public bool queryProducts() { bool success = false; int invoice_no = 0; int sum_of_qty = 0; decimal sum_of_price = 0M; if (int.TryParse(txtEncode.Text, out invoice_no)) { Invoice invoice = new Invoice() { InvoiceId = invoice_no }; ProductInvoice pI = new ProductInvoice() { invoice = invoice }; if (!dbController.checkIfAlreadyConsumed(pI)) { lblPOSmsg.Text = "PV-" + invoice.InvoiceId.ToString("00000"); carts = dbController.getListOfProductInvoice(invoice); int quantity = carts.Count; dt.Clear(); foreach (ProductInvoice productInvoice in carts) { currentProduct = dbController.getProductFromBarcode(productInvoice.product.Barcode); sum_of_qty += productInvoice.QuantitySold; sum_of_price += productInvoice.GroupPrice; addRowInDatagrid(productInvoice); } poSlbl2.Text = sum_of_price.ToString("N"); totalAmount = sum_of_price; success = true; } else { MessageBox.Show("Invoice transaction was already used or do not exist."); } } txtEncode.Clear(); return(success); }