How can I post the same data to two different handlers depending on the button clicked? - asp.net-core

Question

[See updates at bottom]
I have a Razor page with a form on it. I want to have two buttons on that form, that perform a slightly different action - both using the same posted form data.
I tried using the asp-page-handler helper on the second button, but it doesn't seem to add anything to the HTML (I would expect it to add a formaction attribute to the <button> element, but it doesn't add anything at all).
Here's an example page:
#page "{id?}"
#model IndexModel
#tagHelperPrefix x:
#addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers
<p>Current value is #Model.Foo</p>
<x:form method="post">
<input type="text" name="foo" />
<button type="submit">Default</button>
<button type="submit" x:asp-page-handler="Alternative">Alternative</button>
</x:form>
... and here's the corresponding page model:
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.RazorPages;
namespace MyWebApplication.Pages
{
public class IndexModel : PageModel
{
[BindProperty]
public string Foo { get; set; }
public void OnGet(int? id)
{
}
public void OnPostAsync(string foo)
{
Foo = foo;
}
public void OnPostAlternativeAsync(string foo)
{
Foo = foo.ToUpper();
}
}
}
This is rendered as:
...where the generated HTML for the form is:
<form method="post">
<input type="text" name="foo" />
<button type="submit">Default</button>
<button type="submit" x:asp-page-handler="Alternative">Alternative</button>
</form>
The fact that the x:asp-page-handler attribute is still in the generated HTML makes me think that the Razor engine hasn't recognized it. I've tried taking off the x: prefix, but that didn't help.
What am I doing wrong?
UPDATE
OK, I tried removing the tag prefix and removing the #tagHelperPrefix line, and that made a difference. A formaction is added to the second <button> element as expected.
However:
that's really annoying - the #tagHelperPrefix is not something I want to lose, and
now both buttons are triggering the "Alternative" action, even though only one of them has the formaction!
Here's the new generated HTML:
<form method="post">
<input type="text" name="foo" />
<button type="submit">Default</button>
<button type="submit" formaction="/?handler=Alternative">Alternative</button>
</form>
SECOND UPDATE
OK, so If I put asp-page-handler="" on the "default" button, then each button goes to the correct handler, which is fine.
The last question that remains, then, is: how can I make this work with the tag helper prefix?

Answer 1

[Answering my own question in case this helps others.]
It turns out that:
The tag-helper-prefix only applies to elements, not attributes, so it should be asp-page-handler="..." rather than x:asp-page-handler="..." even if the tag-helper-prefix is x:.
Those asp- attributes are only recognized within a tag that is tag-helper-enabled - which is all elements when no tag-helper-prefix is specified, or only elements with the tag-helper-prefix where one is specified. In my case, I had to change <button ...> to <x:button ...>.
If you specify asp-page-handler for one button, you need to specify it on all the buttons, even if you specify it as "" to get the default action.