using System;

public class Program

{

    public static void Main()

    {

        EventHandler @event = EventHandlerHelper.Once((object sender, EventArgs e) =>

        {

            Console.WriteLine("Somente uma vez");

        });

        @event.Invoke(null, null);

        @event.Invoke(null, null);

    }

}

​

public static class Functional

{

    public static Action<T1, T2> Once<T1, T2>(Action<T1, T2> action)

    {

        var executed = false;

​

        return (T1 arg1, T2 arg2) => {

            if (!executed)

            {

                action(arg1, arg2);

                executed = true;

            }

        };

    }

}

​

public static class EventHandlerHelper

{

    public static EventHandler Once(Action<object, EventArgs> action)

    {

        return Functional.Once(action).Invoke;

    }

}