/// <summary>
        /// Check Login
        /// </summary>
        /// <param name="email"></param>
        /// <param name="pass"></param>
        /// <returns>NULL : false</returns>
        /// <returns># NULL : true</returns>
        public string CheckLogin(string email, string pass)
        {
            string hoten = null;
            int    flag  = CheckMail(email);

            if (flag == 0)
            {
                return(null);
            }
            if (flag == 1)
            {
                SinhVienDAO context = new SinhVienDAO();
                SinhVien    sv      = context.GetLoginSinhVien(email, pass);
                if (sv == null)
                {
                    return(null);
                }
                hoten = "SINHVIEN:" + sv.MaSV;
                return(hoten);
            }
            if (flag == 2)
            {
                GiangVien gv = GiangVienDAO.GetLoginGiangVien(email, pass);
                if (gv == null)
                {
                    return(null);
                }
                hoten = "GIANGVIEN:" + gv.MaGV;
                return(hoten);
            }
            return(null);
        }