using System;

​

class Base

{

    protected int x = 4;

}

​

class Der:Base

{

    public void foo(Base b)

    {

        Console.WriteLine(b.x);

    }

}

​

class Sib:Base

{

}

public class Program

{

    public static void Main()

    {

        Der d = new Der();

        Sib s = new Sib();

        d.foo(s);

    }

}