fix: Account for form attribute on checkbox tag input helper #49201 (#49323)

* fix: account for form attribute on checkbox tag input helper

* add xmldoc for FormName

* updated Public API specs

* formatting

Author: m0ddixx

src/Mvc/Mvc.TagHelpers/src/InputTagHelper.cs

@@ -116,6 +116,15 @@ public InputTagHelper(IHtmlGenerator generator)
     [HtmlAttributeName("type")]
     public string InputTypeName { get; set; }
 
+    /// <summary>
+    /// The name of the associated form
+    /// </summary>
+    /// <remarks>
+    /// Used to associate a hidden checkbox tag to the respecting form when <see cref="CheckBoxHiddenInputRenderMode"/> is not <see cref="CheckBoxHiddenInputRenderMode.None"/>.
+    /// </remarks>
+    [HtmlAttributeName("form")]
+    public string FormName { get; set; }
+
     /// <summary>
     /// The name of the &lt;input&gt; element.
     /// </summary>
@@ -161,6 +170,11 @@ public override void Process(TagHelperContext context, TagHelperOutput output)
             output.CopyHtmlAttribute(nameof(Value), context);
         }
 
+        if (FormName != null)
+        {
+            output.CopyHtmlAttribute("form", context);
+        }
+
         // Note null or empty For.Name is allowed because TemplateInfo.HtmlFieldPrefix may be sufficient.
         // IHtmlGenerator will enforce name requirements.
         var metadata = For.Metadata;
@@ -328,6 +342,16 @@ private TagBuilder GenerateCheckBox(
                     hiddenForCheckboxTag.MergeAttribute("name", Name);
                 }
 
+                if (output.Attributes.TryGetAttribute("form", out var formAttribute))
+                {
+                    // If the original checkbox has a form attribute, the hidden field should respect it and the
+                    // attribute should be passed on
+                    if (formAttribute.Value is string formAttributeString)
+                    {
+                        hiddenForCheckboxTag.MergeAttribute("form", formAttributeString);
+                    }
+                }
+
                 if (ViewContext.CheckBoxHiddenInputRenderMode == CheckBoxHiddenInputRenderMode.EndOfForm && ViewContext.FormContext.CanRenderAtEndOfForm)
                 {
                     ViewContext.FormContext.EndOfFormContent.Add(hiddenForCheckboxTag);

src/Mvc/Mvc.TagHelpers/src/PublicAPI.Unshipped.txt

@@ -1 +1,3 @@
 #nullable enable
+~Microsoft.AspNetCore.Mvc.TagHelpers.InputTagHelper.FormName.get -> string
+~Microsoft.AspNetCore.Mvc.TagHelpers.InputTagHelper.FormName.set -> void

src/Mvc/Mvc.TagHelpers/test/InputTagHelperTest.cs

@@ -1319,6 +1319,69 @@ public async Task ProcessAsync_CallsGenerateTextBox_WithExpectedParametersForHid
         Assert.Equal(expectedTagName, output.TagName);
     }
 
+    [Fact]
+    public async Task ProcessAsync_GenerateCheckBox_WithHiddenInputExpectedFormAttribute()
+    {
+        var propertyName = "-expression-";
+        var expectedTagName = "input";
+        var formName = "form-name";
+        var expectedEndOfFormContent = $"<input form=\"HtmlEncode[[{formName}]]\" name=\"HtmlEncode[[{propertyName}]]\" " +
+            "type=\"HtmlEncode[[hidden]]\" value=\"HtmlEncode[[false]]\" />";
+        var inputTypeName = "checkbox";
+        var expectedAttributes = new TagHelperAttributeList
+            {
+                { "name",  propertyName },
+                { "type", inputTypeName },
+                { "form", formName },
+                { "value", "true" },
+            };
+
+        var metadataProvider = new EmptyModelMetadataProvider();
+        var htmlGenerator = new TestableHtmlGenerator(metadataProvider);
+        var model = false;
+        var modelExplorer = metadataProvider.GetModelExplorerForType(typeof(bool), model);
+        var modelExpression = new ModelExpression(name: string.Empty, modelExplorer: modelExplorer);
+        var viewContext = TestableHtmlGenerator.GetViewContext(model, htmlGenerator, metadataProvider);
+        viewContext.FormContext.CanRenderAtEndOfForm = true;
+        viewContext.CheckBoxHiddenInputRenderMode = CheckBoxHiddenInputRenderMode.EndOfForm;
+
+        var tagHelper = new InputTagHelper(htmlGenerator)
+        {
+            For = modelExpression,
+            InputTypeName = inputTypeName,
+            Name = propertyName,
+            FormName = formName,
+            ViewContext = viewContext,
+        };
+
+        var attributes = new TagHelperAttributeList
+            {
+                { "name", propertyName },
+                { "type", inputTypeName },
+                { "form", formName}
+            };
+
+        var context = new TagHelperContext(attributes, new Dictionary<object, object>(), "test");
+        var output = new TagHelperOutput(
+            expectedTagName,
+            new TagHelperAttributeList(),
+            getChildContentAsync: (useCachedResult, encoder) => Task.FromResult<TagHelperContent>(result: null))
+        {
+            TagMode = TagMode.SelfClosing,
+        };
+
+        // Act
+        await tagHelper.ProcessAsync(context, output);
+
+        // Assert
+        Assert.Equal(expectedAttributes, output.Attributes);
+        Assert.False(output.IsContentModified);
+        Assert.Equal(expectedTagName, output.TagName);
+
+        Assert.Equal(expectedEndOfFormContent, string.Join("", viewContext.FormContext.EndOfFormContent.Select(html => HtmlContentUtilities.HtmlContentToString(html))));
+        Assert.True(string.IsNullOrEmpty(HtmlContentUtilities.HtmlContentToString(output.PostElement)));
+    }
+
     [Theory]
     [InlineData(null, "password", null)]
     [InlineData(null, "Password", "not-null")]