Silverlight4.0 TreeViewItem Drag Drop

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C#

C#4.0

Silverlight4

Hi All,
I have a question 'how to cancel drag.'

In my case. when customer drag treeViewItem to target, show messageBox, if 'Cancel' button click, give up drag operation.

Follow is my code, but it dose not work.pls help!

XAML:

   <my:TreeViewDragDropTarget x:Name="tvddtCategroy" AllowDrop="true" AllowedSourceEffects="Move" ItemDroppedOnTarget="tvddtCategroy_ItemDroppedOnTarget">
      <controls:TreeView x:Name="tvCategory" BorderBrush="White" AllowDrop="True" 
      </controls:TreeView>
</my:TreeViewDragDropTarget>

C#:

private void tvddtCategroy_ItemDroppedOnTarget(object sender, ItemDragEventArgs e)
        {
            if (MessageBox.Show("Are you sure to move this category?", "", MessageBoxButton.OKCancel) == MessageBoxResult.Cancel)
            {
                e.Cancel = true;
                e.Handled = true;
            }
        }