using System;

public class A

{

    public delegate void E(int val);

    public event E e;

    public void go()

    {

        if (e!= null)

            e.Invoke(3);

    }

}

​

public class B

{

    public event A.E e;

    public void init()

    {

        var a = new A();

        a.go();

        a.e += e;

        a.go();

    }

}

​

public class Program

{

    public static void truc(int val)

    {

        Console.WriteLine("Hello World");

    }

    public static void Main()

    {

        var b = new B();

        b.e += truc;

        b.init();

    }

}