using System;

using System.Collections.Generic;       

public class Program{

        public static void Main(string[] args){

            var fbObservable = new NotificationProvider("Facebook");

            var githubObservable = new NotificationProvider("GitHub");

​

            var observer = new NotificationSubscriber("Florin");

            observer.Subscribe(fbObservable);

            //observer.Unsubscribe();

​

            observer.Subscribe(githubObservable);

            //observer.Unsubscribe();

​

            var observer2 = new NotificationSubscriber("Piagio");

            observer2.Subscribe(fbObservable);

​

            fbObservable.EventNotification("Event notification 1 !");

            githubObservable.EventNotification("Event notification!");

        }

    }

​

public class NotificationProvider : IObservable<SomeEvent>{

    public string ProviderName { get; private set; }

    // Maintain a list of observers

    private List<IObserver<SomeEvent>> _observers;

​

    public NotificationProvider(string _providerName){

        ProviderName = _providerName;

        _observers = new List<IObserver<SomeEvent>>();

    }

​

    // Define Unsubscriber class

    private class Unsubscriber : IDisposable{

        private List<IObserver<SomeEvent>> _observers;

        private IObserver<SomeEvent> _observer;

​

        public Unsubscriber(List<IObserver<SomeEvent>> observers,

                            IObserver<SomeEvent> observer){

            this._observers = observers;

            this._observer = observer;

        }

​

        public void Dispose(){

            if (!(_observer == null)) _observers.Remove(_observer);

        }

    }

​

    // Define Subscribe method

    public IDisposable Subscribe(IObserver<SomeEvent> observer){

        if (!_observers.Contains(observer))

            _observers.Add(observer);

        return new Unsubscriber(_observers, observer);

    }

​

    // Notify observers when event occurs

    public void EventNotification(string description){

        foreach (var observer in _observers){

            observer.OnNext(new SomeEvent(ProviderName, description,

                            DateTime.Now));

        }

    }

}

​

public class NotificationSubscriber : IObserver<SomeEvent>{

    public string SubscriberName { get; private set; }

    private IDisposable _unsubscriber;

​

    public NotificationSubscriber(string _subscriberName){

        SubscriberName = _subscriberName;

    }

​

    public virtual void Subscribe(IObservable<SomeEvent> provider){

        // Subscribe to the Observable

        if (provider != null)

            _unsubscriber = provider.Subscribe(this);

    }

​

    public virtual void OnCompleted(){

        Console.WriteLine("Done");

    }

​

    public virtual void OnError(Exception e){

        Console.WriteLine($"Error: {e.Message}");

    }

​

    public virtual void OnNext(SomeEvent ev){

        Console.WriteLine($"Hey {SubscriberName} -> you received {ev.EventProviderName} {ev.Description} @ {ev.Date} ");

    }

​

    public virtual void Unsubscribe(){

        _unsubscriber.Dispose();

    }

}

​

public class SomeEvent{

    public string EventProviderName { get; set; }

    public string Description { get; set; }

    public DateTime Date { get; set; }

​

    public SomeEvent(string _eventProviderName, string _description, DateTime _date){

        EventProviderName = _eventProviderName;

        Description = _description;

        Date = _date;

    }

}