protected void submitBtn_Click(object sender, EventArgs e) { int quantity = Int32.Parse(quantityTxt.Text); int id = Int32.Parse(Request.QueryString["id"]); if (quantityTxt.Text == "") { errorLb.Text = "Quantity must be filled"; } else if (quantity <= 0) { errorLb.Text = "Quantity must be greater than zero"; } else if (!CartHandler.checkQuantity(id, quantity)) { errorLb.Text = "Quantity exceeds the limit"; } else { addProduct(); } }