title: Linq - OfType<> not working as expected
body: <p>I have the following code which detects all the elements in a Silverlight application beneath a certain point</p>
  
  <p>then filters them to be only those of a particular type - <code>CardButton</code></p>
  
  <pre><code>IEnumerable&lt;UIElement&gt; elementsBeneathCursor =
                  VisualTreeHelper.FindElementsInHostCoordinates(new Point(xPosn, yPosn), Application.Current.RootVisual);
              IEnumerable&lt;CardButton&gt; cardsBeneathCursor = elementsBeneathCursor.OfType&lt;CardButton&gt;();
  </code></pre>
  
  <p>Even though when I inspect <code>elementsBeneathCursor</code> in the debugger, I can see there are 2 elements of type <code>CardButton</code>
  Yet when I apply the <code>OfType&lt;&gt;</code> filter the resultant list is null</p>
  
  <p>what's going wrong?</p>