Validate input arrays

Hi,

Can i know how to validate input arrays in Codeingiter 4?

PHP Code:

<?= form_dropdown(array('name' => 'upload_purpose[]', 'class' => 'form-control upload_purpose', 'options' => $cc_fileds)); ?>

PHP Code:

$rules = [
 'upload_purpose[]' => ['label' => 'Upload Fields', 'rules' =>'required'],
]; 


I tried above an it doesn't work. it always displays the error message even though parameters were passed