MeowWoof.cs

class Program
{
    class Animal
    {
    }

    class Cat : Animal
    {
        public void Meow()
        {
        }
    }

    class Dog : Animal
    {
        public void Woof()
        {
        }
    }

    static void Main(string[] args)
    {
        var cats = new Cat[] {new Cat(), new Cat()};
        Animal[] animals = cats;
        animals[1] = new Dog();
        cats[1].Meow();
    }
}