Add ValidationSummaryTagHelper.

samples/TagHelperSample.Web/Views/Home/Create.cshtml

@@ -10,13 +10,13 @@
     <div class="form-horizontal">
         @* validation summary tag helper will target just <div/> elements and append the list of errors *@
         @* - i.e. this helper, like <select/> helper has ContentBehavior.Append *@
-        @* validation-model-errors-only="true" implies validation-summary="true" *@
-        @* helper does nothing if model is valid and (client-side validation is disabled or validation-model-errors-only="true") *@
+        @* helper does nothing if model is valid and (client-side validation is disabled or validation-summary="ModelOnly") *@
         @* don't need a bound attribute to match Html.ValidationSummary()'s headerTag parameter; users wrap message as they wish *@
         @* initially at least, will not remove the <div/> if list isn't generated *@
         @* - should helper remove the <div/> if list isn't generated? *@
         @* - (Html.ValidationSummary returns empty string despite non-empty message parameter) *@
-        <div validation-summary="true" validation-model-errors-only="true">
+        @* Acceptable values are: "None", "ModelOnly" and "All" *@
+        <div validation-summary="ModelOnly">
             <span style="color:red">This is my message</span>
         </div>
 

samples/TagHelperSample.Web/Views/Home/Edit.cshtml

@@ -6,7 +6,7 @@
 
 <form>
     <div class="form-horizontal">
-        <div validation-summary="true"/>
+        <div validation-summary="All"/>
         <input type="hidden" for="Id" />
 
         <div class="form-group">

src/Microsoft.AspNet.Mvc.TagHelpers/Properties/AssemblyInfo.cs

@@ -0,0 +1,6 @@
+// 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.Runtime.CompilerServices;
+
+[assembly: InternalsVisibleTo("Microsoft.AspNet.Mvc.TagHelpers.Test")]

src/Microsoft.AspNet.Mvc.TagHelpers/Resources.resx

@@ -126,4 +126,7 @@
   <data name="FormTagHelper_CannotDetermineAction" xml:space="preserve">
     <value>Cannot determine an {1} for {0}. A {0} with a URL-based {1} must not have attributes starting with {3} or a {2} attribute.</value>
   </data>
+  <data name="ValidationSummaryTagHelper_InvalidValidationSummaryValue" xml:space="preserve">
+    <value>Cannot parse '{1}' value '{2}' for {0}. Acceptable values are '{3}', '{4}' and '{5}'.</value>
+  </data>
 </root>

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

@@ -0,0 +1,26 @@
+// 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.
+
+namespace Microsoft.AspNet.Mvc.TagHelpers
+{
+    /// <summary>
+    /// Acceptable validation summary rendering modes.
+    /// </summary>
+    public enum ValidationSummary
+    {
+        /// <summary>
+        /// No validation summary.
+        /// </summary>
+        None,
+
+        /// <summary>
+        /// Validation summary with model-level errors only (excludes all property errors).
+        /// </summary>
+        ModelOnly,
+
+        /// <summary>
+        /// Validation summary with all errors.
+        /// </summary>
+        All
+    }
+}

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

@@ -0,0 +1,74 @@
+// 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.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;div&gt; elements with a <c>validation-summary</c> 
+    /// attribute.
+    /// </summary>
+    [TagName("div")]
+    [ContentBehavior(ContentBehavior.Append)]
+    public class ValidationSummaryTagHelper : TagHelper
+    {
+        // Protected to ensure subclasses are correctly activated. Internal for ease of use when testing.
+        [Activate]
+        protected internal ViewContext ViewContext { get; set; }
+
+        // Protected to ensure subclasses are correctly activated. Internal for ease of use when testing.
+        [Activate]
+        protected internal IHtmlGenerator Generator { get; set; }
+
+        // TODO: Change to ValidationSummary enum once https://github.com/aspnet/Razor/issues/196 has been completed.
+        /// <summary>
+        /// If <c>All</c> or <c>ModelOnly</c>, appends a validation summary. Acceptable values are defined by the 
+        /// <see cref="ValidationSummary"/> enum.
+        /// </summary>
+        [HtmlAttributeName("validation-summary")]
+        public string ValidationSummaryValue { get; set; }
+
+        /// <inheritdoc />
+        /// Does nothing if <see cref="ValidationSummaryValue"/> is <c>null</c>, empty or "None".
+        public override void Process(TagHelperContext context, TagHelperOutput output)
+        {
+            if (!string.IsNullOrEmpty(ValidationSummaryValue))
+            {
+                ValidationSummary validationSummaryValue;
+                if (!Enum.TryParse(ValidationSummaryValue, ignoreCase: true, result: out validationSummaryValue))
+                {
+                    throw new InvalidOperationException(
+                        Resources.FormatValidationSummaryTagHelper_InvalidValidationSummaryValue(
+                            "<div>",
+                            "validation-summary",
+                            ValidationSummaryValue,
+                            ValidationSummary.All,
+                            ValidationSummary.ModelOnly,
+                            ValidationSummary.None));
+                }
+                else if (validationSummaryValue == ValidationSummary.None)
+                {
+                    return;
+                }
+
+                var validationModelErrorsOnly = validationSummaryValue == ValidationSummary.ModelOnly;
+                var tagBuilder = Generator.GenerateValidationSummary(
+                    ViewContext,
+                    excludePropertyErrors: validationModelErrorsOnly,
+                    message: null,
+                    headerTag: null,
+                    htmlAttributes: null);
+
+                if (tagBuilder != null)
+                {
+                    output.MergeAttributes(tagBuilder);
+                    output.Content += tagBuilder.InnerHtml;
+                }
+            }
+        }
+    }
+}