codeigniter-file-upload-required-validation

We write for validation:: 

$this->form_validation->set_rules('name', 'Name', 'trim|required');

$this->form_validation->set_rules('code', 'Code', 'trim|required');

$this->form_validation->set_rules('userfile', 'Document', 'required');

Now have to write like, :

$this->form_validation->set_rules('name', 'Name', 'trim|required');

$this->form_validation->set_rules('code', 'Code', 'trim|required');

if (empty($_FILES['userfile']['name']))

{

    $this->form_validation->set_rules('userfile', 'Document', 'required');

}