[Fixes #4047] SelectTagHelpers: asp-items shouldn't require/depend on asp-for.

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

@@ -15,9 +15,11 @@ using Microsoft.AspNetCore.Razor.TagHelpers;
 namespace Microsoft.AspNetCore.Mvc.TagHelpers
 {
     /// <summary>
-    /// <see cref="ITagHelper"/> implementation targeting &lt;select&gt; elements with an <c>asp-for</c> attribute.
+    /// <see cref="ITagHelper"/> implementation targeting &lt;select&gt; elements with <c>asp-for</c> and/or
+    /// <c>asp-items</c> attribute(s).
     /// </summary>
     [HtmlTargetElement("select", Attributes = ForAttributeName)]
+    [HtmlTargetElement("select", Attributes = ItemsAttributeName)]
     public class SelectTagHelper : TagHelper
     {
         private const string ForAttributeName = "asp-for";
@@ -70,6 +72,13 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                 throw new ArgumentNullException(nameof(context));
             }
 
+            if (For == null)
+            {
+                // Informs contained elements that they're running within a targeted <select/> element.
+                context.Items[typeof(SelectTagHelper)] = null;
+                return;
+            }
+
             // Note null or empty For.Name is allowed because TemplateInfo.HtmlFieldPrefix may be sufficient.
             // IHtmlGenerator will enforce name requirements.
             if (For.Metadata == null)
@@ -101,9 +110,6 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
 
         /// <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)
@@ -119,6 +125,13 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             // Ensure GenerateSelect() _never_ looks anything up in ViewData.
             var items = Items ?? Enumerable.Empty<SelectListItem>();
 
+            if (For == null)
+            {
+                var options = Generator.GenerateGroupsAndOptions(optionLabel: null, selectList: items);
+                output.PostContent.AppendHtml(options);
+                return;
+            }
+
             var tagBuilder = Generator.GenerateSelect(
                 ViewContext,
                 For.ModelExplorer,

src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewFeatures/DefaultHtmlGenerator.cs

@@ -1457,7 +1457,8 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
             return newSelectList;
         }
 
-        private IHtmlContent GenerateGroupsAndOptions(string optionLabel, IEnumerable<SelectListItem> selectList)
+        /// <inheritdoc />
+        public IHtmlContent GenerateGroupsAndOptions(string optionLabel, IEnumerable<SelectListItem> selectList)
         {
             var listItemBuilder = new HtmlContentBuilder();
 

src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewFeatures/IHtmlGenerator.cs

@@ -307,6 +307,19 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
             bool allowMultiple,
             object htmlAttributes);
 
+        /// <summary>
+        /// Generates &lt;optgroup&gt; and &lt;option&gt; elements.
+        /// </summary>
+        /// <param name="optionLabel">Optional text for a default empty &lt;option&gt; element.</param>
+        /// <param name="selectList">
+        /// A collection of <see cref="SelectListItem"/> objects used to generate &lt;optgroup&gt; and &lt;option&gt;
+        /// elements.
+        /// </param>
+        /// <returns>
+        /// An <see cref="IHtmlContent"/> instance for &lt;optgroup&gt; and &lt;option&gt; elements.
+        /// </returns>
+        IHtmlContent GenerateGroupsAndOptions(string optionLabel, IEnumerable<SelectListItem> selectList);
+
         TagBuilder GenerateTextArea(
             ViewContext viewContext,
             ModelExplorer modelExplorer,

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

@@ -356,6 +356,80 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             Assert.Equal(savedValue, items.Select(item => item.Value));
         }
 
+        [Fact]
+        public async Task ProcessAsync_WithItems_AndNoModelExpression_GeneratesExpectedOutput()
+        {
+            // Arrange
+            var originalAttributes = new TagHelperAttributeList
+            {
+                { "class", "form-control" },
+            };
+            var originalPostContent = "original content";
+
+            var expectedAttributes = new TagHelperAttributeList(originalAttributes);
+            var selectItems = new SelectList(Enumerable.Range(0, 5));
+            var expectedOptions = "<option>HtmlEncode[[0]]</option>" + Environment.NewLine
+                + "<option>HtmlEncode[[1]]</option>" + Environment.NewLine
+                + "<option>HtmlEncode[[2]]</option>" + Environment.NewLine
+                + "<option>HtmlEncode[[3]]</option>" + Environment.NewLine
+                + "<option>HtmlEncode[[4]]</option>" + Environment.NewLine;
+
+            var expectedPreContent = "original pre-content";
+            var expectedContent = "original content";
+            var expectedPostContent = originalPostContent + expectedOptions;
+            var expectedTagName = "select";
+
+            var tagHelperContext = new TagHelperContext(
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
+                items: new Dictionary<object, object>(),
+                uniqueId: "test");
+            var output = new TagHelperOutput(
+                expectedTagName,
+                originalAttributes,
+                getChildContentAsync: (useCachedResult, encoder) =>
+                {
+                    var tagHelperContent = new DefaultTagHelperContent();
+                    tagHelperContent.AppendHtml("Something");
+                    return Task.FromResult<TagHelperContent>(tagHelperContent);
+                })
+            {
+                TagMode = TagMode.SelfClosing,
+            };
+            output.PreContent.AppendHtml(expectedPreContent);
+            output.Content.AppendHtml(expectedContent);
+            output.PostContent.AppendHtml(originalPostContent);
+
+            var metadataProvider = new TestModelMetadataProvider();
+            var htmlGenerator = new TestableHtmlGenerator(metadataProvider);
+            var viewContext = TestableHtmlGenerator.GetViewContext(
+                model: null,
+                htmlGenerator: htmlGenerator,
+                metadataProvider: metadataProvider);
+
+            var tagHelper = new SelectTagHelper(htmlGenerator)
+            {
+                Items = selectItems,
+                ViewContext = viewContext,
+            };
+
+            // Act
+            tagHelper.Init(tagHelperContext);
+            await tagHelper.ProcessAsync(tagHelperContext, output);
+
+            // Assert
+            Assert.Equal(TagMode.SelfClosing, output.TagMode);
+            Assert.Equal(expectedAttributes, output.Attributes);
+            Assert.Equal(expectedPreContent, output.PreContent.GetContent());
+            Assert.Equal(expectedContent, output.Content.GetContent());
+            Assert.Equal(expectedPostContent, HtmlContentUtilities.HtmlContentToString(output.PostContent));
+            Assert.Equal(expectedTagName, output.TagName);
+
+            var kvp = Assert.Single(tagHelperContext.Items);
+            Assert.Equal(typeof(SelectTagHelper), kvp.Key);
+            Assert.Null(kvp.Value);
+        }
+
         [Theory]
         [MemberData(nameof(GeneratesExpectedDataSet))]
         public async Task ProcessAsyncInTemplate_WithItems_GeneratesExpectedOutput_DoesNotChangeSelectList(