private void button3_Click(object sender, EventArgs e) { label13.Text = ""; if (s != null && label8.Text != "" && textBox2.Text != "" && richTextBox1.Text != "") { Stock m = Stock.checkInventory(s.Product.ProductId); if (m == null) { label13.Text = label8.Text; } else { StockTransaction st = StockTransaction.transact(Convert.ToInt32(label8.Text), Convert.ToInt16(textBox2.Text), richTextBox1.Text); label13.Text = "Transaction successful. Remaining Quantity:" + st.QuantityAfter; } } else { label13.Text = "Enter valid Quantity."; } }