A while loop isn't in itself an iterator. There's a difference between

  while(collection.Count > 0) 
  {
   collection.Delete(0);
  }

.. which is a working implementation of clear, though probably superfluous.

and...

  int i = 0;
  while(i < collection.Count) 
  {
   collection.Delete(i);
   i++;
  }

which does try to walk along the list and is broken.

Also, .Net has a specific exception to stop you modifying a collection while an enumerator is walking along it. Google for "Collection was modified; enumeration operation may not execute"