Event事件

语法代码

class Program
    {
        static void Main(string[] args)
        {
            var e = new EventTest(5);
            e.SetValue(100);
            e.ChangeNum += new EventTest.NumDelegate(EventTest.EventFired);
            e.SetValue(100);
            e.SetValue(200);
            Console.ReadLine();
        }
    }
    class EventTest
    {
        private int value;
        public delegate void NumDelegate();
        public event NumDelegate ChangeNum;
        public EventTest(int num)
        {
            SetValue(num);
        }
        public void SetValue(int Num)
        {
            if (value != Num)
            {
                value = Num;
                OnNumChanged();
            }
            else
            {
                Console.WriteLine("没改变");
            }
        }
        public static void EventFired()
        {
            Console.WriteLine("绑定了");
        }
        protected virtual void OnNumChanged()
        {
            if (ChangeNum != null)
            {
                ChangeNum();
            }
            else
            {
                Console.WriteLine("没绑定");
            }
        }
    }

加event 关键字后 

只能进行 += 或 -=操作，而不能=操作