAttribute",
+                    EndWriteTagHelperAttributeMethodName = "EndWriteTagHelperAttribute",
 
                     // Can't use nameof because IHtmlHelper is (also) not accessible here.
                     MarkAsHtmlEncodedMethodName = HtmlHelperPropertyName + ".Raw",

src/Microsoft.AspNetCore.Mvc.Razor/Properties/Resources.Designer.cs

@@ -478,6 +478,22 @@ namespace Microsoft.AspNetCore.Mvc.Razor
             return string.Format(CultureInfo.CurrentCulture, GetString("ViewLocationFormatsIsRequired"), p0);
         }
 
+        /// <summary>
+        /// Nesting of TagHelper attribute writing scopes is not supported.
+        /// </summary>
+        internal static string RazorPage_NestingAttributeWritingScopesNotSupported
+        {
+            get { return GetString("RazorPage_NestingAttributeWritingScopesNotSupported"); }
+        }
+
+        /// <summary>
+        /// Nesting of TagHelper attribute writing scopes is not supported.
+        /// </summary>
+        internal static string FormatRazorPage_NestingAttributeWritingScopesNotSupported()
+        {
+            return GetString("RazorPage_NestingAttributeWritingScopesNotSupported");
+        }
+
         private static string GetString(string name, params string[] formatterNames)
         {
             var value = _resourceManager.GetString(name);

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

@@ -40,6 +40,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor
         private IViewBufferScope _bufferScope;
         private bool _ignoreBody;
         private HashSet<string> _ignoredSections;
+        private TextWriter _pageWriter;
 
         public RazorPage()
         {
@@ -231,6 +232,61 @@ namespace Microsoft.AspNetCore.Mvc.Razor
             return tagHelperContent;
         }
 
+        /// <summary>
+        /// Starts a new scope for writing <see cref="ITagHelper"/> attribute values.
+        /// </summary>
+        /// <remarks>
+        /// All writes to the <see cref="Output"/> or <see cref="ViewContext.Writer"/> after calling this method will
+        /// be buffered until <see cref="EndWriteTagHelperAttribute"/> is called.
+        /// The content will be buffered using a shared <see cref="StringWriter"/> within this <see cref="RazorPage"/> 
+        /// Nesting of <see cref="BeginWriteTagHelperAttribute"/> and <see cref="EndWriteTagHelperAttribute"/> method calls
+        /// is not supported.
+        /// </remarks>
+        public void BeginWriteTagHelperAttribute()
+        {
+            if (_pageWriter != null)
+            {
+                throw new InvalidOperationException(Resources.RazorPage_NestingAttributeWritingScopesNotSupported);
+            }
+
+            _pageWriter = ViewContext.Writer;
+
+            if (_valueBuffer == null)
+            {
+                _valueBuffer = new StringWriter();
+            }
+
+            // We need to replace the ViewContext's Writer to ensure that all content (including content written
+            // from HTML helpers) is redirected.
+            ViewContext.Writer = _valueBuffer;
+
+        }
+
+        /// <summary>
+        /// Ends the current writing scope that was started by calling <see cref="BeginWriteTagHelperAttribute"/>.
+        /// </summary>
+        /// <returns>The content buffered by the shared <see cref="StringWriter"/> of this <see cref="RazorPage"/>.</returns>
+        /// <remarks>
+        /// This method assumes that there will be no nesting of <see cref="BeginWriteTagHelperAttribute"/> 
+        /// and <see cref="EndWriteTagHelperAttribute"/> method calls.
+        /// </remarks>
+        public string EndWriteTagHelperAttribute()
+        {
+            if (_pageWriter == null)
+            {
+                throw new InvalidOperationException(Resources.RazorPage_ThereIsNoActiveWritingScopeToEnd);
+            }
+
+            var content = _valueBuffer.ToString();
+            _valueBuffer.GetStringBuilder().Clear();
+
+            // Restore previous writer.
+            ViewContext.Writer = _pageWriter;
+            _pageWriter = null;
+
+            return content;
+        }
+
         /// <summary>
         /// Writes the specified <paramref name="value"/> with HTML encoding to <see cref="Output"/>.
         /// </summary>

src/Microsoft.AspNetCore.Mvc.Razor/Resources.resx

@@ -206,4 +206,7 @@
   <data name="ViewLocationFormatsIsRequired" xml:space="preserve">
     <value>'{0}' cannot be empty. These locations are required to locate a view for rendering.</value>
   </data>
+  <data name="RazorPage_NestingAttributeWritingScopesNotSupported" xml:space="preserve">
+    <value>Nesting of TagHelper attribute writing scopes is not supported.</value>
+  </data>
 </root>

test/Microsoft.AspNetCore.Mvc.Razor.Host.Test/TestFiles/Output/Runtime/ModelExpressionTagHelper.cs

@@ -13,7 +13,7 @@ namespace AspNetCore
     {
         #line hidden
         #pragma warning disable 0414
-        private global::Microsoft.AspNetCore.Razor.TagHelpers.TagHelperContent __tagHelperStringValueBuffer = null;
+        private string __tagHelperStringValueBuffer = null;
         #pragma warning restore 0414
         private global::Microsoft.AspNetCore.Razor.Runtime.TagHelpers.TagHelperExecutionContext __tagHelperExecutionContext = null;
         private global::Microsoft.AspNetCore.Razor.Runtime.TagHelpers.TagHelperRunner __tagHelperRunner = null;

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

@@ -225,6 +225,64 @@ namespace Microsoft.AspNetCore.Mvc.Razor
             await page.ExecuteAsync();
         }
 
+        [Fact]
+        public async Task EndWriteTagHelperAttribute_RestoresPageWriter()
+        {
+            // Arrange
+            var page = CreatePage(v =>
+            {
+                v.BeginWriteTagHelperAttribute();
+                v.Write("Hello World!");
+                v.EndWriteTagHelperAttribute();
+            });
+            var originalWriter = page.Output;
+
+            // Act
+            await page.ExecuteAsync();
+
+            // Assert
+            Assert.NotNull(originalWriter);
+            Assert.Same(originalWriter, page.Output);
+        }
+
+        [Fact]
+        public async Task EndWriteTagHelperAttribute_ReturnsAppropriateContent()
+        {
+            // Arrange
+            var viewContext = CreateViewContext();
+            var page = CreatePage(v =>
+            {
+                v.HtmlEncoder = new HtmlTestEncoder();
+                v.BeginWriteTagHelperAttribute();
+                v.Write("Hello World!");
+                var returnValue = v.EndWriteTagHelperAttribute();
+
+                // Assert
+                var content = Assert.IsType<string>(returnValue);
+                Assert.Equal("HtmlEncode[[Hello World!]]", content);
+            });
+
+            // Act & Assert
+            await page.ExecuteAsync();
+        }
+
+        [Fact]
+        public async Task BeginWriteTagHelperAttribute_NestingWritingScopesThrows()
+        {
+            // Arrange
+            var viewContext = CreateViewContext();
+            var page = CreatePage(v =>
+            {
+                v.BeginWriteTagHelperAttribute();
+                v.BeginWriteTagHelperAttribute();
+                v.Write("Hello World!");
+            });
+
+            // Act & Assert
+            var ex = await Assert.ThrowsAsync<InvalidOperationException>(() => page.ExecuteAsync());
+            Assert.Equal("Nesting of TagHelper attribute writing scopes is not supported.", ex.Message);
+        }
+
         // This is an integration test for ensuring that ViewBuffer segments used by
         // TagHelpers can be merged back into the 'main' segment where possible.
         [Fact]