namespace Class1
{    
    //定义事件引发时，需要传的参数
    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;}
        }

}

//事件所用的委托(链表)
    delegate void NewMailEventHandler(object sender, NewMailEventArgs e);

//提供事件的类
    class MailManager
    {
        public event NewMailEventHandler NewMail;
        //通知已订阅事件的对象
        protected virtual void OnNewMail(NewMailEventArgs e)
        {
            NewMailEventHandler temp = NewMail; //MulticastDelegate一个委托链表
            //通知所有已订阅事件的对象
            if(temp != null)
                temp(this,e); //通过事件NewMail(一种特殊的委托)逐一回调客户端的方法

}
        //提供一个方法，引发事件
        public void SimulateNewMail(string from, string to, string subject)
        {
            NewMailEventArgs e = new NewMailEventArgs(from,to,subject);
            OnNewMail(e);
        }
    }

//使用事件
    class Fax
    {
        public Fax(MailManager mm)
        {
            //Subscribe 
            mm.NewMail += new NewMailEventHandler(Fax_NewMail);
        }
        private void Fax_NewMail(object sender, NewMailEventArgs e)
        {
            Console.WriteLine("Message arrived at Fax...");
            Console.WriteLine("From={0}, To={1}, Subject='{2}'",e.From,e.To,e.Subject);
        }
        public void Unregister(MailManager mm)
        {
            mm.NewMail -= new NewMailEventHandler(Fax_NewMail);
        }
    }
    class Print
    {
        public Print(MailManager mm)
        {
            //Subscribe ,在mm.NewMail的委托链表中加入Print_NewMail方法
            mm.NewMail += new NewMailEventHandler(Print_NewMail);
        }
        private void Print_NewMail(object sender, NewMailEventArgs e)
        {
            Console.WriteLine("Message arrived at Print...");
            Console.WriteLine("From={0}, To={1}, Subject='{2}'",e.From,e.To,e.Subject);
        }
        public void Unregister(MailManager mm)
        {
            mm.NewMail -= new NewMailEventHandler(Print_NewMail);
        }
    }

class ExcelProgram
    {
        [STAThread]
        static void Main(string[] args)
        {    
            MailManager mm = new MailManager();
            if(true)
            {
                Fax fax = new Fax(mm);
                Print prt = new Print(mm);
            }