static member

using System;

namespace ООП
{
    class Program
    {
        static void Main(string[] args)
        {
            User user1 = new User();
            User user2 = new User();

            Console.WriteLine(user1.Id);
            Console.WriteLine(user2.Id);
        }
    }

    class User
    {
        public static int Ids;
        public int Id;

        public User()
        {
            Id = ++Ids;
        }
    }
}