static void Main(string[] args)
{
var subject = new Subject();
var observerA = new Observer("observerA");
var observerB = new Observer("observerB");
var observerC = new Observer("observerC");
var observerD = new Observer("observerD");
observerA.Subscribe(subject);
observerB.Subscribe(subject);
// change the subjects number
subject.SetNumber(10);
observerC.Subscribe(subject);
observerB.Unsubscribe();
// weirdly I can subscribe observerD here but to unsubscribe I need to keep the unsubscribe object returned
Console.WriteLine("Subscribing observerD from somewhere else!");
var unsubcriberD = subject.Subscribe(observerD);
subject.SetNumber(99);
Console.WriteLine("Unsubscribing observerD from somewhere else!");
unsubcriberD.Dispose();
subject.SetNumber(321);
Console.WriteLine("Press any key to exit...");
Console.ReadKey();
}