using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<User> users = new List<User>();

            users.Add(new User() { id = 1, code = "A" });

            users.Add(new User() { id = 2, code = null });

            users.Add(new User() { id = 3, code = "C" });

            users.Add(new User() { id = 4, code = "C" });

            users.Add(new User() { id = 5, code = "B" });

​

            string[] possibleValues = new string[] { null, "A", "B" };

​

            var result = users.Where(x => possibleValues.Any(z => z == x.code));

​

            foreach (var item in result)

            {

                Console.WriteLine("Id: {0}", item.id);

            }

    }

}

​

public class User

    {

        public int id { get; set; }

        public string code { get; set; }

    }