觸發事件，檢查是否注冊過事件的方法

下面的代碼，會發現事件沒有被注冊

 class PRogram    {        static void Main(string[] args)        {            EventCheck check = new EventCheck();            check.OnDataArrived(new DataArrivedEventArgs() { data="Hello World!"});            Console.ReadKey();        }        static void check_DataArrived(object sender, DataArrivedEventArgs e)        {            Console.WriteLine(e.data);        }    }    class EventCheck    {        //第三步 use the delegate type to declare a variable that can refer to any method with the same signature as the delegate.        //To associate the event with the method that will handle the event, add an instance of the delegate to the event.         //The event handler is called whenever the event occurs, unless you remove the delegate.        internal event DataArrivedEventHandler DataArrived;                //第四步        internal void OnDataArrived(DataArrivedEventArgs e)        {            try            {                //DataArrivedEventHandler handler = DataArrived;                //if (handler != null)                //{                //    handler(this, e);                //}                DataArrived += EventCheck_DataArrived;                if (DataArrived != null)                {                    DataArrived -= EventCheck_DataArrived;                    DataArrived(this, e);                }            }            catch (Exception ex)            {                Console.WriteLine("遇到異常{0}", ex.Message);            }        }        void EventCheck_DataArrived(object sender, DataArrivedEventArgs e)        {            Console.WriteLine(e.data);        }    }    //第二步   Represents the method that will handle an event when the event provides data.    //This method's first parameter is of type Object and refers to the instance that raises the event.     //Its second parameter is derived from type EventArgs and holds the event data.    //If the event does not generate event data, the second parameter is simply the value of the EventArgs.Empty field.     //Otherwise, the second parameter is a type derived from EventArgs and supplies any fields or properties needed to hold the event data.    delegate void DataArrivedEventHandler(object sender,DataArrivedEventArgs e);  //正式寫代碼的時候，可以使用EventHandler<TEventArgs>泛型委托    //第一步       //create a custom event data class, create a class that derives from the EventArgs class and provide the properties to store the necessary data.     //The name of your custom event data class should end with EventArgs.    class DataArrivedEventArgs : EventArgs    {       internal string data;    }

修改第四步的代碼，先將事件復制到臨時變量里面，然后通過這個臨時變量來處理，就不會有問題

//第四步        internal void OnDataArrived(DataArrivedEventArgs e)        {            try            {                //DataArrivedEventHandler handler = DataArrived;                //if (handler != null)                //{                //    handler(this, e);                //}                DataArrived += EventCheck_DataArrived;                DataArrivedEventHandler handler = DataArrived;                if (handler != null)                {                    DataArrived -= EventCheck_DataArrived;                    handler(this, e);                }            }            catch (Exception ex)            {                Console.WriteLine("遇到異常{0}", ex.Message);            }        }