title: How to make a custom control validatable?
body: <p>I have a custom control based on a <code>Panel</code>. It's merely a simple panel with 3 DropDownLists. If I add a <code>CompareValidator</code> to my WebForm and set the <code>ControlToValidate</code> property to the ID of my custom control I get the following error:</p>
  
  <p><code>Control '{0}' referenced by the ControlToValidate property of '{1}' cannot be validated.</code></p>
  
  <p>I understand why, the CompareValidator cannot sensibly validate anything against the <code>Panel</code>. So what do I need to add to my custom control to make it possible for a validator to be able to validate it?</p>
  
  <p>I have tried adding a <code>Text</code> property to my Panel which returns a combination of the Text properties of the 3 DropDownLists. No avail.</p>
  
  <p>I have tried looking for certain interfaces (<code>ITextControl</code>) that validatable controls (<code>TextBox</code>) implement and add them to my custom control. No avail.</p>
  
  <p>How can I make my custom control based on a Panel, validatable by the regular .net validators? (RequiredFieldValidator, CompareValidator, etc.)</p>