Addressed code review comments.

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

@@ -27,6 +27,11 @@ public class RenderAtEndOfFormTagHelper : TagHelper
         /// <inheritdoc />
         public override void Init(TagHelperContext context)
         {
+            if (context == null)
+            {
+                throw new ArgumentNullException(nameof(context));
+            }
+
             // Push the new FormContext.
             ViewContext.FormContext = new FormContext
             {

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

@@ -21,6 +21,8 @@ public class SelectTagHelper : TagHelper
     {
         private const string ForAttributeName = "asp-for";
         private const string ItemsAttributeName = "asp-items";
+        private bool _allowMultiple;
+        private IReadOnlyCollection<string> _currentValues;
 
         /// <summary>
         /// Creates a new <see cref="SelectTagHelper"/>.
@@ -59,27 +61,16 @@ public override int Order
         [HtmlAttributeName(ItemsAttributeName)]
         public IEnumerable<SelectListItem> Items { get; set; }
 
-        /// <inheritdoc />
-        /// <remarks>Does nothing if <see cref="For"/> is <c>null</c>.</remarks>
-        /// <exception cref="InvalidOperationException">
-        /// Thrown if <see cref="Items"/> is non-<c>null</c> but <see cref="For"/> is <c>null</c>.
-        /// </exception>
-        public override void Process(TagHelperContext context, TagHelperOutput output)
+        public override void Init(TagHelperContext context)
         {
             if (context == null)
             {
                 throw new ArgumentNullException(nameof(context));
             }
 
-            if (output == null)
-            {
-                throw new ArgumentNullException(nameof(output));
-            }
-
             // 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)
+            if (For.Metadata == null)
             {
                 throw new InvalidOperationException(Resources.FormatTagHelpers_NoProvidedMetadata(
                     "<select>",
@@ -92,36 +83,54 @@ public override void Process(TagHelperContext context, TagHelperOutput output)
             // "SelectExpressionNotEnumerable" InvalidOperationException during generation.
             // Metadata.IsEnumerableType is similar but does not take runtime type into account.
             var realModelType = For.ModelExplorer.ModelType;
-            var allowMultiple = typeof(string) != realModelType &&
+            _allowMultiple = typeof(string) != realModelType &&
                 typeof(IEnumerable).GetTypeInfo().IsAssignableFrom(realModelType.GetTypeInfo());
+            _currentValues = Generator.GetCurrentValues(
+                ViewContext,
+                For.ModelExplorer,
+                expression: For.Name,
+                allowMultiple: _allowMultiple);
+
+            // Whether or not (not being highly unlikely) we generate anything, could update contained <option/>
+            // elements. Provide selected values for <option/> tag helpers. They'll run next.
+            context.Items[typeof(SelectTagHelper)] = _currentValues;
+        }
+
+        /// <inheritdoc />
+        /// <remarks>Does nothing if <see cref="For"/> is <c>null</c>.</remarks>
+        /// <exception cref="InvalidOperationException">
+        /// Thrown if <see cref="Items"/> is non-<c>null</c> but <see cref="For"/> is <c>null</c>.
+        /// </exception>
+        public override void Process(TagHelperContext context, TagHelperOutput output)
+        {
+            if (context == null)
+            {
+                throw new ArgumentNullException(nameof(context));
+            }
+
+            if (output == null)
+            {
+                throw new ArgumentNullException(nameof(output));
+            }
 
             // Ensure GenerateSelect() _never_ looks anything up in ViewData.
             var items = Items ?? Enumerable.Empty<SelectListItem>();
 
-            var currentValues = Generator.GetCurrentValues(
-                ViewContext,
-                For.ModelExplorer,
-                expression: For.Name,
-                allowMultiple: allowMultiple);
             var tagBuilder = Generator.GenerateSelect(
                 ViewContext,
                 For.ModelExplorer,
                 optionLabel: null,
                 expression: For.Name,
                 selectList: items,
-                currentValues: currentValues,
-                allowMultiple: allowMultiple,
+                currentValues: _currentValues,
+                allowMultiple: _allowMultiple,
                 htmlAttributes: null);
 
             if (tagBuilder != null)
             {
                 output.MergeAttributes(tagBuilder);
                 output.PostContent.Append(tagBuilder.InnerHtml);
             }
-
-            // Whether or not (not being highly unlikely) we generate anything, could update contained <option/>
-            // elements. Provide selected values for <option/> tag helpers. They'll run next.
-            context.Items[typeof(SelectTagHelper)] = currentValues;
         }
     }
 }

test/Microsoft.AspNet.Mvc.FunctionalTests/compiler/resources/HtmlGenerationWebSite.HtmlGeneration_Home.Order.Encoded.html

@@ -41,6 +41,9 @@
                 <option value="HtmlEncode[[Credit]]">Credit</option>
                 <option value="HtmlEncode[[Check]]" selected="HtmlEncode[[selected]]">Check</option>
             </select>
+            <select>
+                <option value="HtmlEncode[[Check]]">Check</option>
+            </select>
         </div>
         <div>
             <label class="order" for="HtmlEncode[[Customer_Number]]">HtmlEncode[[Number]]</label>

test/Microsoft.AspNet.Mvc.FunctionalTests/compiler/resources/HtmlGenerationWebSite.HtmlGeneration_Home.OrderUsingHtmlHelpers.Encoded.html

@@ -40,6 +40,9 @@
             <select id="HtmlEncode[[PaymentMethod]]" multiple="HtmlEncode[[multiple]]" name="HtmlEncode[[PaymentMethod]]"><option value="HtmlEncode[[Credit]]">HtmlEncode[[Credit]]</option>
 <option selected="HtmlEncode[[selected]]" value="HtmlEncode[[Check]]">HtmlEncode[[Check]]</option>
 </select>
+            <select>
+                <option value="Check">Check</option>
+            </select>
         </div>
         <div>
             <label class="HtmlEncode[[order]]" for="HtmlEncode[[Customer_Number]]">HtmlEncode[[Number]]</label>

test/Microsoft.AspNet.Mvc.FunctionalTests/compiler/resources/HtmlGenerationWebSite.HtmlGeneration_Home.OrderUsingHtmlHelpers.html

@@ -40,6 +40,9 @@
             <select id="PaymentMethod" multiple="multiple" name="PaymentMethod"><option value="Credit">Credit</option>
 <option selected="selected" value="Check">Check</option>
 </select>
+            <select>
+                <option value="Check">Check</option>
+            </select>
         </div>
         <div>
             <label class="order" for="Customer_Number">Number</label>

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

@@ -235,6 +235,7 @@ public async Task ProcessAsync_GeneratesExpectedOutput(
             };
 
             // Act
+            tagHelper.Init(tagHelperContext);
             await tagHelper.ProcessAsync(tagHelperContext, output);
 
             // Assert
@@ -332,6 +333,7 @@ public async Task ProcessAsync_WithItems_GeneratesExpectedOutput_DoesNotChangeSe
             };
 
             // Act
+            tagHelper.Init(tagHelperContext);
             await tagHelper.ProcessAsync(tagHelperContext, output);
 
             // Assert
@@ -427,7 +429,6 @@ public async Task ProcessAsyncInTemplate_WithItems_GeneratesExpectedOutput_DoesN
             var savedSelected = items.Select(item => item.Selected).ToList();
             var savedText = items.Select(item => item.Text).ToList();
             var savedValue = items.Select(item => item.Value).ToList();
-
             var tagHelper = new SelectTagHelper(htmlGenerator)
             {
                 For = modelExpression,
@@ -436,6 +437,7 @@ public async Task ProcessAsyncInTemplate_WithItems_GeneratesExpectedOutput_DoesN
             };
 
             // Act
+            tagHelper.Init(tagHelperContext);
             await tagHelper.ProcessAsync(tagHelperContext, output);
 
             // Assert
@@ -533,6 +535,7 @@ public async Task ProcessAsync_CallsGeneratorWithExpectedValues_ItemsAndAttribut
             };
 
             // Act
+            tagHelper.Init(tagHelperContext);
             await tagHelper.ProcessAsync(tagHelperContext, output);
 
             // Assert
@@ -606,6 +609,7 @@ public async Task TagHelper_CallsGeneratorWithExpectedValues_RealModelType(
             };
 
             // Act
+            tagHelper.Init(tagHelperContext);
             await tagHelper.ProcessAsync(tagHelperContext, output);
 
             // Assert

test/WebSites/HtmlGenerationWebSite/Views/HtmlGeneration_Home/OrderUsingHtmlHelpers.cshtml

@@ -43,6 +43,9 @@ Html.BeginForm(actionName: "Submit", controllerName: "HtmlGeneration_Order"))
         <div>
             @Html.LabelFor(m => m.PaymentMethod, htmlAttributes: new { @class = "order" })
             @Html.ListBoxFor(m => m.PaymentMethod, selectList: new SelectList(new[] { new { value = "Credit" }, new { value = "Check" } }, dataValueField: "value", dataTextField: "value"))
+            <select>
+                <option value="Check">Check</option>
+            </select>
         </div>
         <div>
             @Html.LabelFor(m => m.Customer.Number, htmlAttributes: new { @class = "order" })