Provide selected values to `<option/>` tag helpers

- value may remain in the `FormContext` beyond `</select>` end tag but will
  be cleaned up at the `</form>` end tag of the containing `<form/>` element
 - `SelectTagHelper` called prior to helpers for contained `<option/>`s and
   not again later
- adjust mock setups to handle new `GenerateSelect()` call
- add assertions for expected `FormContext.FormData` entry

nit: mention #1468 in a test comment

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

@@ -18,6 +18,16 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
     [ContentBehavior(ContentBehavior.Append)]
     public class SelectTagHelper : TagHelper
     {
+        /// <summary>
+        /// Key used for selected values in <see cref="FormContext.FormData"/>.
+        /// </summary>
+        /// <remarks>
+        /// Value for this dictionary entry will either be <c>null</c> (indicating no <see cref="SelectTagHelper"/> has
+        /// executed within this &lt;form/&gt;) or an <see cref="ICollection{string}"/> instance. Elements of the
+        /// collection are based on current <see cref="ViewDataDictionary.Model"/>.
+        /// </remarks>
+        public static readonly string SelectedValuesFormDataKey = nameof(SelectTagHelper) + "-SelectedValues";
+
         // Protected to ensure subclasses are correctly activated. Internal for ease of use when testing.
         [Activate]
         protected internal IHtmlGenerator Generator { get; set; }
@@ -114,6 +124,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                 // Ensure GenerateSelect() _never_ looks anything up in ViewData.
                 var items = Items ?? Enumerable.Empty<SelectListItem>();
 
+                ICollection<string> selectedValues;
                 var tagBuilder = Generator.GenerateSelect(
                     ViewContext,
                     For.Metadata,
@@ -121,13 +132,18 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                     name: For.Name,
                     selectList: items,
                     allowMultiple: allowMultiple,
-                    htmlAttributes: null);
+                    htmlAttributes: null,
+                    selectedValues: out selectedValues);
 
                 if (tagBuilder != null)
                 {
                     output.SelfClosing = false;
                     output.Merge(tagBuilder);
                 }
+
+                // 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.
+                ViewContext.FormContext.FormData[SelectedValuesFormDataKey] = selectedValues;
             }
         }
     }

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

@@ -79,7 +79,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 
                     // Skip last two test cases because DefaultHtmlGenerator evaluates expression name against
                     // ViewData, not using ModelMetadata.Model. ViewData.Eval() handles simple property paths and some
-                    // dictionary lookups, but not indexing into an array or list. Will file a follow-up bug on this...
+                    // dictionary lookups, but not indexing into an array or list. See #1468...
                     ////{ models, typeof(Model), () => models[1].Text,
                     ////    new NameAndId("[1].Text", "z1__Text"), outerSelected },
                     ////{ models, typeof(NestedModel), () => models[1].NestedModel.Text,
@@ -213,6 +213,14 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             Assert.Equal(expectedContent, output.Content);
             Assert.False(output.SelfClosing);
             Assert.Equal(expectedTagName, output.TagName);
+
+            Assert.NotNull(viewContext.FormContext?.FormData);
+            var keyValuePair = Assert.Single(
+                viewContext.FormContext.FormData,
+                entry => entry.Key == SelectTagHelper.SelectedValuesFormDataKey);
+            Assert.NotNull(keyValuePair.Value);
+            var selectedValues = Assert.IsAssignableFrom<ICollection<string>>(keyValuePair.Value);
+            Assert.InRange(selectedValues.Count, 0, 1);
         }
 
         [Theory]
@@ -278,6 +286,14 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             Assert.Equal(expectedContent, output.Content);
             Assert.False(output.SelfClosing);
             Assert.Equal(expectedTagName, output.TagName);
+
+            Assert.NotNull(viewContext.FormContext?.FormData);
+            var keyValuePair = Assert.Single(
+                viewContext.FormContext.FormData,
+                entry => entry.Key == SelectTagHelper.SelectedValuesFormDataKey);
+            Assert.NotNull(keyValuePair.Value);
+            var selectedValues = Assert.IsAssignableFrom<ICollection<string>>(keyValuePair.Value);
+            Assert.InRange(selectedValues.Count, 0, 1);
         }
 
         [Theory]
@@ -312,6 +328,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 
             var htmlGenerator = new Mock<IHtmlGenerator>(MockBehavior.Strict);
             var viewContext = TestableHtmlGenerator.GetViewContext(model, htmlGenerator.Object, metadataProvider);
+            ICollection<string> selectedValues = new string[0];
             htmlGenerator
                 .Setup(real => real.GenerateSelect(
                     viewContext,
@@ -320,7 +337,8 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                     propertyName, // name
                     expectedItems,
                     expectedAllowMultiple,
-                    null))        // htmlAttributes
+                    null,         // htmlAttributes
+                    out selectedValues))
                 .Returns((TagBuilder)null)
                 .Verifiable();
 
@@ -338,6 +356,12 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 
             // Assert
             htmlGenerator.Verify();
+
+            Assert.NotNull(viewContext.FormContext?.FormData);
+            var keyValuePair = Assert.Single(
+                viewContext.FormContext.FormData,
+                entry => entry.Key == SelectTagHelper.SelectedValuesFormDataKey);
+            Assert.Same(selectedValues, keyValuePair.Value);
         }
 
         [Theory]
@@ -363,6 +387,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 
             var htmlGenerator = new Mock<IHtmlGenerator>(MockBehavior.Strict);
             var viewContext = TestableHtmlGenerator.GetViewContext(model, htmlGenerator.Object, metadataProvider);
+            ICollection<string> selectedValues = new string[0];
             htmlGenerator
                 .Setup(real => real.GenerateSelect(
                     viewContext,
@@ -371,7 +396,8 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                     propertyName, // name
                     It.IsAny<IEnumerable<SelectListItem>>(),
                     allowMultiple,
-                    null))        // htmlAttributes
+                    null,         // htmlAttributes
+                    out selectedValues))
                 .Returns((TagBuilder)null)
                 .Verifiable();
 
@@ -387,6 +413,12 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 
             // Assert
             htmlGenerator.Verify();
+
+            Assert.NotNull(viewContext.FormContext?.FormData);
+            var keyValuePair = Assert.Single(
+                viewContext.FormContext.FormData,
+                entry => entry.Key == SelectTagHelper.SelectedValuesFormDataKey);
+            Assert.Same(selectedValues, keyValuePair.Value);
         }
 
         [Theory]