private void Dispose(bool disposing)
 {
     if (disposing)
     {
         if (_subject != null)
         {
             _subject.Unsubscribe(_observer);
             _subject  = null;
             _observer = null;
         }
     }
 }
 public DisposableToken(RelayObservable <T> subject, IObserver <T> observer)
 {
     _subject  = subject;
     _observer = observer;
 }