Refactor `IHtmlGenerator.GenerateSelect()` and address #2240

- add `IHtmlGenerator.GetCurrentValues()` method
 - bring together bits of `GenerateSelect()` and `UpdateSelectListItemsWithDefaultValue()`
 - get rid of ugly `out` parameter
 - also allows `<option/>` tag helpers to run before `<select/>` helper generation
- match `null` values and `SelectListItem`s with empty values
- match `enum` names correctly
- add doc comments for `IHtmlGenerator.GenerateSelect()` methods

src/Microsoft.AspNet.Mvc.Core/Rendering/Html/DefaultHtmlGenerator.cs

@@ -7,6 +7,7 @@ using System.Collections.Generic;
 using System.Diagnostics;
 using System.Globalization;
 using System.Linq;
+using System.Reflection;
 using System.Text;
 using Microsoft.AspNet.Mvc.Core;
 using Microsoft.AspNet.Mvc.ModelBinding;
@@ -20,6 +21,8 @@ namespace Microsoft.AspNet.Mvc.Rendering
     public class DefaultHtmlGenerator : IHtmlGenerator
     {
         private const string HiddenListItem = @"<li style=""display:none""></li>";
+        private static readonly MethodInfo ConvertEnumFromStringMethod =
+            typeof(DefaultHtmlGenerator).GetTypeInfo().GetDeclaredMethod(nameof(ConvertEnumFromString));
 
         private readonly AntiForgery _antiForgery;
         private readonly IScopedInstance<ActionBindingContext> _bindingContextAccessor;
@@ -377,16 +380,16 @@ namespace Microsoft.AspNet.Mvc.Rendering
             bool allowMultiple,
             object htmlAttributes)
         {
-            ICollection<string> ignored;
+            var currentValues = GetCurrentValues(viewContext, modelExplorer, expression, allowMultiple);
             return GenerateSelect(
                 viewContext,
                 modelExplorer,
                 optionLabel,
                 expression,
                 selectList,
+                currentValues,
                 allowMultiple,
-                htmlAttributes,
+                htmlAttributes);
-                selectedValues: out ignored);
         }
 
         /// <inheritdoc />
@@ -396,9 +399,9 @@ namespace Microsoft.AspNet.Mvc.Rendering
             string optionLabel,
             string expression,
             IEnumerable<SelectListItem> selectList,
+            IReadOnlyCollection<string> currentValues,
             bool allowMultiple,
-            object htmlAttributes,
+            object htmlAttributes)
-            out ICollection<string> selectedValues)
         {
             var fullName = GetFullHtmlFieldName(viewContext, expression);
             if (string.IsNullOrEmpty(fullName))
@@ -407,7 +410,6 @@ namespace Microsoft.AspNet.Mvc.Rendering
             }
 
             // If we got a null selectList, try to use ViewData to get the list of items.
-            var usedViewData = false;
             if (selectList == null)
             {
                 if (string.IsNullOrEmpty(expression))
@@ -419,39 +421,13 @@ namespace Microsoft.AspNet.Mvc.Rendering
                 }
 
                 selectList = GetSelectListItems(viewContext, expression);
-                usedViewData = true;
             }
 
-            var type = allowMultiple ? typeof(string[]) : typeof(string);
+            modelExplorer = modelExplorer ??
-            var defaultValue = GetModelStateValue(viewContext, fullName, type);
+                ExpressionMetadataProvider.FromStringExpression(expression, viewContext.ViewData, _metadataProvider);
-
+            if (currentValues != null)
-            // If ModelState did not contain a current value, fall back to ViewData- or ModelExplorer-supplied value.
-            if (defaultValue == null)
             {
-                if (modelExplorer == null)
+                selectList = UpdateSelectListItemsWithDefaultValue(modelExplorer, selectList, currentValues);
-                {
-                    // Html.DropDownList() and Html.ListBox() helper case.
-                    // Cannot use ViewData if it contains the select list.
-                    if (!usedViewData)
-                    {
-                        defaultValue = viewContext.ViewData.Eval(expression);
-                    }
-                }
-                else
-                {
-                    // <select/>, Html.DropDownListFor() and Html.ListBoxFor() helper case. Do not use ViewData.
-                    defaultValue = modelExplorer.Model;
-                }
-            }
-
-            if (defaultValue != null)
-            {
-                selectList =
-                    UpdateSelectListItemsWithDefaultValue(selectList, defaultValue, allowMultiple, out selectedValues);
-            }
-            else
-            {
-                selectedValues = new string[0];
             }
 
             // Convert each ListItem to an <option> tag and wrap them with <optgroup> if requested.
@@ -758,6 +734,125 @@ namespace Microsoft.AspNet.Mvc.Rendering
                 .SelectMany(v => v.GetClientValidationRules(validationContext));
         }
 
+        /// <inheritdoc />
+        public virtual IReadOnlyCollection<string> GetCurrentValues(
+            [NotNull] ViewContext viewContext,
+            ModelExplorer modelExplorer,
+            string expression,
+            bool allowMultiple)
+        {
+            var fullName = GetFullHtmlFieldName(viewContext, expression);
+            if (string.IsNullOrEmpty(fullName))
+            {
+                throw new ArgumentException(Resources.ArgumentCannotBeNullOrEmpty, nameof(expression));
+            }
+
+            var type = allowMultiple ? typeof(string[]) : typeof(string);
+            var rawValue = GetModelStateValue(viewContext, fullName, type);
+
+            // If ModelState did not contain a current value, fall back to ViewData- or ModelExplorer-supplied value.
+            if (rawValue == null)
+            {
+                if (modelExplorer == null)
+                {
+                    // Html.DropDownList() and Html.ListBox() helper case.
+                    rawValue = viewContext.ViewData.Eval(expression);
+                    if (rawValue is IEnumerable<SelectListItem>)
+                    {
+                        // This ViewData item contains the fallback selectList collection for GenerateSelect().
+                        // Do not try to use this collection.
+                        rawValue = null;
+                    }
+                }
+                else
+                {
+                    // <select/>, Html.DropDownListFor() and Html.ListBoxFor() helper case. Do not use ViewData.
+                    rawValue = modelExplorer.Model;
+                }
+
+                if (rawValue == null)
+                {
+                    return null;
+                }
+            }
+
+            // Convert raw value to a collection.
+            IEnumerable rawValues;
+            if (allowMultiple)
+            {
+                rawValues = rawValue as IEnumerable;
+                if (rawValues == null || rawValues is string)
+                {
+                    throw new InvalidOperationException(
+                        Resources.FormatHtmlHelper_SelectExpressionNotEnumerable(nameof(expression)));
+                }
+            }
+            else
+            {
+                rawValues = new[] { rawValue };
+            }
+
+            modelExplorer = modelExplorer ??
+                ExpressionMetadataProvider.FromStringExpression(expression, viewContext.ViewData, _metadataProvider);
+
+            var enumNames = modelExplorer.Metadata.EnumNamesAndValues;
+            var isTargetEnum = modelExplorer.Metadata.IsEnum;
+            var innerType =
+                Nullable.GetUnderlyingType(modelExplorer.Metadata.ModelType) ?? modelExplorer.Metadata.ModelType;
+
+            // Convert raw value collection to strings.
+            var currentValues = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
+            foreach (var value in rawValues)
+            {
+                // Add original or converted string.
+                var stringValue = (value as string) ?? Convert.ToString(value, CultureInfo.CurrentCulture);
+
+                // Do not add simple names of enum properties here because whitespace isn't relevant for their binding.
+                // Will add matching names just below.
+                if (enumNames == null || !enumNames.ContainsKey(stringValue.Trim()))
+                {
+                    currentValues.Add(stringValue);
+                }
+
+                // Remainder handles isEnum cases. Convert.ToString() returns field names for enum values but select
+                // list may (well, should) contain integer values.
+                var enumValue = value as Enum;
+                if (isTargetEnum && enumValue == null && value != null)
+                {
+                    var valueType = value.GetType();
+                    if (typeof(long).IsAssignableFrom(valueType) || typeof(ulong).IsAssignableFrom(valueType))
+                    {
+                        // E.g. user added an int to a ViewData entry and called a string-based HTML helper.
+                        enumValue = ConvertEnumFromInteger(value, innerType);
+                    }
+                    else if (!string.IsNullOrEmpty(stringValue))
+                    {
+                        // E.g. got a string from ModelState.
+                        var methodInfo = ConvertEnumFromStringMethod.MakeGenericMethod(innerType);
+                        enumValue = (Enum)methodInfo.Invoke(obj: null, parameters: new[] { stringValue });
+                    }
+                }
+
+                if (enumValue != null)
+                {
+                    // Add integer value.
+                    var integerString = enumValue.ToString("d");
+                    currentValues.Add(integerString);
+
+                    // Add all simple names for this value.
+                    var matchingNames = enumNames
+                        .Where(kvp => string.Equals(integerString, kvp.Value, StringComparison.Ordinal))
+                        .Select(kvp => kvp.Key);
+                    foreach (var name in matchingNames)
+                    {
+                        currentValues.Add(name);
+                    }
+                }
+            }
+
+            return (IReadOnlyCollection<string>)currentValues;
+        }
+
         internal static string EvalString(ViewContext viewContext, string key, string format)
         {
             return Convert.ToString(viewContext.ViewData.Eval(key, format), CultureInfo.CurrentCulture);
@@ -990,6 +1085,32 @@ namespace Microsoft.AspNet.Mvc.Rendering
             return UnobtrusiveValidationAttributesGenerator.GetValidationAttributes(clientRules);
         }
 
+        private static Enum ConvertEnumFromInteger(object value, Type targetType)
+        {
+            try
+            {
+                return (Enum)Enum.ToObject(targetType, value);
+            }
+            catch (Exception exception)
+            when (exception is FormatException || exception.InnerException is FormatException)
+            {
+                // The integer was too large for this enum type.
+                return null;
+            }
+        }
+
+        private static object ConvertEnumFromString<TEnum>(string value) where TEnum : struct
+        {
+            TEnum enumValue;
+            if (Enum.TryParse(value, out enumValue))
+            {
+                return enumValue;
+            }
+
+            // Do not return default(TEnum) when parse was unsuccessful.
+            return null;
+        }
+
         private static bool EvalBoolean(ViewContext viewContext, string key)
         {
             return Convert.ToBoolean(viewContext.ViewData.Eval(key), CultureInfo.InvariantCulture);
@@ -1060,48 +1181,26 @@ namespace Microsoft.AspNet.Mvc.Rendering
         }
 
         private static IEnumerable<SelectListItem> UpdateSelectListItemsWithDefaultValue(
+            ModelExplorer modelExplorer,
             IEnumerable<SelectListItem> selectList,
-            object defaultValue,
+            IReadOnlyCollection<string> currentValues)
-            bool allowMultiple,
-            out ICollection<string> selectedValues)
         {
-            IEnumerable defaultValues;
-            if (allowMultiple)
-            {
-                defaultValues = defaultValue as IEnumerable;
-                if (defaultValues == null || defaultValues is string)
-                {
-                    throw new InvalidOperationException(
-                        Resources.FormatHtmlHelper_SelectExpressionNotEnumerable("expression"));
-                }
-            }
-            else
-            {
-                defaultValues = new[] { defaultValue };
-            }
-
-            var values =
-                defaultValues.OfType<object>().Select(value => Convert.ToString(value, CultureInfo.CurrentCulture));
-
-            // ToString() by default returns an enum value's name.  But selectList may use numeric values.
-            var enumValues = defaultValues.OfType<Enum>().Select(value => value.ToString());
-            values = values.Concat(enumValues);
-
-            selectedValues = new HashSet<string>(values, StringComparer.OrdinalIgnoreCase);
-
             // Perform deep copy of selectList to avoid changing user's Selected property values.
             var newSelectList = new List<SelectListItem>();
             foreach (SelectListItem item in selectList)
             {
-                var newItem = new SelectListItem
+                var value = item.Value ?? item.Text;
+                var selected = currentValues.Contains(value);
+                var copy = new SelectListItem
                 {
                     Disabled = item.Disabled,
                     Group = item.Group,
-                    Selected = selectedValues.Contains(item.Value ?? item.Text),
+                    Selected = selected,
                     Text = item.Text,
                     Value = item.Value,
                 };
-                newSelectList.Add(newItem);
+
+                newSelectList.Add(copy);
             }
 
             return newSelectList;

src/Microsoft.AspNet.Mvc.Core/Rendering/Html/HtmlHelper.cs

@@ -691,8 +691,8 @@ namespace Microsoft.AspNet.Mvc.Rendering
                 ViewContext,
                 modelExplorer,
                 optionLabel,
-                expression: expression,
+                expression,
-                selectList: selectList,
+                selectList,
                 allowMultiple: false,
                 htmlAttributes: htmlAttributes);
             if (tagBuilder == null)

src/Microsoft.AspNet.Mvc.Core/Rendering/Html/IHtmlGenerator.cs

@@ -159,6 +159,40 @@ namespace Microsoft.AspNet.Mvc.Rendering
             object routeValues,
             object htmlAttributes);
 
+        /// <summary>
+        /// Generate a &lt;select&gt; element for the <paramref name="expression"/>.
+        /// </summary>
+        /// <param name="viewContext">A <see cref="ViewContext"/> instance for the current scope.</param>
+        /// <param name="modelExplorer">
+        /// <see cref="ModelExplorer"/> for the <paramref name="expression"/>. If <c>null</c>, determines validation
+        /// attributes using <paramref name="viewContext"/> and the <paramref name="expression"/>.
+        /// </param>
+        /// <param name="optionLabel">Optional text for a default empty &lt;option&gt; element.</param>
+        /// <param name="expression">Expression name, relative to the current model.</param>
+        /// <param name="selectList">
+        /// A collection of <see cref="SelectListItem"/> objects used to populate the &lt;select&gt; element with
+        /// &lt;optgroup&gt; and &lt;option&gt; elements. If <c>null</c>, finds this collection at
+        /// <c>ViewContext.ViewData[expression]</c>.
+        /// </param>
+        /// <param name="allowMultiple">
+        /// If <c>true</c>, includes a <c>multiple</c> attribute in the generated HTML. Otherwise generates a
+        /// single-selection &lt;select&gt; element.
+        /// </param>
+        /// <param name="htmlAttributes">
+        /// An <see cref="object"/> that contains the HTML attributes for the &lt;select&gt; element. Alternatively, an
+        /// <see cref="IDictionary{string, object}"/> instance containing the HTML attributes.
+        /// </param>
+        /// <returns>A new <see cref="TagBuilder"/> describing the &lt;select&gt; element.</returns>
+        /// <remarks>
+        /// <para>
+        /// Combines <see cref="TemplateInfo.HtmlFieldPrefix"/> and <paramref name="expression"/> to set
+        /// &lt;select&gt; element's "name" attribute. Sanitizes <paramref name="expression"/> to set element's "id"
+        /// attribute.
+        /// </para>
+        /// <para>
+        /// See <see cref="GetCurrentValues"/> for information about how current values are determined.
+        /// </para>
+        /// </remarks>
         TagBuilder GenerateSelect(
             [NotNull] ViewContext viewContext,
             ModelExplorer modelExplorer,
@@ -168,15 +202,55 @@ namespace Microsoft.AspNet.Mvc.Rendering
             bool allowMultiple,
             object htmlAttributes);
 
+        /// <summary>
+        /// Generate a &lt;select&gt; element for the <paramref name="expression"/>.
+        /// </summary>
+        /// <param name="viewContext">A <see cref="ViewContext"/> instance for the current scope.</param>
+        /// <param name="modelExplorer">
+        /// <see cref="ModelExplorer"/> for the <paramref name="expression"/>. If <c>null</c>, determines validation
+        /// attributes using <paramref name="viewContext"/> and the <paramref name="expression"/>.
+        /// </param>
+        /// <param name="optionLabel">Optional text for a default empty &lt;option&gt; element.</param>
+        /// <param name="expression">Expression name, relative to the current model.</param>
+        /// <param name="selectList">
+        /// A collection of <see cref="SelectListItem"/> objects used to populate the &lt;select&gt; element with
+        /// &lt;optgroup&gt; and &lt;option&gt; elements. If <c>null</c>, finds this collection at
+        /// <c>ViewContext.ViewData[expression]</c>.
+        /// </param>
+        /// <param name="currentValues">
+        /// An <see cref="IReadOnlyCollection{string}"/> containing values for &lt;option&gt; elements to select. If
+        /// <c>null</c>, selects &lt;option&gt; elements based on <see cref="SelectListItem.Selected"/> values in
+        /// <paramref name="selectList"/>.
+        /// </param>
+        /// <param name="allowMultiple">
+        /// If <c>true</c>, includes a <c>multiple</c> attribute in the generated HTML. Otherwise generates a
+        /// single-selection &lt;select&gt; element.
+        /// </param>
+        /// <param name="htmlAttributes">
+        /// An <see cref="object"/> that contains the HTML attributes for the &lt;select&gt; element. Alternatively, an
+        /// <see cref="IDictionary{string, object}"/> instance containing the HTML attributes.
+        /// </param>
+        /// <returns>A new <see cref="TagBuilder"/> describing the &lt;select&gt; element.</returns>
+        /// <remarks>
+        /// <para>
+        /// Combines <see cref="TemplateInfo.HtmlFieldPrefix"/> and <paramref name="expression"/> to set
+        /// &lt;select&gt; element's "name" attribute. Sanitizes <paramref name="expression"/> to set element's "id"
+        /// attribute.
+        /// </para>
+        /// <para>
+        /// See <see cref="GetCurrentValues"/> for information about how the <paramref name="currentValues"/>
+        /// collection may be created.
+        /// </para>
+        /// </remarks>
         TagBuilder GenerateSelect(
             [NotNull] ViewContext viewContext,
             ModelExplorer modelExplorer,
             string optionLabel,
             string expression,
             IEnumerable<SelectListItem> selectList,
+            IReadOnlyCollection<string> currentValues,
             bool allowMultiple,
-            object htmlAttributes,
+            object htmlAttributes);
-            out ICollection<string> selectedValues);
 
         TagBuilder GenerateTextArea(
             [NotNull] ViewContext viewContext,
@@ -216,5 +290,45 @@ namespace Microsoft.AspNet.Mvc.Rendering
             [NotNull] ViewContext viewContext,
             ModelExplorer modelExplorer,
             string expression);
+
+        /// <summary>
+        /// Gets the collection of current values for the given <paramref name="expression"/>.
+        /// </summary>
+        /// <param name="viewContext">A <see cref="ViewContext"/> instance for the current scope.</param>
+        /// <param name="modelExplorer">
+        /// <see cref="ModelExplorer"/> for the <paramref name="expression"/>. If <c>null</c>, calculates the
+        /// <paramref name="expression"/> result using <see cref="ViewDataDictionary.Eval(string)"/>.
+        /// </param>
+        /// <param name="expression">Expression name, relative to the current model.</param>
+        /// <param name="allowMultiple">
+        /// If <c>true</c>, require a collection <paramref name="expression"/> result. Otherwise, treat result as a
+        /// single value.
+        /// </param>
+        /// <returns>
+        /// <para>
+        /// <c>null</c> if no <paramref name="expression"/> result is found. Otherwise an
+        /// <see cref="IReadOnlyCollection{string}"/> containing current values for the given
+        /// <paramref name="expression"/>.
+        /// </para>
+        /// <para>
+        /// Converts the <paramref name="expression"/> result to a <see cref="string"/>. If that result is an
+        /// <see cref="System.Collections.IEnumerable"/> type, instead converts each item in the collection and returns
+        /// them separately.
+        /// </para>
+        /// <para>
+        /// If the <paramref name="expression"/> result or the element type is an <see cref="System.Enum"/>, returns a
+        /// <see cref="string"/> containing the integer representation of the <see cref="System.Enum"/> value as well
+        /// as all <see cref="System.Enum"/> names for that value. Otherwise returns the default <see cref="string"/>
+        /// conversion of the value.
+        /// </para>
+        /// </returns>
+        /// <remarks>
+        /// See <see cref="GenerateSelect"/> for information about how the return value may be used.
+        /// </remarks>
+        IReadOnlyCollection<string> GetCurrentValues(
+            [NotNull] ViewContext viewContext,
+            ModelExplorer modelExplorer,
+            string expression,
+            bool allowMultiple);
     }
 }

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

@@ -79,16 +79,20 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             // Ensure GenerateSelect() _never_ looks anything up in ViewData.
             var items = Items ?? Enumerable.Empty<SelectListItem>();
 
-            ICollection<string> selectedValues;
+            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,
-                htmlAttributes: null,
+                htmlAttributes: null);
-                selectedValues: out selectedValues);
 
             if (tagBuilder != null)
             {
@@ -98,7 +102,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 
             // 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;
+            ViewContext.FormContext.FormData[SelectedValuesFormDataKey] = currentValues;
         }
     }
 }

test/Microsoft.AspNet.Mvc.Core.Test/Rendering/DefaultHtmlGeneratorTest.cs

@@ -0,0 +1,646 @@
+// 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.Generic;
+using System.Globalization;
+using System.IO;
+using System.Linq;
+using Microsoft.AspNet.DataProtection;
+using Microsoft.AspNet.Mvc.ModelBinding;
+using Microsoft.Framework.OptionsModel;
+using Microsoft.Framework.WebEncoders;
+using Moq;
+using Xunit;
+
+namespace Microsoft.AspNet.Mvc.Rendering
+{
+    public class DefaultHtmlGeneratorTest
+    {
+        [Theory]
+        [InlineData(false)]
+        [InlineData(true)]
+        public void GetCurrentValues_WithEmptyViewData_ReturnsNull(bool allowMultiple)
+        {
+            // Arrange
+            var metadataProvider = new TestModelMetadataProvider();
+            var htmlGenerator = GetGenerator(metadataProvider);
+            var viewContext = GetViewContext<Model>(model: null, metadataProvider: metadataProvider);
+
+            // Act
+            var result = htmlGenerator.GetCurrentValues(
+                viewContext,
+                modelExplorer: null,
+                expression: nameof(Model.Name),
+                allowMultiple: allowMultiple);
+
+            // Assert
+            Assert.Null(result);
+        }
+
+        [Theory]
+        [InlineData(false)]
+        [InlineData(true)]
+        public void GetCurrentValues_WithNullExpressionResult_ReturnsNull(bool allowMultiple)
+        {
+            // Arrange
+            var metadataProvider = new TestModelMetadataProvider();
+            var htmlGenerator = GetGenerator(metadataProvider);
+            var viewContext = GetViewContext<Model>(model: null, metadataProvider: metadataProvider);
+            var modelExplorer = metadataProvider.GetModelExplorerForType(typeof(string), model: null);
+
+            // Act
+            var result = htmlGenerator.GetCurrentValues(
+                viewContext,
+                modelExplorer,
+                expression: nameof(Model.Name),
+                allowMultiple: allowMultiple);
+
+            // Assert
+            Assert.Null(result);
+        }
+
+        [Theory]
+        [InlineData(false)]
+        [InlineData(true)]
+        public void GetCurrentValues_WithSelectListInViewData_ReturnsNull(bool allowMultiple)
+        {
+            // Arrange
+            var metadataProvider = new TestModelMetadataProvider();
+            var htmlGenerator = GetGenerator(metadataProvider);
+            var viewContext = GetViewContext<Model>(model: null, metadataProvider: metadataProvider);
+            viewContext.ViewData[nameof(Model.Name)] = Enumerable.Empty<SelectListItem>();
+
+            // Act
+            var result = htmlGenerator.GetCurrentValues(
+                viewContext,
+                modelExplorer: null,
+                expression: nameof(Model.Name),
+                allowMultiple: allowMultiple);
+
+            // Assert
+            Assert.Null(result);
+        }
+
+        [Theory]
+        [InlineData("some string")] // treated as if it were not IEnumerable
+        [InlineData(23)]
+        [InlineData(RegularEnum.Three)]
+        public void GetCurrentValues_AllowMultipleWithNonEnumerableInViewData_Throws(object value)
+        {
+            // Arrange
+            var metadataProvider = new TestModelMetadataProvider();
+            var htmlGenerator = GetGenerator(metadataProvider);
+            var viewContext = GetViewContext<Model>(model: null, metadataProvider: metadataProvider);
+            viewContext.ViewData[nameof(Model.Name)] = value;
+
+            // Act & Assert
+            var exception = Assert.Throws<InvalidOperationException>(() => htmlGenerator.GetCurrentValues(
+                viewContext,
+                modelExplorer: null,
+                expression: nameof(Model.Name),
+                allowMultiple: true));
+            Assert.Equal(
+                "The parameter 'expression' must evaluate to an IEnumerable when multiple selection is allowed.",
+                exception.Message);
+        }
+
+        // rawValue, allowMultiple -> expected current values
+        public static TheoryData<object, bool, IReadOnlyCollection<string>> GetCurrentValues_StringAndCollectionData
+        {
+            get
+            {
+                return new TheoryData<object, bool, IReadOnlyCollection<string>>
+                {
+                    // ModelStateDictionary converts single values to arrays and visa-versa.
+                    { string.Empty, false, new [] { string.Empty } },
+                    { string.Empty, true, new [] { string.Empty } },
+                    { "some string", false,  new [] { "some string" } },
+                    { "some string", true, new [] { "some string" } },
+                    { new [] { "some string" }, false, new [] { "some string" } },
+                    { new [] { "some string" }, true, new [] { "some string" } },
+                    { new [] { "some string", "some other string" }, false, new [] { "some string" } },
+                    {
+                        new [] { "some string", "some other string" },
+                        true,
+                        new [] { "some string", "some other string" }
+                    },
+                    // { new string[] { null }, false, null } would fall back to other sources.
+                    { new string[] { null }, true, new [] { string.Empty } },
+                    { new [] { string.Empty }, false, new [] { string.Empty } },
+                    { new [] { string.Empty }, true, new [] { string.Empty } },
+                    {
+                        new [] { null, "some string", "some other string" },
+                        true,
+                        new [] { string.Empty, "some string", "some other string" }
+                    },
+                    // ignores duplicates
+                    {
+                        new [] { null, "some string", null, "some other string", null, "some string", null },
+                        true,
+                        new [] { string.Empty, "some string", "some other string" }
+                    },
+                    // ignores case of duplicates
+                    {
+                        new [] { "some string", "SoMe StriNg", "Some String", "soME STRing", "SOME STRING" },
+                        true,
+                        new [] { "some string" }
+                    },
+                };
+            }
+        }
+
+        [Theory]
+        [MemberData(nameof(GetCurrentValues_StringAndCollectionData))]
+        public void GetCurrentValues_WithModelStateEntryAndViewData_ReturnsModelStateEntry(
+            object rawValue,
+            bool allowMultiple,
+            IReadOnlyCollection<string> expected)
+        {
+            // Arrange
+            var metadataProvider = new TestModelMetadataProvider();
+            var htmlGenerator = GetGenerator(metadataProvider);
+            var model = new Model { Name = "ignored property value" };
+
+            var viewContext = GetViewContext<Model>(model, metadataProvider);
+            viewContext.ViewData[nameof(Model.Name)] = "ignored ViewData value";
+
+
+            var valueProviderResult = new ValueProviderResult(
+                rawValue,
+                attemptedValue: null,
+                culture: CultureInfo.InvariantCulture);
+            viewContext.ModelState.SetModelValue(nameof(Model.Name), valueProviderResult);
+
+            // Act
+            var result = htmlGenerator.GetCurrentValues(
+                viewContext,
+                modelExplorer: null,
+                expression: nameof(Model.Name),
+                allowMultiple: allowMultiple);
+
+            // Assert
+            Assert.NotNull(result);
+            Assert.Equal<string>(expected, result);
+        }
+
+        [Theory]
+        [MemberData(nameof(GetCurrentValues_StringAndCollectionData))]
+        public void GetCurrentValues_WithModelStateEntryModelExplorerAndViewData_ReturnsModelStateEntry(
+            object rawValue,
+            bool allowMultiple,
+            IReadOnlyCollection<string> expected)
+        {
+            // Arrange
+            var metadataProvider = new TestModelMetadataProvider();
+            var htmlGenerator = GetGenerator(metadataProvider);
+            var model = new Model { Name = "ignored property value" };
+
+            var viewContext = GetViewContext<Model>(model, metadataProvider);
+            viewContext.ViewData[nameof(Model.Name)] = "ignored ViewData value";
+
+            var modelExplorer = metadataProvider.GetModelExplorerForType(typeof(string), "ignored model value");
+
+            var valueProviderResult = new ValueProviderResult(
+                rawValue,
+                attemptedValue: null,
+                culture: CultureInfo.InvariantCulture);
+            viewContext.ModelState.SetModelValue(nameof(Model.Name), valueProviderResult);
+
+            // Act
+            var result = htmlGenerator.GetCurrentValues(
+                viewContext,
+                modelExplorer,
+                expression: nameof(Model.Name),
+                allowMultiple: allowMultiple);
+
+            // Assert
+            Assert.NotNull(result);
+            Assert.Equal<string>(expected, result);
+        }
+
+        // rawValue -> expected current values
+        public static TheoryData<string, string[]> GetCurrentValues_StringData
+        {
+            get
+            {
+                return new TheoryData<string, string[]>
+                {
+                    // 1. If given a ModelExplorer, GetCurrentValues does not use ViewData even if expression result is
+                    // null.
+                    // 2. Otherwise if ViewData entry exists, GetCurrentValue does not fall back to ViewData.Model even
+                    // if entry is null.
+                    // 3. Otherwise, GetCurrentValue does not fall back anywhere else even if ViewData.Model is null.
+                    { null, null },
+                    { string.Empty, new [] { string.Empty } },
+                    { "some string", new [] { "some string" } },
+                };
+            }
+        }
+
+        [Theory]
+        [MemberData(nameof(GetCurrentValues_StringData))]
+        public void GetCurrentValues_WithModelExplorerAndViewData_ReturnsExpressionResult(
+            string rawValue,
+            IReadOnlyCollection<string> expected)
+        {
+            // Arrange
+            var metadataProvider = new TestModelMetadataProvider();
+            var htmlGenerator = GetGenerator(metadataProvider);
+            var model = new Model { Name = "ignored property value" };
+
+            var viewContext = GetViewContext<Model>(model, metadataProvider);
+            viewContext.ViewData[nameof(Model.Name)] = "ignored ViewData value";
+
+            var modelExplorer = metadataProvider.GetModelExplorerForType(typeof(string), rawValue);
+
+            var valueProviderResult = new ValueProviderResult(
+                rawValue: null,
+                attemptedValue: null,
+                culture: CultureInfo.InvariantCulture);
+            viewContext.ModelState.SetModelValue(nameof(Model.Name), valueProviderResult);
+
+            // Act
+            var result = htmlGenerator.GetCurrentValues(
+                viewContext,
+                modelExplorer,
+                expression: nameof(Model.Name),
+                allowMultiple: false);
+
+            // Assert
+            Assert.Equal<string>(expected, result);
+        }
+
+        [Theory]
+        [MemberData(nameof(GetCurrentValues_StringData))]
+        public void GetCurrentValues_WithViewData_ReturnsViewDataEntry(
+            object rawValue,
+            IReadOnlyCollection<string> expected)
+        {
+            // Arrange
+            var metadataProvider = new TestModelMetadataProvider();
+            var htmlGenerator = GetGenerator(metadataProvider);
+            var model = new Model { Name = "ignored property value" };
+
+            var viewContext = GetViewContext<Model>(model, metadataProvider);
+            viewContext.ViewData[nameof(Model.Name)] = rawValue;
+
+            var valueProviderResult = new ValueProviderResult(
+                rawValue: null,
+                attemptedValue: null,
+                culture: CultureInfo.InvariantCulture);
+            viewContext.ModelState.SetModelValue(nameof(Model.Name), valueProviderResult);
+
+            // Act
+            var result = htmlGenerator.GetCurrentValues(
+                viewContext,
+                modelExplorer: null,
+                expression: nameof(Model.Name),
+                allowMultiple: false);
+
+            // Assert
+            Assert.Equal<string>(expected, result);
+        }
+
+        [Theory]
+        [MemberData(nameof(GetCurrentValues_StringData))]
+        public void GetCurrentValues_WithModel_ReturnsModel(string rawValue, IReadOnlyCollection<string> expected)
+        {
+            // Arrange
+            var metadataProvider = new TestModelMetadataProvider();
+            var htmlGenerator = GetGenerator(metadataProvider);
+            var model = new Model { Name = rawValue };
+
+            var viewContext = GetViewContext<Model>(model, metadataProvider);
+
+            var valueProviderResult = new ValueProviderResult(
+                rawValue: null,
+                attemptedValue: null,
+                culture: CultureInfo.InvariantCulture);
+            viewContext.ModelState.SetModelValue(nameof(Model.Name), valueProviderResult);
+
+            // Act
+            var result = htmlGenerator.GetCurrentValues(
+                viewContext,
+                modelExplorer: null,
+                expression: nameof(Model.Name),
+                allowMultiple: false);
+
+            // Assert
+            Assert.Equal<string>(expected, result);
+        }
+
+        // rawValue -> expected current values
+        public static TheoryData<string[], string[]> GetCurrentValues_StringCollectionData
+        {
+            get
+            {
+                return new TheoryData<string[], string[]>
+                {
+                    { new string[] { null }, new [] { string.Empty } },
+                    { new [] { string.Empty }, new [] { string.Empty } },
+                    { new [] { "some string" }, new [] { "some string" } },
+                    { new [] { "some string", "some other string" }, new [] { "some string", "some other string" } },
+                    {
+                        new [] { null, "some string", "some other string" },
+                        new [] { string.Empty, "some string", "some other string" }
+                    },
+                    // ignores duplicates
+                    {
+                        new [] { null, "some string", null, "some other string", null, "some string", null },
+                        new [] { string.Empty, "some string", "some other string" }
+                    },
+                    // ignores case of duplicates
+                    {
+                        new [] { "some string", "SoMe StriNg", "Some String", "soME STRing", "SOME STRING" },
+                        new [] { "some string" }
+                    },
+                };
+            }
+        }
+
+        [Theory]
+        [MemberData(nameof(GetCurrentValues_StringCollectionData))]
+        public void GetCurrentValues_CollectionWithModelExplorerAndViewData_ReturnsExpressionResult(
+            string[] rawValue,
+            IReadOnlyCollection<string> expected)
+        {
+            // Arrange
+            var metadataProvider = new TestModelMetadataProvider();
+            var htmlGenerator = GetGenerator(metadataProvider);
+            var model = new Model { Collection = { "ignored property value" } };
+
+            var viewContext = GetViewContext<Model>(model, metadataProvider);
+            viewContext.ViewData[nameof(Model.Collection)] = new[] { "ignored ViewData value" };
+
+            var modelExplorer =
+                metadataProvider.GetModelExplorerForType(typeof(List<string>), new List<string>(rawValue));
+
+            var valueProviderResult = new ValueProviderResult(
+                rawValue: null,
+                attemptedValue: null,
+                culture: CultureInfo.InvariantCulture);
+            viewContext.ModelState.SetModelValue(nameof(Model.Collection), valueProviderResult);
+
+            // Act
+            var result = htmlGenerator.GetCurrentValues(
+                viewContext,
+                modelExplorer,
+                expression: nameof(Model.Collection),
+                allowMultiple: true);
+
+            // Assert
+            Assert.Equal<string>(expected, result);
+        }
+
+        [Theory]
+        [MemberData(nameof(GetCurrentValues_StringCollectionData))]
+        public void GetCurrentValues_CollectionWithViewData_ReturnsViewDataEntry(
+            object[] rawValue,
+            IReadOnlyCollection<string> expected)
+        {
+            // Arrange
+            var metadataProvider = new TestModelMetadataProvider();
+            var htmlGenerator = GetGenerator(metadataProvider);
+            var model = new Model { Collection = { "ignored property value" } };
+
+            var viewContext = GetViewContext<Model>(model, metadataProvider);
+            viewContext.ViewData[nameof(Model.Collection)] = rawValue;
+
+            var valueProviderResult = new ValueProviderResult(
+                rawValue: null,
+                attemptedValue: null,
+                culture: CultureInfo.InvariantCulture);
+            viewContext.ModelState.SetModelValue(nameof(Model.Collection), valueProviderResult);
+
+            // Act
+            var result = htmlGenerator.GetCurrentValues(
+                viewContext,
+                modelExplorer: null,
+                expression: nameof(Model.Collection),
+                allowMultiple: true);
+
+            // Assert
+            Assert.Equal<string>(expected, result);
+        }
+
+        [Theory]
+        [MemberData(nameof(GetCurrentValues_StringCollectionData))]
+        public void GetCurrentValues_CollectionWithModel_ReturnsModel(
+            string[] rawValue,
+            IReadOnlyCollection<string> expected)
+        {
+            // Arrange
+            var metadataProvider = new TestModelMetadataProvider();
+            var htmlGenerator = GetGenerator(metadataProvider);
+            var model = new Model();
+            model.Collection.AddRange(rawValue);
+
+            var viewContext = GetViewContext<Model>(model, metadataProvider);
+
+            var valueProviderResult = new ValueProviderResult(
+                rawValue: null,
+                attemptedValue: null,
+                culture: CultureInfo.InvariantCulture);
+            viewContext.ModelState.SetModelValue(nameof(Model.Collection), valueProviderResult);
+
+            // Act
+            var result = htmlGenerator.GetCurrentValues(
+                viewContext,
+                modelExplorer: null,
+                expression: nameof(Model.Collection),
+                allowMultiple: true);
+
+            // Assert
+            Assert.Equal<string>(expected, result);
+        }
+
+        // property name, rawValue -> expected current values
+        public static TheoryData<string, object, string[]> GetCurrentValues_ValueToConvertData
+        {
+            get
+            {
+                return new TheoryData<string, object, string[]>
+                {
+                    { nameof(Model.FlagsEnum), FlagsEnum.All, new [] { "-1", "All" } },
+                    { nameof(Model.FlagsEnum), FlagsEnum.FortyTwo, new [] { "42", "FortyTwo" } },
+                    { nameof(Model.FlagsEnum), FlagsEnum.None, new [] { "0", "None" } },
+                    { nameof(Model.FlagsEnum), FlagsEnum.Two, new [] { "2", "Two" } },
+                    { nameof(Model.FlagsEnum), string.Empty, new [] { string.Empty } },
+                    { nameof(Model.FlagsEnum), "All", new [] { "-1", "All" } },
+                    { nameof(Model.FlagsEnum), "FortyTwo", new [] { "42", "FortyTwo" } },
+                    { nameof(Model.FlagsEnum), "None", new [] { "0", "None" } },
+                    { nameof(Model.FlagsEnum), "Two", new [] { "2", "Two" } },
+                    { nameof(Model.FlagsEnum), "Two, Four", new [] { "Two, Four", "6" } },
+                    { nameof(Model.FlagsEnum), "garbage", new [] { "garbage" } },
+                    { nameof(Model.FlagsEnum), "0", new [] { "0", "None" } },
+                    { nameof(Model.FlagsEnum), "   43", new [] { "   43", "43" } },
+                    { nameof(Model.FlagsEnum), "-5   ", new [] { "-5   ", "-5" } },
+                    { nameof(Model.FlagsEnum), 0, new [] { "0", "None" } },
+                    { nameof(Model.FlagsEnum), 1, new [] { "1", "One" } },
+                    { nameof(Model.FlagsEnum), 43, new [] { "43" } },
+                    { nameof(Model.FlagsEnum), -5, new [] { "-5" } },
+                    { nameof(Model.FlagsEnum), int.MaxValue, new [] { "2147483647" } },
+                    { nameof(Model.FlagsEnum), (uint)int.MaxValue + 1, new [] { "2147483648" } },
+                    { nameof(Model.FlagsEnum), uint.MaxValue, new [] { "4294967295" } },  // converted to string & used
+
+                    { nameof(Model.Id), string.Empty, new [] { string.Empty } },
+                    { nameof(Model.Id), "garbage", new [] { "garbage" } },                  // no compatibility checks
+                    { nameof(Model.Id), "0", new [] { "0" } },
+                    { nameof(Model.Id), "  43", new [] { "  43" } },
+                    { nameof(Model.Id), "-5  ", new [] { "-5  " } },
+                    { nameof(Model.Id), 0, new [] { "0" } },
+                    { nameof(Model.Id), 1, new [] { "1" } },
+                    { nameof(Model.Id), 43, new [] { "43" } },
+                    { nameof(Model.Id), -5, new [] { "-5" } },
+                    { nameof(Model.Id), int.MaxValue, new [] { "2147483647" } },
+                    { nameof(Model.Id), (uint)int.MaxValue + 1, new [] { "2147483648" } },  // no limit checks
+                    { nameof(Model.Id), uint.MaxValue, new [] { "4294967295" } },           // no limit checks
+
+                    { nameof(Model.NullableEnum), RegularEnum.Zero, new [] { "0", "Zero" } },
+                    { nameof(Model.NullableEnum), RegularEnum.One, new [] { "1", "One" } },
+                    { nameof(Model.NullableEnum), RegularEnum.Two, new [] { "2", "Two" } },
+                    { nameof(Model.NullableEnum), RegularEnum.Three, new [] { "3", "Three" } },
+                    { nameof(Model.NullableEnum), string.Empty, new [] { string.Empty } },
+                    { nameof(Model.NullableEnum), "Zero", new [] { "0", "Zero" } },
+                    { nameof(Model.NullableEnum), "Two", new [] { "2", "Two" } },
+                    { nameof(Model.NullableEnum), "One, Two", new [] { "One, Two", "3", "Three" } },
+                    { nameof(Model.NullableEnum), "garbage", new [] { "garbage" } },
+                    { nameof(Model.NullableEnum), "0", new [] { "0", "Zero" } },
+                    { nameof(Model.NullableEnum), "   43", new [] { "   43", "43" } },
+                    { nameof(Model.NullableEnum), "-5   ", new [] { "-5   ", "-5" } },
+                    { nameof(Model.NullableEnum), 0, new [] { "0", "Zero" } },
+                    { nameof(Model.NullableEnum), 1, new [] { "1", "One" } },
+                    { nameof(Model.NullableEnum), 43, new [] { "43" } },
+                    { nameof(Model.NullableEnum), -5, new [] { "-5" } },
+                    { nameof(Model.NullableEnum), int.MaxValue, new [] { "2147483647" } },
+                    { nameof(Model.NullableEnum), (uint)int.MaxValue + 1, new [] { "2147483648" } },
+                    { nameof(Model.NullableEnum), uint.MaxValue, new [] { "4294967295" } },
+
+                    { nameof(Model.RegularEnum), RegularEnum.Zero, new [] { "0", "Zero" } },
+                    { nameof(Model.RegularEnum), RegularEnum.One, new [] { "1", "One" } },
+                    { nameof(Model.RegularEnum), RegularEnum.Two, new [] { "2", "Two" } },
+                    { nameof(Model.RegularEnum), RegularEnum.Three, new [] { "3", "Three" } },
+                    { nameof(Model.RegularEnum), string.Empty, new [] { string.Empty } },
+                    { nameof(Model.RegularEnum), "Zero", new [] { "0", "Zero" } },
+                    { nameof(Model.RegularEnum), "Two", new [] { "2", "Two" } },
+                    { nameof(Model.RegularEnum), "One, Two", new [] { "One, Two", "3", "Three" } },
+                    { nameof(Model.RegularEnum), "garbage", new [] { "garbage" } },
+                    { nameof(Model.RegularEnum), "0", new [] { "0", "Zero" } },
+                    { nameof(Model.RegularEnum), "   43", new [] { "   43", "43" } },
+                    { nameof(Model.RegularEnum), "-5   ", new [] { "-5   ", "-5" } },
+                    { nameof(Model.RegularEnum), 0, new [] { "0", "Zero" } },
+                    { nameof(Model.RegularEnum), 1, new [] { "1", "One" } },
+                    { nameof(Model.RegularEnum), 43, new [] { "43" } },
+                    { nameof(Model.RegularEnum), -5, new [] { "-5" } },
+                    { nameof(Model.RegularEnum), int.MaxValue, new [] { "2147483647" } },
+                    { nameof(Model.RegularEnum), (uint)int.MaxValue + 1, new [] { "2147483648" } },
+                    { nameof(Model.RegularEnum), uint.MaxValue, new [] { "4294967295" } },
+                };
+            }
+        }
+
+        [Theory]
+        [MemberData(nameof(GetCurrentValues_ValueToConvertData))]
+        public void GetCurrentValues_ValueConvertedAsExpected(
+            string propertyName,
+            object rawValue,
+            IReadOnlyCollection<string> expected)
+        {
+            // Arrange
+            var metadataProvider = TestModelMetadataProvider.CreateDefaultProvider();
+            var htmlGenerator = GetGenerator(metadataProvider);
+            var viewContext = GetViewContext<Model>(model: null, metadataProvider: metadataProvider);
+
+            var valueProviderResult = new ValueProviderResult(
+                rawValue,
+                attemptedValue: null,
+                culture: CultureInfo.InvariantCulture);
+            viewContext.ModelState.SetModelValue(propertyName, valueProviderResult);
+
+            // Act
+            var result = htmlGenerator.GetCurrentValues(
+                viewContext,
+                modelExplorer: null,
+                expression: propertyName,
+                allowMultiple: false);
+
+            // Assert
+            Assert.Equal<string>(expected, result);
+        }
+
+        // GetCurrentValues uses only the IModelMetadataProvider passed to the DefaultHtmlGenerator constructor.
+        private static IHtmlGenerator GetGenerator(IModelMetadataProvider metadataProvider)
+        {
+            var mvcOptionsAccessor = new Mock<IOptions<MvcOptions>>();
+            mvcOptionsAccessor.SetupGet(accessor => accessor.Options).Returns(new MvcOptions());
+            var htmlEncoder = Mock.Of<IHtmlEncoder>();
+            var dataOptionsAccessor = new Mock<IOptions<DataProtectionOptions>>();
+            dataOptionsAccessor.SetupGet(accessor => accessor.Options).Returns(new DataProtectionOptions());
+            var antiForgery = new AntiForgery(
+                Mock.Of<IClaimUidExtractor>(),
+                Mock.Of<IDataProtectionProvider>(),
+                Mock.Of<IAntiForgeryAdditionalDataProvider>(),
+                mvcOptionsAccessor.Object,
+                htmlEncoder,
+                dataOptionsAccessor.Object);
+
+            return new DefaultHtmlGenerator(
+                antiForgery,
+                Mock.Of<IScopedInstance<ActionBindingContext>>(),
+                metadataProvider,
+                Mock.Of<IUrlHelper>(),
+                htmlEncoder);
+        }
+
+        // GetCurrentValues uses only the ModelStateDictionary and ViewDataDictionary from the passed ViewContext.
+        private static ViewContext GetViewContext<TModel>(TModel model, IModelMetadataProvider metadataProvider)
+        {
+            var actionContext = new ActionContext();
+            var viewData = new ViewDataDictionary<TModel>(metadataProvider, actionContext.ModelState)
+            {
+                Model = model,
+            };
+
+            return new ViewContext(
+                actionContext,
+                Mock.Of<IView>(),
+                viewData,
+                Mock.Of<ITempDataDictionary>(),
+                TextWriter.Null);
+        }
+
+        public enum RegularEnum
+        {
+            Zero,
+            One,
+            Two,
+            Three,
+        }
+
+        public enum FlagsEnum
+        {
+            None = 0,
+            One = 1,
+            Two = 2,
+            Four = 4,
+            FortyTwo = 42,
+            All = -1,
+        }
+
+        private class Model
+        {
+            public int Id { get; set; }
+
+            public string Name { get; set; }
+
+            public RegularEnum RegularEnum { get; set; }
+
+            public FlagsEnum FlagsEnum { get; set; }
+
+            public RegularEnum? NullableEnum { get; set; }
+
+            public List<string> Collection { get; } = new List<string>();
+        }
+    }
+}

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

@@ -243,12 +243,9 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             Assert.Equal(expectedTagName, output.TagName);
 
             Assert.NotNull(viewContext.FormContext?.FormData);
-            var keyValuePair = Assert.Single(
+            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]
@@ -341,12 +338,9 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             Assert.Equal(expectedTagName, output.TagName);
 
             Assert.NotNull(viewContext.FormContext?.FormData);
-            var keyValuePair = Assert.Single(
+            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);
 
             Assert.Equal(savedDisabled, items.Select(item => item.Disabled));
             Assert.Equal(savedGroup, items.Select(item => item.Group));
@@ -446,12 +440,9 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             Assert.Equal(expectedTagName, output.TagName);
 
             Assert.NotNull(viewContext.FormContext?.FormData);
-            var keyValuePair = Assert.Single(
+            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);
 
             Assert.Equal(savedDisabled, items.Select(item => item.Disabled));
             Assert.Equal(savedGroup, items.Select(item => item.Group));
@@ -504,17 +495,25 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var modelExpression = new ModelExpression(string.Empty, modelExplorer);
             viewContext.ViewData.TemplateInfo.HtmlFieldPrefix = propertyName;
 
-            ICollection<string> selectedValues = new string[0];
+            var currentValues = new string[0];
+            htmlGenerator
+                .Setup(real => real.GetCurrentValues(
+                    viewContext,
+                    modelExplorer,
+                    string.Empty,   // expression
+                    false))         // allowMultiple
+                .Returns(currentValues)
+                .Verifiable();
             htmlGenerator
                 .Setup(real => real.GenerateSelect(
                     viewContext,
                     modelExplorer,
-                    null,         // optionLabel
+                    null,           // optionLabel
-                    string.Empty, // name
+                    string.Empty,   // expression
                     expectedItems,
-                    false,        // allowMultiple
+                    currentValues,
-                    null,         // htmlAttributes
+                    false,          // allowMultiple
-                    out selectedValues))
+                    null))          // htmlAttributes
                 .Returns((TagBuilder)null)
                 .Verifiable();
 
@@ -536,7 +535,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var keyValuePair = Assert.Single(
                 viewContext.FormContext.FormData,
                 entry => entry.Key == SelectTagHelper.SelectedValuesFormDataKey);
-            Assert.Same(selectedValues, keyValuePair.Value);
+            Assert.Same(currentValues, keyValuePair.Value);
         }
 
         [Theory]
@@ -569,17 +568,25 @@ 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];
+            var currentValues = new string[0];
+            htmlGenerator
+                .Setup(real => real.GetCurrentValues(
+                    viewContext,
+                    modelExplorer,
+                    propertyName,   // expression
+                    allowMultiple))
+                .Returns(currentValues)
+                .Verifiable();
             htmlGenerator
                 .Setup(real => real.GenerateSelect(
                     viewContext,
                     modelExplorer,
-                    null,         // optionLabel
+                    null,           // optionLabel
-                    propertyName, // name
+                    propertyName,   // expression
                     It.IsAny<IEnumerable<SelectListItem>>(),
+                    currentValues,
                     allowMultiple,
-                    null,         // htmlAttributes
+                    null))          // htmlAttributes
-                    out selectedValues))
                 .Returns((TagBuilder)null)
                 .Verifiable();
 
@@ -600,7 +607,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var keyValuePair = Assert.Single(
                 viewContext.FormContext.FormData,
                 entry => entry.Key == SelectTagHelper.SelectedValuesFormDataKey);
-            Assert.Same(selectedValues, keyValuePair.Value);
+            Assert.Same(currentValues, keyValuePair.Value);
         }
 
         public class NameAndId