class Program
    {
        static void Main(string[] args)
        {
            MailManager mail = new MailManager();
            new Fax(mail);
            mail.SimulateNewMail("123", "234", "456");

            Console.ReadKey();
        }

    }

    internal class NewMailEventArgs : EventArgs
    {
        private readonly string m_from, m_to, m_subject;
        public NewMailEventArgs(string from,string to,string subject)
        {
            this.m_to = to;
            this.m_from = from;
            this.m_subject = subject;
        }

        public string Form { get { return m_from; } }
        public string To { get { return m_to; } }
        public string Subject { get { return m_subject; } }
    }

    internal   class MailManager
    {
        public event EventHandler<NewMailEventArgs> NewMail;


        protected virtual void OnNewMail(NewMailEventArgs e)
        {

            EventHandler<NewMailEventArgs> temp = Interlocked.CompareExchange(ref NewMail, null, null);
            if (temp!=null)
            {
                temp(this,e);
            }
        }


        public void SimulateNewMail(string from,string to,string subject)
        {
            NewMailEventArgs e = new NewMailEventArgs(from,to,subject);
            OnNewMail(e);
        }
    }

    internal sealed class Fax
    {

        public Fax(MailManager mm)
        {
            mm.NewMail += FaxMsg;
        }

        private void FaxMsg(object sender, NewMailEventArgs e)
        {
            Console.WriteLine("Faxing Mail message:");
            Console.WriteLine("to={0},from={1},subject={2}",e.To,e.Form,e.Subject);
        }

        public  void  Unregister(MailManager mm)
        {
            mm.NewMail -= FaxMsg;
        }
    }