Add <select/> tag helper

- #1248

Author: dougbu

src/Microsoft.AspNet.Mvc.TagHelpers/Properties/Resources.Designer.cs

@@ -135,7 +135,10 @@
   <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">
+  <data name="SelectTagHelper_CannotDetermineContentWhenOnlyItemsSpecified" xml:space="preserve">
+    <value>Cannot determine body for {0}. '{2}' must be null if '{1}' is null.</value>
+  </data>
+  <data name="TagHelpers_InvalidValue_ThreeAcceptableValues" xml:space="preserve">
     <value>Cannot parse '{1}' value '{2}' for {0}. Acceptable values are '{3}', '{4}' and '{5}'.</value>
   </data>
   <data name="TagHelpers_NoProvidedMetadata" xml:space="preserve">

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

@@ -0,0 +1,134 @@
+// 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 System.Collections;
+using System.Collections.Generic;
+using System.Linq;
+using Microsoft.AspNet.Mvc.ModelBinding;
+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;select&gt; elements.
+    /// </summary>
+    [ContentBehavior(ContentBehavior.Append)]
+    public class SelectTagHelper : TagHelper
+    {
+        // Protected to ensure subclasses are correctly activated. Internal for ease of use when testing.
+        [Activate]
+        protected internal IHtmlGenerator Generator { get; set; }
+
+        // Protected to ensure subclasses are correctly activated. Internal for ease of use when testing.
+        [Activate]
+        protected internal ViewContext ViewContext { get; set; }
+
+        /// <summary>
+        /// An expression to be evaluated against the current model.
+        /// </summary>
+        public ModelExpression For { get; set; }
+
+        /// <summary>
+        /// Specifies that multiple options can be selected at once.
+        /// </summary>
+        /// <remarks>
+        /// Passed through to the generated HTML if value is <c>multiple</c>. Converted to <c>multiple</c> or absent if
+        /// value is <c>true</c> or <c>false</c>. Other values are not acceptable. Also used to determine the correct
+        /// "selected" attributes for generated &lt;option&gt; elements.
+        /// </remarks>
+        public string Multiple { get; set; }
+
+        /// <summary>
+        /// A collection of <see cref="SelectListItem"/> objects used to populate the &lt;select&gt; element with
+        /// &lt;optgroup&gt; and &lt;option&gt; elements.
+        /// </summary>
+        public IEnumerable<SelectListItem> Items { get; set; }
+
+        /// <inheritdoc />
+        /// <remarks>Does nothing if <see cref="For"/> is <c>null</c>.</remarks>
+        public override void Process(TagHelperContext context, TagHelperOutput output)
+        {
+            if (For == null)
+            {
+                // Regular HTML <select/> element. Just make sure Items wasn't specified.
+                if (Items != null)
+                {
+                    var message = Resources.FormatSelectTagHelper_CannotDetermineContentWhenOnlyItemsSpecified(
+                        "<select>",
+                        nameof(For).ToLowerInvariant(),
+                        nameof(Items).ToLowerInvariant());
+                    throw new InvalidOperationException(message);
+                }
+
+                // Pass through attribute that is also a well-known HTML attribute.
+                if (!string.IsNullOrEmpty(Multiple))
+                {
+                    output.CopyHtmlAttribute(nameof(Multiple), context);
+                }
+            }
+            else
+            {
+                // Note null or empty For.Name is allowed because TemplateInfo.HtmlFieldPrefix may be sufficient.
+                // IHtmlGenerator will enforce name requirements.
+                var metadata = For.Metadata;
+                if (metadata == null)
+                {
+                    throw new InvalidOperationException(Resources.FormatTagHelpers_NoProvidedMetadata(
+                        "<select>",
+                        nameof(For).ToLowerInvariant(),
+                        nameof(IModelMetadataProvider),
+                        For.Name));
+                }
+
+                bool allowMultiple;
+                if (string.IsNullOrEmpty(Multiple))
+                {
+                    // Base allowMultiple on the instance or declared type of the expression.
+                    var realModelType = For.Metadata.RealModelType;
+                    allowMultiple =
+                        typeof(string) != realModelType && typeof(IEnumerable).IsAssignableFrom(realModelType);
+                }
+                else if (string.Equals(Multiple, "multiple", StringComparison.OrdinalIgnoreCase))
+                {
+                    allowMultiple = true;
+
+                    // Copy exact attribute name and value the user entered. Must be done prior to any copying from a
+                    // TagBuilder. Not done in next case because "true" and "false" aren't valid for the HTML 5
+                    // attribute.
+                    output.CopyHtmlAttribute(nameof(Multiple), context);
+                }
+                else if (!bool.TryParse(Multiple.ToLowerInvariant(), out allowMultiple))
+                {
+                    throw new InvalidOperationException(Resources.FormatTagHelpers_InvalidValue_ThreeAcceptableValues(
+                        "<select>",
+                        nameof(Multiple).ToLowerInvariant(),
+                        Multiple,
+                        bool.FalseString.ToLowerInvariant(),
+                        bool.TrueString.ToLowerInvariant(),
+                        nameof(Multiple).ToLowerInvariant())); // acceptable value (as well as attribute name)
+                }
+
+                // Ensure GenerateSelect() _never_ looks anything up in ViewData.
+                var items = Items ?? Enumerable.Empty<SelectListItem>();
+
+                var tagBuilder = Generator.GenerateSelect(
+                    ViewContext,
+                    For.Metadata,
+                    optionLabel: null,
+                    name: For.Name,
+                    selectList: items,
+                    allowMultiple: allowMultiple,
+                    htmlAttributes: null);
+
+                if (tagBuilder != null)
+                {
+                    output.SelfClosing = false;
+                    output.Merge(tagBuilder);
+                }
+            }
+        }
+    }
+}

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

@@ -42,7 +42,7 @@ public override void Process(TagHelperContext context, TagHelperOutput output)
                 if (!Enum.TryParse(ValidationSummaryValue, ignoreCase: true, result: out validationSummaryValue))
                 {
                     throw new InvalidOperationException(
-                        Resources.FormatValidationSummaryTagHelper_InvalidValidationSummaryValue(
+                        Resources.FormatTagHelpers_InvalidValue_ThreeAcceptableValues(
                             "<div>",
                             "validation-summary",
                             ValidationSummaryValue,