public Unsubscriber(ObservableSink sink, IObserver <LogEvent> observer)
 {
     if (sink == null)
     {
         throw new ArgumentNullException(nameof(sink));
     }
     if (observer == null)
     {
         throw new ArgumentNullException(nameof(observer));
     }
     _sink     = sink;
     _observer = observer;
 }
 public Unsubscriber(ObservableSink sink, IObserver <LogEvent> observer)
 {
     _sink     = sink ?? throw new ArgumentNullException(nameof(sink));
     _observer = observer ?? throw new ArgumentNullException(nameof(observer));
 }