public void AddUser(string nickName, string password, string PrimaryMail, string userPhoto = null)
        {
            if (!IsDuplicateNickname(nickName))
            {
                var newUser = new User();
                newUser.NickName = nickName;
                newUser.Password = password;
                newUser.PrimaryMail = PrimaryMail;
                newUser.UserPhoto = userPhoto;
                using (var session = ConfigureRepository.SessionFactory.OpenSession())
                {
                    using (var transaction = session.BeginTransaction())
                    {
                        try
                        {
                            session.Save(newUser);
                            users.Add(newUser);
                            transaction.Commit();
                        }
                        catch (HibernateException)
                        {
                            transaction.Rollback();
                        }
                    }
                }
            }
            else throw new DuplicateValueException();
        }