Addressed code review comments.

Author: NTaylorMullen

src/Microsoft.AspNet.Mvc.TagHelpers/ValidationMessageTagHelper.cs

@@ -34,10 +34,16 @@ public override void Process(TagHelperContext context, TagHelperOutput output)
                 var tagBuilder = Generator.GenerateValidationMessage(ViewContext,
                                                                      For.Name,
                                                                      message: output.Content,
-                                                                     tag: output.TagName,
+                                                                     tag: null,
                                                                      htmlAttributes: null);
 
-                output.Merge(tagBuilder);
+                if (tagBuilder != null)
+                {
+                    output.MergeAttributes(tagBuilder);
+
+                    // We passed in the content, if there's something there it'll be set appropriately.
+                    output.Content = tagBuilder.InnerHtml;
+                }
             }
         }
     }

test/Microsoft.AspNet.Mvc.TagHelpers.Test/ValidationMessageTagHelperTest.cs

@@ -4,8 +4,10 @@
 using System.Collections.Generic;
 using System.IO;
 using System.Reflection;
+using System.Threading.Tasks;
 using Microsoft.AspNet.Http;
 using Microsoft.AspNet.Mvc.ModelBinding;
+using Microsoft.AspNet.Mvc.Razor;
 using Microsoft.AspNet.Mvc.Rendering;
 using Microsoft.AspNet.Razor.Runtime.TagHelpers;
 using Microsoft.AspNet.Routing;
@@ -17,50 +19,94 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
     public class ValidationMessageTagHelperTest
     {
         [Fact]
-        public void Process_CallsIntoGenerateValidationMessageWithExpectedParameters()
+        public async Task ProcessAsync_GeneratesExpectedOutput()
+        {
+            // Arrange
+            var metadataProvider = new DataAnnotationsModelMetadataProvider();
+            var modelExpression = CreateModelExpression("Name");
+            var validationMessageTagHelper = new ValidationMessageTagHelper
+            {
+                For = modelExpression
+            };
+
+            var tagHelperContext = new TagHelperContext(
+                allAttributes: new Dictionary<string, object>
+                {
+                    { "id", "myvalidationmessage" },
+                    { "for", modelExpression },
+                });
+            var output = new TagHelperOutput(
+                "original tag name",
+                attributes: new Dictionary<string, string>
+                {
+                    { "id", "myvalidationmessage" }
+                },
+                content: "Something");
+
+            var htmlGenerator = new TestableHtmlGenerator(metadataProvider);
+            var viewContext = TestableHtmlGenerator.GetViewContext(model: null,
+                                                                   htmlGenerator: htmlGenerator,
+                                                                   metadataProvider: metadataProvider);
+
+            var activator = new DefaultTagHelperActivator();
+            activator.Activate(validationMessageTagHelper, viewContext);
+
+            // Act
+            await validationMessageTagHelper.ProcessAsync(tagHelperContext, output);
+
+            // Assert
+            Assert.Equal(4, output.Attributes.Count);
+            var attribute = Assert.Single(output.Attributes, kvp => kvp.Key.Equals("id"));
+            Assert.Equal("myvalidationmessage", attribute.Value);
+            attribute = Assert.Single(output.Attributes, kvp => kvp.Key.Equals("class"));
+            Assert.Equal("field-validation-valid", attribute.Value);
+            attribute = Assert.Single(output.Attributes, kvp => kvp.Key.Equals("data-valmsg-for"));
+            Assert.Equal("Name", attribute.Value);
+            attribute = Assert.Single(output.Attributes, kvp => kvp.Key.Equals("data-valmsg-replace"));
+            Assert.Equal("false", attribute.Value);
+            Assert.Equal("Something", output.Content);
+            Assert.Equal("original tag name", output.TagName);
+        }
+
+        [Fact]
+        public async Task ProcessAsync_CallsIntoGenerateValidationMessageWithExpectedParameters()
         {
             // Arrange
             var validationMessageTagHelper = new ValidationMessageTagHelper
             {
                 For = CreateModelExpression("Hello")
             };
-            var called = false;
             var output = new TagHelperOutput(
                 "span",
                 attributes: new Dictionary<string, string>(),
                 content: "Content of validation message");
 
             var expectedViewContext = CreateViewContext();
             var generator = new Mock<IHtmlGenerator>();
-            generator.Setup(mock =>
-                mock.GenerateValidationMessage(It.IsAny<ViewContext>(),
-                                               It.IsAny<string>(),
-                                               It.IsAny<string>(),
-                                               It.IsAny<string>(),
-                                               It.IsAny<object>()))
-                    .Callback<ViewContext, string, string, string, object>(
-                        (viewContext, name, message, tag, htmlAttributes) =>
-                        {
-                            called = true;
-
-                            Assert.Same(expectedViewContext, viewContext);
-                            Assert.Equal("Hello", name);
-                            Assert.Equal("Content of validation message", message);
-                            Assert.Equal("span", tag);
-                            Assert.Null(htmlAttributes);
-                        })
-                    .Returns(new TagBuilder("span"));
+            var setup = generator.Setup(mock =>
+                mock.GenerateValidationMessage(expectedViewContext,
+                                               "Hello",
+                                               "Content of validation message",
+                                               null,
+                                               null));
+            setup.Returns(new TagBuilder("span"));
+            setup.Verifiable();
 
             SetViewContextAndGenerator(validationMessageTagHelper, expectedViewContext, generator.Object);
 
             // Act & Assert
-            validationMessageTagHelper.Process(context: null, output: output);
+            await validationMessageTagHelper.ProcessAsync(context: null, output: output);
 
-            Assert.True(called);
+            generator.Verify();
+            Assert.Equal("span", output.TagName);
+            Assert.Empty(output.Attributes);
+
+            // Our mock doesn't set any TagBuilder content.
+            Assert.Empty(output.Content);
         }
 
         [Fact]
-        public void Process_MergesTagBuilderFromGenerateValidationMessage()
+        public async Task ProcessAsync_MergesTagBuilderFromGenerateValidationMessage()
         {
             // Arrange
             var validationMessageTagHelper = new ValidationMessageTagHelper
@@ -81,21 +127,21 @@ public void Process_MergesTagBuilderFromGenerateValidationMessage()
 
             var expectedViewContext = CreateViewContext();
             var generator = new Mock<IHtmlGenerator>(MockBehavior.Strict);
-            generator.Setup(mock =>
+            var setup = generator.Setup(mock =>
                 mock.GenerateValidationMessage(It.IsAny<ViewContext>(),
                                                It.IsAny<string>(),
                                                It.IsAny<string>(),
                                                It.IsAny<string>(),
-                                               It.IsAny<object>()))
-                    .Returns(tagBuilder);
+                                               It.IsAny<object>()));
+            setup.Returns(tagBuilder);
 
             SetViewContextAndGenerator(validationMessageTagHelper, expectedViewContext, generator.Object);
 
             // Act
-            validationMessageTagHelper.Process(context: null, output: output);
+            await validationMessageTagHelper.ProcessAsync(context: null, output: output);
 
             // Assert
-            Assert.Equal(output.TagName, "span2");
+            Assert.Equal(output.TagName, "span");
             Assert.Equal(2, output.Attributes.Count);
             var attribute = Assert.Single(output.Attributes, kvp => kvp.Key.Equals("data-foo"));
             Assert.Equal("bar", attribute.Value);
@@ -105,7 +151,7 @@ public void Process_MergesTagBuilderFromGenerateValidationMessage()
         }
 
         [Fact]
-        public void Process_DoesNothingIfNullFor()
+        public async Task ProcessAsync_DoesNothingIfNullFor()
         {
             // Arrange
             var validationMessageTagHelper = new ValidationMessageTagHelper();
@@ -119,9 +165,10 @@ public void Process_DoesNothingIfNullFor()
 
             SetViewContextAndGenerator(validationMessageTagHelper, expectedViewContext, generator.Object);
 
-            // Act & Assert
-            validationMessageTagHelper.Process(context: null, output: output);
+            // Act
+            await validationMessageTagHelper.ProcessAsync(context: null, output: output);
 
+            // Assert
             Assert.Equal("span", output.TagName);
             Assert.Empty(output.Attributes);
             Assert.Equal("Content of validation message", output.Content);