private void grdHoaDon_CellClick(object sender, DataGridViewCellEventArgs e)
 {
     if (e.RowIndex >= 0)
     {
         DataGridViewRow row      = this.grdHoaDon.Rows[e.RowIndex];
         DataTable       dtHoaDon = BUS_HD.HoaDonTheoMa(row.Cells[0].Value.ToString());
         cboMaKH.SelectedValue = dtHoaDon.Rows[0]["MaKH"].ToString();
     }
 }