title: DependencyProperty getter/setter not being called
body: <p>I am trying to create a Custom control derived from a standard Grid. 
  I added a ObservableCollection as a DependencyProperty of the Custom control. However, the get/set of it is never reached. Can I have some guidelines in creating a DependencyProperty that works correctly with and ObservableCollection?</p>
  
  <pre><code>public class MyGrid : Grid
  {
      public ObservableCollection&lt;string&gt; Items
      {
          get
          {
              return (ObservableCollection&lt;string&gt;)GetValue(ItemsProperty);
          }
          set
          {
              SetValue(ItemsProperty, value);
          }
      }
  
  public static  DependencyProperty ItemsProperty =
                  DependencyProperty.Register("Items", typeof(ObservableCollection&lt;string&gt;), 
          typeof(MyGrid), new UIPropertyMetadata(null, OnItemsChanged));
  
  }
  </code></pre>