class Monster
    {
        public int Hp { get; protected set; }
        public string Name { get; protected set; }

        public virtual void Attack()
        {
            Console.WriteLine("Monster attacking!");
        }

    }

    class Skeleton : Monster
    {
        public Skeleton()
        {
            Hp = 20;
            Name = "Skeleton";
        }

        public override void Attack()
        {
            Console.WriteLine("Skeleton attacking!");
        }
    }

Monster skeleton = new Skeleton();

Skeleton skeleton = new Skeleton();