private void btnThanhToan_Click(object sender, EventArgs e)
        {
            try
            {
                if (hd.KTraTonTai(txtMaHD.Text) == false)
                {
                    for (int i = 0; i < dgvCTHD.Rows.Count - 1; i++)
                    {
                        string mahh    = dgvCTHD.Rows[i].Cells[0].Value.ToString();
                        string soluong = dgvCTHD.Rows[i].Cells[2].Value.ToString();
                        string dongia  = dgvCTHD.Rows[i].Cells[3].Value.ToString();

                        hd.ThemHHVaoHoaDon(txtMaHD.Text, mahh, int.Parse(soluong), float.Parse(dongia));
                        hh.CapNhatSoLuongTonHH(mahh, int.Parse(soluong));
                    }
                    float tongtien = 0;
                    for (int i = 0; i < dgvCTHD.Rows.Count - 1; i++)
                    {
                        tongtien += float.Parse(dgvCTHD.Rows[i].Cells[3].Value.ToString());
                    }
                    hd.CapNhatHD(txtMaHD.Text, tongtien);
                    MessageBox.Show("Thanh toán thành công");
                    InHD();
                    for (int i = 0; i < dgvCTHD.Rows.Count - 1; i = 0)
                    {
                        dgvCTHD.Rows.RemoveAt(i);
                    }
                    btnThanhToan.Enabled = false;
                    block();
                    ClearDL();
                    dgvMH.DataSource = hh.getDSHangHoa();
                }
            }
            catch
            {
                MessageBox.Show("Lỗi!!");
            }
        }