class A :CollectionBase
    {
        public void add(B b)
        {
            List.Add(b);

        }
        public void remove(B b)
        {
            List.Remove(b);

        }
        public B this[int index]
        {
            set
            {
                List[index] = value;
            }
            get
            {
                return (B)List[index];
            }
        }

    }