设计模式：观察者模式

{
    public abstract class Subject
    {
        private IList<Observer> observers = new List<Observer>();
        public void Attach(Observer observer)
        {
            observers.Add(observer);
        }
        public void Detach(Observer observer)
        {
            observers.Remove(observer);
        }
        public void Notify()
        {
            foreach(Observer observer in observers)
            {
                observer.Update();
            }
        }
    }
    public abstract class Observer
    {
        public abstract void Update();
    }
    public class ConcreteSubject : Subject
    {
        private string subjectState;
        public string SubjectState
        {
            get { return subjectState; }
            set
            {
                subjectState = value;
            }
        }
    }
    public class ConcreteObserver:Observer
    {
        private string name;
        private string observerState;
        private ConcreteSubject subject;
        public ConcreteObserver(ConcreteSubject subject,string name)
        {
            this.subject = subject;
            this.name = name;
        }
        public override void Update()
        {
            observerState = subject.SubjectState;
            Console.WriteLine("观察者{0}的新状态是{1}", name, observerState);
        }
        public ConcreteSubject Subject
        {
            get { return subject; }
            set { subject = value; }
        }
    }
}

            ConcreteSubject subject = new ConcreteSubject();
            subject.Attach(new ConcreteObserver(subject, "X"));
            subject.Attach(new ConcreteObserver(subject, "Y"));
            subject.Attach(new ConcreteObserver(subject, "Z"));
            subject.SubjectState = "ABC";
            subject.Notify();