public bool Insert()
        {
            string strInsertHD = "insert into HOADON(MaKH,NgayDat) Values(";
            strInsertHD  = Order.MaKH   ",";
            strInsertHD  = "'"   Order.NgayDat   "')";            
            int result = 0;
            if (_conn.State == ConnectionState.Closed)
                if (!this.Connect())
                    return false;
            SqlCommand command = new SqlCommand(strInsertHD, _conn);
            try
            {
                result = command.ExecuteNonQuery(); 
                if (result > 0)
                {
                    string mahd = this.GetMaHD().ToString();
                    foreach(OrderDetail tmp in Order.OrderDetails)
                    {
                        string strInsertCTHD = "insert into CTHOADON(MaHD,TenSP,SoLuong,DonGia) Values(";
                        strInsertCTHD  = mahd   ",";
                        strInsertCTHD  = "N'"   tmp.TenSP   "',";
                        strInsertCTHD  = tmp.SoLuong.ToString()   ",";
                        strInsertCTHD  = tmp.DonGia.ToString()   ")";
                        command = new SqlCommand(strInsertCTHD, _conn);
                        result = command.ExecuteNonQuery();
                    }
                }
            }
            catch (Exception ex)
            {
                _error = ex.Message;
            }

            if (result <= 0)
                return false;

            return true;
        }