Reactive

// handcrafted observables
// not recommended

void Main()
{
	var numbers = new NumbersObservable(5);
	var subscription = numbers.Subscribe(new ConsoleObserver<int>("numbers"));
	
	var numbersExt = new NumbersObservable(5);
	numbersExt.SubscribeConsole("numbersExt");
}

public class NumbersObservable : IObservable<int>
{
	
	private readonly int _amount;
	
	public NumbersObservable(int amount)
	{
		_amount = amount;
	}
	
	public IDisposable Subscribe(IObserver<int> observer)
	{
		for (int i = 0; i < _amount; i++)
		{
			observer.OnNext(i);
		}
		
		observer.OnCompleted();
		return Disposable.Empty;
	}
	
}

public class ConsoleObserver<T> : IObserver<T>
{
	private readonly string _name;
	public ConsoleObserver(string name = "")
	{
		_name = name;
	}
	
	public void OnNext(T value)
	{
		Console.WriteLine("{0} - OnNext({1})", _name, value);
	}

	public void OnError(Exception error)
	{
		Console.WriteLine("{0} - OnError", _name);
		Console.WriteLine("\t {0}", error);
	}
	
	public void OnCompleted()
	{
		Console.WriteLine("{0} - OnCompleted", _name);
	}
	
}


public static class Extensions
{
	public static IDisposable SubscribeConsole<T>(this IObservable<T> observable, string name = "")
	{
		return observable.Subscribe(new ConsoleObserver<T>(name));
	}
}

// Observable.Create()

void Main()
{
	// using System.Reactive.Linq
	var observable = Observable.Create<int>(observer =>
	{
		for (int i = 0; i < _amount; i++)
		{
			observer.OnNext(i);
		}
		
		observer.OnCompleted();
		return Disposable.Empty;
	});
}

Source