title: How to add validation to PropertyGrid's CollectionEditor?
body: <p>I'm using PropertyGrid to edit an object containing a collection.
  Collection is edited using the CollectionEditor.
  I have to make sure elements in collection are unique.</p>
  
  <p>How can I add validation to CollectionEditor:</p>
  
  <ol>
  <li>By either overloading CollectionEditor's OnFormClosing</li>
  <li>Or adding validation for creating/editing items?</li>
  </ol>