static void Main(string[] args)
        {
            Asteroid          a  = new Asteroid();
            ExplodingAsteroid ea = new ExplodingAsteroid();
            SpaceShip         s  = new SpaceShip();
            TieFighter        tf = new TieFighter();

            a.CollideWith(s);
            a.CollideWith(tf);
            ea.CollideWith(s);
            ea.CollideWith(tf);

            Console.WriteLine();
            a = ea;
            s = tf;

            a.CollideWith(s);
        }
 public override void CollideWith(SpaceShip s)
 {
     s.CollideWith(this);
 }
 public virtual void CollideWith(SpaceShip s)
 {
     s.CollideWith(this);
 }