using System;

public class Program

{

    public static void Main()

    {

        Foo1 x = new Foo1();

        x.foo(new SpecialString());

        x.foo(new BasicString());

    }

}

​

class BasicString {

}

​

class SpecialString: BasicString {

}

​

​

interface Foo {

    void foo(SpecialString a);

}

​

class Base {

    public void foo(BasicString a) {

    }

}

​

class Foo1: Base, Foo {

    public void foo(SpecialString a) {

    }

}

​