Add ValidationMessage TagHelper.

#1250

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

@@ -0,0 +1,44 @@
+// 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 Microsoft.AspNet.Mvc.Rendering;
+using Microsoft.AspNet.Razor.Runtime.TagHelpers;
+using Microsoft.AspNet.Razor.TagHelpers;
+
+namespace Microsoft.AspNet.Mvc.TagHelpers
+{
+    /// <summary>
+    /// <see cref="ITagHelper"/> implementation targeting &lt;span&gt; elements with <c>validation-for</c> attributes.
+    /// </summary>
+    [TagName("span")]
+    [ContentBehavior(ContentBehavior.Modify)]
+    public class ValidationMessageTagHelper : TagHelper
+    {
+        [Activate]
+        private ViewContext ViewContext { get; set; }
+
+        [Activate]
+        private IHtmlGenerator Generator { get; set; }
+
+        /// <summary>
+        /// An name to be validated on the current model.
+        /// </summary>
+        [HtmlAttributeName("validation-for")]
+        public ModelExpression For { get; set; }
+
+        /// <inheritdoc />
+        public override void Process(TagHelperContext context, TagHelperOutput output)
+        {
+            if (For != null)
+            {
+                var tagBuilder = Generator.GenerateValidationMessage(ViewContext,
+                                                                     For.Name,
+                                                                     message: output.Content,
+                                                                     tag: output.TagName,
+                                                                     htmlAttributes: null);
+
+                output.Merge(tagBuilder);
+            }
+        }
+    }
+}