class Program
    {
        static void Main(string[] args)
        {
            for (int i = 101 - 1; i >= 0; i--)
            {
                Single.AddUser(i);
            }
            foreach (var user in Single.GETEnumerable())
            {
                Console.WriteLine(user.Id);
            }
            Single.User d = Single.GETEnumerable().First(a => a.Id == 0);
            Console.WriteLine("before-"+d.Id);
            Single.ReplaceId(0,200);
            Console.WriteLine("after-" + d.Id);

            while (true)
            {
                
            }
        }
    }

    class Single
    {
        private static readonly List<Admin> Admins = new List<Admin> ();
        public static IEnumerable<User> GETEnumerable()
        {
            return Admins;
        }

        public static void AddUser(int id)
        {
            Admins.Add(new Admin {Id = id});
        }

        public static void ReplaceId(int old, int @new)
        {
            Admins.First(s => s.Id == old).Id = @new;
        }
        internal class Decorator
        {
            protected int _id;
        }

        internal class User : Decorator
        {
            public int Id => _id;
        }
        private class Admin : User
        {
            public new int Id
            {
                get { return _id; }
                set { _id = value; }
            }
        }
    }