通常定义事件 都是通过自定义委托的方式来实现， 今天使用EventHandler   委托来定义事件：

 

public class NewMailEventArgs : EventArgs    {        private readonly string m_from;        private readonly string m_to;        private readonly string m_subject;        public NewMailEventArgs(string from, string to, string subject)        {            m_from = from;            m_to = to;            m_subject = subject;        }        public string From { get { return m_from; } }        public string To { get { return m_to; } }        public string Subject { get { return m_subject; } }      }    //定义事件成员    public class MailManager    {        public event EventHandler
     
       NewMail;        public virtual void OnNewMail(NewMailEventArgs e)        {            //Volatile线程 Threading.Tasks            EventHandler
      
        temp = Volatile.Read(ref NewMail);            if (temp != null)            {                temp(this, e);            }        }        public void SimulateNewMail(string from, string to, string subject)        {            //构造一个对象来容纳想传给通知接受者的信息            NewMailEventArgs e = new NewMailEventArgs(from, to, subject);                        //调用虚方法通知对象事件已发生            //如果没有类型重写该方法，我们的对象将通知事件的所有等级对象            OnNewMail(e);        }      }    public sealed class Fax {        /// 
       
        /// 将MailManager 对象传给构造器        /// 
        /// 
               public Fax(MailManager mail)        {            mail.NewMail += FaxMsg;        }              /// 
       
        /// 注销事件        /// 
        /// 
               public void UnFax(MailManager mail)        {            mail.NewMail -= FaxMsg;        }        //新电子邮件到达时，mailManager调用这个方法        public void FaxMsg(object sender, NewMailEventArgs e)        {            Console.WriteLine("Faxing mail message:");            Console.WriteLine(" From={0},To={1},Subject={2}", e.From, e.To, e.Subject);        }     }    static void Main(string[] args)        {                         MailManager mail = new MailManager();            Fax fax = new Fax(mail);            mail.SimulateNewMail("from0", "to1", "subject2");        }