class Program    {        public static void Main()        {            // Define a provider and two observers.            LocationTracker provider = new LocationTracker();            LocationReporter reporter1 = new LocationReporter("FixedGPS");            reporter1.Subscribe(provider);            LocationReporter reporter2 = new LocationReporter("MobileGPS");            reporter2.Subscribe(provider);            provider.TrackLocation(new Location(47.6456, -122.1312));            reporter1.Unsubscribe();            provider.TrackLocation(new Location(47.6677, -122.1199));            provider.TrackLocation(null);            provider.EndTransmission();            Console.ReadLine();            // The example displays output similar to the following:            //      FixedGPS: The current location is 47.6456, -122.1312            //      MobileGPS: The current location is 47.6456, -122.1312            //      MobileGPS: The current location is 47.6677, -122.1199            //      MobileGPS: The location cannot be determined.            //      The Location Tracker has completed transmitting data to MobileGPS.        }    }    public struct Location    {        double lat, lon;        public Location(double latitude, double longitude)        {            this.lat = latitude;            this.lon = longitude;        }        public double Latitude        { get { return this.lat; } }        public double Longitude        { get { return this.lon; } }    }    public class LocationReporter : IObserver
     
          {        private IDisposable unsubscriber;        private string instName;        public LocationReporter(string name)        {            this.instName = name;        }        public string Name        { get { return this.instName; } }        public virtual void Subscribe(IObservable
      
        provider)        {            if (provider != null)                unsubscriber = provider.Subscribe(this);        }        public virtual void OnCompleted()        {            Console.WriteLine("The Location Tracker has completed transmitting data to {0}.", this.Name);            this.Unsubscribe();        }        public virtual void OnError(Exception e)        {            Console.WriteLine("{0}: The location cannot be determined.", this.Name);        }        public virtual void OnNext(Location value)        {            Console.WriteLine("{2}: The current location is {0}, {1}", value.Latitude, value.Longitude, this.Name);        }        public virtual void Unsubscribe()        {            unsubscriber.Dispose();        }    }    public class LocationTracker : IObservable
       
            {        public LocationTracker()        {            observers = new List
        
         
          >();        }        private List
          
           
            > observers; public IDisposable Subscribe(IObserver
            
              observer) { if (!observers.Contains(observer)) observers.Add(observer); return new Unsubscriber(observers, observer); } private class Unsubscriber : IDisposable { private List
             
              
               > _observers; private IObserver
               
                 _observer; public Unsubscriber(List
                
                 
                  > observers, IObserver
                  
                    observer) { this._observers = observers; this._observer = observer; } public void Dispose() { if (_observer != null && _observers.Contains(_observer)) _observers.Remove(_observer); } } public void TrackLocation(Nullable
                   
                     loc) { foreach (var observer in observers) { if (!loc.HasValue) observer.OnError(new LocationUnknownException()); else observer.OnNext(loc.Value); } } public void EndTransmission() { foreach (var observer in observers.ToArray()) if (observers.Contains(observer)) observer.OnCompleted(); observers.Clear(); } } public class LocationUnknownException : Exception { internal LocationUnknownException() { } }