using System;

public class Program

{

    public static void Main()

    {

        X x1 = new X();

        X x2 = new X();

        x1.otherX = x2;

        x1.TestOtherX();

        Y y1 = new Y();

        Y y2 = new Y();

        y1.y = y2;

        y1.TestOtherY();

    }

}

​

public class X{

    public X otherX;

    public void TestOtherX(){otherX.Test();}

    protected void Test(){Console.WriteLine("OK!");}

}

​

public class Y : X{

    public Y y;

    public void TestOtherY(){y.Test();}

}