Checkbox Tag-Helper ignores form attribute when hidden input is generated

[m0ddixx]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

I have the following code:

<label form="document-generator-single-form" asp-for="DeadlineToResubmissionDate" class="form-label"></label>
<input form="document-generator-single-form" asp-for="DeadlineToResubmissionDate" type="checkbox" class="form-check"/>
<span form="document-generator-single-form" asp-validation-for="DeadlineToResubmissionDate" class="invalid-feedback"></span>

This code generates a hidden field, which i suppose should represent the payload for this input field when it's not selected:

<input name="DeadlineToResubmissionDate" type="hidden" value="false">

However the form attribute, which should point to the id of the submission form is missing here.
I also conducted the Request-Payload and the DeadlineToResubmissionDate property is missing from the request completely.

Expected Behavior

Either the hidden input should respect the given form from the Tag-Helper, or something is happening inside here, I am missing:

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

Lines 317 to 338 in 53aad98

	if (hiddenForCheckboxTag != null)
	{
	var renderingMode =
	output.TagMode == TagMode.SelfClosing ? TagRenderMode.SelfClosing : TagRenderMode.StartTag;
	hiddenForCheckboxTag.TagRenderMode = renderingMode;
	if (!hiddenForCheckboxTag.Attributes.ContainsKey("name") &&
	!string.IsNullOrEmpty(Name))
	{
	// The checkbox and hidden elements should have the same name attribute value. Attributes will
	// match if both are present because both have a generated value. Reach here in the special case
	// where user provided a non-empty fallback name.
	hiddenForCheckboxTag.MergeAttribute("name", Name);
	}
	if (ViewContext.CheckBoxHiddenInputRenderMode == CheckBoxHiddenInputRenderMode.EndOfForm && ViewContext.FormContext.CanRenderAtEndOfForm)
	{
	ViewContext.FormContext.EndOfFormContent.Add(hiddenForCheckboxTag);
	}
	else
	{
	output.PostElement.AppendHtml(hiddenForCheckboxTag);
	}

Steps To Reproduce

No response

Exceptions (if any)

No response

.NET Version

6.0.411

Anything else?

No response

[m0ddixx]

I'm trying something like the follwoing inside the if statement

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

Line 313 in 53aad98

if (ViewContext.CheckBoxHiddenInputRenderMode != CheckBoxHiddenInputRenderMode.None)

if (htmlAttributes.ContainsKey("form"))
{
    // If the original checkbox has a form attribute, the hidden field should respect it and the
    // attribute should be passed on
    htmlAttributes.TryGetValue("form", out var formAttribute);
    if (formAttribute is string formAttributeString)
    {
        hiddenForCheckboxTag.MergeAttribute("form", formAttributeString);
    }
}

I think I'm missing something while constructing the tests, because I can't get the form attribute into htmlAttributes.

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[javiercn]

@m0ddixx thanks for contacting us.

It's very likely the tag helper doesn't account for the form parameter when emitting the hidden field. It was not a commonly supported browser feature at the time this was implemented.

We would be happy to accept a PR if you want to provide a simple fix for it.