public bool addTaiKhoan(TaiKhoan tk)
        {
            String        Sql  = "INSERT INTO taikhoan(username,passwords,fullname,loai) VALUES(@username,@password,@fullname,@loai)";
            SqlConnection conn = DBConnection.Instance.getConnection();

            try {
                cmd = new SqlCommand(Sql, conn);
                conn.Open();
                cmd.Parameters.Add("@username", SqlDbType.NVarChar).Value = tk.getUsername();
                cmd.Parameters.Add("@password", SqlDbType.NVarChar).Value = tk.getPasswords();
                cmd.Parameters.Add("@fullname", SqlDbType.NVarChar).Value = tk.getFullname();
                cmd.Parameters.Add("@loai", SqlDbType.NVarChar).Value     = tk.getLoai();
                cmd.ExecuteNonQuery();
                conn.Close();
            }catch (Exception e)
            {
                return(false);
            }
            return(true);
        }