explicit-implicit operator

       public class Base
    {   
        public long _id;

        public Base(long id)
        {
            this._id = id;
        }
    }
    public class A : Base
    {
        public A(long id) : base(id)
        {

        }
    }
    public class B : Base
    {
        public B(long id) : base(id)
        {

        }
        public static implicit operator A(B id)
        {
            return new A(id._id);
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            B _b = new B(10);

            A _a = _b;

            Base _base = _a;
        }
    }