private void themsp_Click(object sender, EventArgs e)
 {
     if (txbMaHoaDon.Text != "")
     {
         ChiTietHoaDon chiTietHoaDon = new ChiTietHoaDon();
         chiTietHoaDon.mhd = txbMaHoaDon.Text;
         chiTietHoaDon.ShowDialog();
         tongtien.Text = dao.getPrice(txbMaHoaDon.Text);
     }
 }