using System;

public class Program

{

    public static void Main()

    {

        Derived d = new Derived();

        d.Foo(a: 1, b: 2);

        Base b = d;

        d.Foo(a: 1, b: 2);

    }

    public class Base

    {

        public virtual void Foo(int a, int b) {}

    }

    public class Derived : Base

    {

        public override void Foo(int b, int a) {

            Console.WriteLine("a = {0}, b = {1}", a, b);

        }

    }

}