How to detect mouse release during drag-drop
===

I have a custom WPF control which handles drag/drop. I override OnDragOver so that the control will not accept the dropped object if it is busy doing something else:

    protected override void OnDragOver(DragEventArgs e)
    {
         base.OnDragOver(e);
    
         if (isBusy)     
              e.Effects = DragDropEffects.None;
         else
              e.Effects = DragDropEffects.Move;
      
         e.Handled = true;
    }

In another control which initiates the drag&drop, there is some UI element which is disabled when the operation starts and supposed to be enabled if the operation is cancelled or when the mouse is released on the target while the above target says the operation is not allowed. 

What events can I use on the source control to check for the 2nd condition? 

