Observer pattern c#

Receive update from subject, we create an interface we call IObserver.

public interface IObserver
{
    // Receive update from subject
    void Update(ISubject subject);
}

We create another interface we call the ISubject. The method of this interface are Attach, Detach and Notify. Attach attaches the observer to the subject. Detach detaches them. Notify will notify of the process.

public interface ISubject
{
    void Attach(IObserver observer);
    void Detach(IObserver observer);
    void Notify();
}

Let's create a couple of Observers we call Observer1 and Observer2. Concrete Observers react to the updates issued by the Subject they had been attached to.

class Observer1 : IObserver
{
    public void Update(ISubject subject)
    {            
        if ((subject as Subject).State < 3)
        {
           Console.WriteLine("Concrete Observer1: Reacted to the event.");
        }
    }
}

Let's also create the second Observer Observer2.

class Observer2 : IObserver
{
    public void Update(ISubject subject)
    {
        if ((subject as Subject).State == 0 || (subject as Subject).State >= 2)
        {
            Console.WriteLine("Observer2: Reacted to the event.");
        }
    }
}

Now let's put this all together in the Main method found in the Program class.

static void Main(string[] args)
{
    var subject = new Subject();
    var observer1 = new Observer1();
    subject.Attach(observer1);
    var observer2 = new Observer2();
    subject.Attach(observer2);

    subject.SomeBusinessLogic();
    subject.SomeBusinessLogic();
    subject.Detach(observer2);
    subject.SomeBusinessLogic();
    }
}

When we compiled this and run it, we should get

Subject: Attached an observer.
Subject: Attached an observer.

Subject: I'm doing something important.
Subject: My state has just changed to: 1
Subject: Notifying observers...
Concrete Observer1: Reacted to the event.

Subject: I'm doing something important.
Subject: My state has just changed to: 4
Subject: Notifying observers...
Observer2: Reacted to the event.
Subject: Detached an observer.

Subject: I'm doing something important.
Subject: My state has just changed to: 6
Subject: Notifying observers...

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