Add ContentBehaviorAttribute for TagHelpers.

- Added detection of custom ContentBehaviors via the ContentBehaviorAttribute in the TagHelperDescriptorFactory.
- Updated some comments in the ContentBehavior enum.
- Add tests to validate custom content behavior resolution.

#122

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

@@ -0,0 +1,33 @@
+// Copyright (c) Microsoft Open Technologies, Inc. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using Microsoft.AspNet.Razor.TagHelpers;
+
+namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
+{
+    /// <summary>
+    /// Used to override <see cref="TagHelper"/>s functionality for when its
+    /// <see cref="TagHelper.Process(TagHelperOutput, TagHelperContext)"/> or
+    /// <see cref="TagHelper.ProcessAsync(TagHelperOutput, TagHelperContext)"/> is invoked with a 
+    /// <see cref="TagHelperOutput"/>.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = false)]
+    public sealed class ContentBehaviorAttribute : Attribute
+    {
+        /// <summary>
+        /// Instantiates a new instance of the <see cref="ContentBehaviorAttribute"/> class.
+        /// </summary>
+        /// <param name="contentBehavior">The <see cref="Razor.TagHelpers.ContentBehavior"/> for the
+        /// <see cref="TagHelper"/> to use.</param>
+        public ContentBehaviorAttribute(ContentBehavior contentBehavior)
+        {
+            ContentBehavior = contentBehavior;
+        }
+
+        /// <summary>
+        /// A <see cref="Razor.TagHelpers.ContentBehavior"/> for the <see cref="TagHelper"/> to follow.
+        /// </summary>
+        public ContentBehavior ContentBehavior { get; private set; }
+    }
+}

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

@@ -67,10 +67,12 @@ private static TagHelperAttributeDescriptor ToTagHelperAttributeDescriptor(Prope
             return new TagHelperAttributeDescriptor(property.Name, property);
         }
 
-        // TODO: Make the content behavior pull from a ContentBehaviorAttribute: https://github.com/aspnet/Razor/issues/122
         private static ContentBehavior GetContentBehavior(Type type)
         {
-            return DefaultContentBehavior;
+            var typeInfo = type.GetTypeInfo();
+            var contentBehaviorAttribute = typeInfo.GetCustomAttribute<ContentBehaviorAttribute>(inherit: false);
+
+            return contentBehaviorAttribute?.ContentBehavior ?? DefaultContentBehavior;
         }
 
         private static bool IsValidTagHelperProperty(PropertyInfo property)

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

@@ -6,6 +6,10 @@
     },
     "frameworks": {
         "net45": { },
-        "aspnetcore50": { }
+        "aspnetcore50": { 
+            "dependencies": {
+                "System.Reflection.Extensions": "4.0.0.0"
+            }
+        }
     }
 }

src/Microsoft.AspNet.Razor/TagHelpers/ContentBehavior.cs

@@ -9,15 +9,15 @@ namespace Microsoft.AspNet.Razor.TagHelpers
     public enum ContentBehavior
     {
         /// <summary>
-        /// Indicates that the tag helper will not modify its inner HTML in any way. This is the default
+        /// Indicates that a tag helper will not modify its content in any way. This is the default
         /// <see cref="ContentBehavior"/>.
         /// </summary>
         /// <remarks>Children of the current tag helper will execute after the current tag helper.</remarks>
         None,
 
         /// <summary>
-        /// Indicates that the tag helper wants anything within its tag builder's inner HTML to be
-        /// appended to content its children generate.
+        /// Indicates that the tag helper wants anything within its outputs content to be appended to what its children
+        /// generate.
         /// </summary>
         /// <remarks>Children of the current tag helper will execute before the current tag helper.</remarks>
         Append,
@@ -30,15 +30,15 @@ public enum ContentBehavior
         Modify,
 
         /// <summary>
-        /// Indicates that the tag helper wants anything within its tag builder's inner HTML to be
-        /// prepended to the content its children generate.
+        /// Indicates that the tag helper wants anything within its outputs content to be prepended to what its 
+        /// children generate.
         /// </summary>
         /// <remarks>Children of the current tag helper will execute after the current tag helper.</remarks>
         Prepend,
 
         /// <summary>
-        /// Indicates that the tag helper wants anything within its tag builder's inner HTML to
-        /// replace any HTML inside of it.
+        /// Indicates that the tag helper wants anything within its outputs content to replace the HTML its children
+        /// generate.
         /// </summary>
         /// <remarks>Children of the current tag helper will not execute.</remarks>
         Replace,

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

@@ -86,5 +86,48 @@ public void DescriptorFactory_OnlyUnderstandsPropertiesWithPublicGetAndSet()
             // Assert
             Assert.Equal(descriptor, expectedDescriptor, DefaultDescriptorComparer);
         }
+
+
+        [Fact]
+        public void DescriptorResolver_ResolvesCustomContentBehaviors()
+        {
+            // Arrange            
+            var expectedDescriptor = new TagHelperDescriptor(
+                "CustomContentBehavior",
+                typeof(CustomContentBehaviorTagHelper).FullName,
+                ContentBehavior.Append);
+
+            // Act
+            var descriptor = TagHelperDescriptorFactory.CreateDescriptor(typeof(CustomContentBehaviorTagHelper));
+
+            // Assert
+            Assert.Equal(descriptor, expectedDescriptor, DefaultDescriptorComparer);
+        }
+
+        [Fact]
+        public void DescriptorResolver_DoesntResolveInheritedCustomContentBehaviors()
+        {
+            // Arrange            
+            var expectedDescriptor = new TagHelperDescriptor(
+                "InheritedCustomContentBehavior",
+                typeof(InheritedCustomContentBehaviorTagHelper).FullName,
+                ContentBehavior.None);
+
+            // Act
+            var descriptor = TagHelperDescriptorFactory.CreateDescriptor(
+                typeof(InheritedCustomContentBehaviorTagHelper));
+
+            // Assert
+            Assert.Equal(descriptor, expectedDescriptor, DefaultDescriptorComparer);
+        }
+
+        [ContentBehavior(ContentBehavior.Append)]
+        private class CustomContentBehaviorTagHelper
+        {
+        }
+
+        private class InheritedCustomContentBehaviorTagHelper : CustomContentBehaviorTagHelper
+        {
+        }
     }
 }