private void comboBoxSalesNumber_SelectedIndexChanged(object sender, EventArgs e)
 {
     if (comboBoxSalesNumber.SelectedIndex != -1 && comboBoxSalesNumber.SelectedIndex != 0)
     {
         SalesId = Convert.ToInt32(comboBoxSalesNumber.SelectedValue.ToString());
         retrieve.ShowSalesLines(dataGridViewSalesLines, PoLineIdGv, ProductIdGv, ProductNameGv, LineNo, ProductPriceGv, ProductQuantityGv, ProductTotalPrice, UnitGv, Convert.ToInt32(comboBoxSalesNumber.SelectedValue.ToString()));
         labelGrandTotalValue.Text = (retrieve.GetSalesGrossTotal(Convert.ToInt32(comboBoxSalesNumber.SelectedValue.ToString()))).ToString();
     }
 }