abstract class Animal {}

abstract class Pet : Animal {

    public string name;

}

class Cat : Pet {}

class Dog : Pet {}

class Fox : Animal {}

​

class PetCage {

    public Pet content;

}

class FoxCage: PetCage {

    new public Fox content; // Fox is'n Pet!

}

​

public class Program {

​

    public static void Main() {

        var cage = new PetCage();

        cage = new FoxCage();

        cage.content = new Cat(); // Hello, Cat in my FoxCage.

    }

​

}