using System.Collections.ObjectModel;
using System.Linq;

namespace WindowsFormsApplication1
{
    public class User
    {
        public string Username { get; set; }
        public string Password { get; set; }

        public static Collection<User> getListOfUsers()
        {
            return new Collection<User>
                       {
                           new User{Username = "tommy", Password = "secret"},
                           new User{Username = "brendan", Password = "secret"},
                           new User{Username = "jack", Password = "secret"},
                       };
        }
    }

    public static class UserHelper
    {
        public static bool validateLogin(this Collection<User> users, string username, string password)
        {
            return users.FirstOrDefault(x => x.Username == username).Password == password;
        }
    }
}