title: asp:RequiredFieldValidator does not validate hidden fields
body: <p>It seems that ASP.NET validators do not validate hidden fields. I get messages like this:</p>
  
  <blockquote>
    <p>Control 'hiddenField' referenced by the ControlToValidate property of 'hiddenFieldValidator' cannot be validated.</p>
  </blockquote>
  
  <p>I have an <code>&lt;asp:HiddenField&gt;</code> in my page which gets filled client side with some value. I need this to be present once on the server so I added a <code>RequiredFieldValidator</code> to it.</p>
  
  <p>And it does not work!</p>
  
  <p>As I see it, as a workaround, I can:</p>
  
  <p><strong>1.</strong> use a custom validator and not tie it to the hidden field, just call a method on <code>OnServerValidate</code>;</p>
  
  <p><strong>2.</strong> Use a <code>&lt;asp:TextBox&gt;</code> with a CSS style <code>display:none</code> and it should work.</p>
  
  <p>But I want to make sure I am not missing something here. Is it possible or not to validate a hidden field in the same way as the other text fields? O maybe a third, more elegant option?</p>
  
  <p>TIA!</p>