// When one of our custom devices is removed, we want to make sure that if
 // it is the '.current' device, we null out '.current'.
 protected override void OnRemoved()
 {
     base.OnRemoved();
     if (current == this)
     {
         current = null;
     }
 }
 public override void MakeCurrent()
 {
     base.MakeCurrent();
     current = this;
 }