using System;

​

namespace Example

{

    //Define an interface that we can pass to the handler

    public interface IUnsubscribe

    {

        void Unsubscribe(TransientEventHandler handler);

    }

    //Define the handler that accepts our new interface

    public delegate  void TransientEventHandler(object sender, EventArgs e, IUnsubscribe unsubscribe);

    //Implement the interface in the bus itself

    public class EventBus : IUnsubscribe

    {

        protected event TransientEventHandler Clicked;

        //Allow regular handlers

        public void Subscribe(EventHandler handler)

        {

            Clicked += delegate (object sender, EventArgs e, IUnsubscribe bus) { handler(sender, e); };

        }

        //Allow transient handlers

        public void Subscribe(TransientEventHandler handler)

        {

            Clicked += handler;

        }

        //Raise the event

        public void OnClicked()

        {

            if (Clicked != null)

            {

                Clicked(this, new EventArgs(), this);

            }

        }

        //Support unsubscription

        public void Unsubscribe(TransientEventHandler handler)

        {

            Clicked -= handler;

        }

    }

    //Now test it all

    public class Program

    {

        static private bool unsub = false;

        private static void SimulateClick(EventBus bus)

        {

            bus.OnClicked();

        }

​

        public static void MyHandler(object sender, EventArgs e, IUnsubscribe unsubscribe)

        {

            Console.WriteLine("MyHandler - Handling click");

            if (unsub)

            {

                Console.WriteLine("MyHandler - Unsubscribing");

                unsubscribe.Unsubscribe(MyHandler);

            }

        }

        public static void Main()

        {

            var e = new EventBus();

            e.Subscribe(MyHandler);

            Console.WriteLine("Main - sending click");

            SimulateClick(e);

            Console.WriteLine("Main - telling handler to unsubscribe next time it runs");

            unsub = true;

            Console.WriteLine("Main - sending click");

            SimulateClick(e);

            Console.WriteLine("Main - sending click");

            SimulateClick(e);

            Console.WriteLine("Main - sending click");

            SimulateClick(e);

        }

    }

}