using System;

​

abstract class Animal {}

abstract class Pet : Animal {}

class Cat : Pet {}

class Dog : Pet {}

class Fox : Animal {}

​

interface ICageIn<in T> { T content { set; } }

interface ICageOut<out T> { T content { get; } }

interface ICageInOut<T> { T content { get; set; } }

​

class Cage<T> : ICageIn<T>, ICageOut<T>, ICageInOut<T> { public T content { get; set; } }

​

public class Program {

​

    static void touchPet( ICageOut<Pet> cage ) {

        Console.WriteLine( cage.content );

    }

​

    static void pushPet( ICageIn<Pet> cage ) {

        cage.content = new Dog();

    }

​

    static void replacePet( ICageInOut<Pet> cage ) {

        touchPet( cage as ICageOut<Pet> );

        pushPet( cage as ICageIn<Pet> );

    }

    void enshurePet( Cage<Pet> cage ) {

        if( cage.content is Pet ) return;

        pushPet( cage as ICageIn<Pet> );

    }

​

    public static void Main() {

        var animalCage = new Cage<Animal>();

        var petCage = new Cage<Pet>();

        var catCage = new Cage<Cat>();

        var dogCage = new Cage<Dog>();

        var foxCage = new Cage<Fox>();

​

        touchPet( animalCage ); // forbid :-)

        touchPet( petCage ); // allow :-)

        touchPet( catCage ); // allow :-)

        touchPet( dogCage ); // allow :-)

        touchPet( foxCage ); // forbid :-)

​

        pushPet( animalCage ); // allow :-)

        pushPet( petCage ); // allow :-)

        pushPet( catCage ); // forbid :-)

        pushPet( dogCage ); // forbid :-)

        pushPet( foxCage ); // forbid :-)

        replacePet( animalCage ); // forbid :-)

        replacePet( petCage ); // allow :-)

        replacePet( catCage ); // forbid :-)

        replacePet( dogCage ); // forbid :-)

        replacePet( foxCage ); // forbid :-)

    }

​

}