static void Main(string[] args)
{
var observerA = new ConcreteObserverA();
var observerB = new ConcreteObserverB();
var subject = new Subject();
subject.RegisterObserver(observerA);
subject.RegisterObserver(observerB);
subject.NotifyObservers();
Console.WriteLine("--------------------");
subject.UnregisterObserver(observerA);
subject.NotifyObservers();
}