Using getBlockPrefix with null creates invalid html5 tag on form

[ktwbc]

There are use cases for returning null in a FormType definition with the call getBlockPrefix, for example to for a GET form to only use the form values in the resulting URL without a form name inside square brackets.

A typical case would be to simply add

 public function getBlockPrefix()
{
    return null;
}

to the formType.

However, this name is also being used to build the opening

tag when using
{{ form_start(form) }}
in the twig template. As a result of the null name, your form tag is created as:

<form name="" method="get">

The empty name "" is invalid HTML5 and will fail an HTML scan.

It would be preferable to simply leave the name off altogether in this case, i.e.

<form method="get">

if blockprefix is null.

[ogizanagi]

You shouldn't overwrite getBlockPrefix for this use case, but instead create your form with an empty name:

$form = $this->get('form.factory')-> createNamed('', MyFormType::class, $data, $options);
// or:
$formBuilder = $this->get('form.factory')-> createNamedBuilder('', MyFormType::class, $data, $options);

Actually, your solution was only working previously because of the backward compatibility layer in 2.8 branch.
The block prefix is used for form theming, in order to choose which block should be rendered. It must not be null.

However, it won't solve the issue you've mentioned, because of this line in the form_div_layout, which does not perform any check on the name emptiness before rendering the attribute.

Perhaps you could submit a PR on the 2.7 — actually 2.8 according to the consensus for a similar case in #13852 — branch in order to suggest it as a bug fix ?

[xabbuh]

Closing as this is not a bug as @ogizanagi explained.