观察者模式

public abstract class Subject
    {
        protected IList<Observer> observers = new List<Observer>();

        protected int state = 0;

        public abstract void Notify();

        public abstract void Attach(Observer observer);

        public abstract void Detach(Observer observer);

        public abstract int GetState();

        public abstract void ChangeState(int state);
    }

    public class ConcurentSubject : Subject
    {
        public override void Attach(Observer observer)
        {
            observers.Add(observer);
        }

        public override void Detach(Observer observer)
        {
            //remove observer from observers
        }

        public override void Notify()
        {
            foreach (var item in observers)
            {
                item.Update(this);
            }
        }

        public override int GetState()
        {
            return state;
        }

        public override void ChangeState(int state)
        {
            this.state = state;
        }
    }

public abstract class Observer
    {
        public abstract void Update(Subject sub);
    }

    public class ConcurentObserver : Observer
    {
        public override void Update(Subject sub)
        {
            int state = sub.GetState();
            Console.WriteLine("new state is {0}", state);
        }
    }

public class Client
    {
        public static void Do()
        {
            Subject subject = new ConcurentSubject();
            Observer observer1 = new ConcurentObserver();
            Observer observer2 = new ConcurentObserver();
            subject.Attach(observer1);
            subject.Attach(observer2);
            subject.ChangeState(1);
            subject.Notify();
            subject.ChangeState(2);
            subject.Notify();
            Console.ReadKey();
        }
    }