protected void reportShowButton_Click(object sender, EventArgs e) { DateTime lowerDate = DateTime.Parse(lowerDateTextBox.Value.ToString()); DateTime upperDate = DateTime.Parse(upperDateTextBox.Value.ToString()); BillManager billManager = new BillManager(); List <ViewUnpaidBill> viewUnpaidBillList = billManager.GetUnpaidBillsByDate(lowerDate, upperDate); billShowGridView.DataSource = viewUnpaidBillList; billShowGridView.DataBind(); double totalAmount = 0; foreach (ViewUnpaidBill viewUnpaidBill in viewUnpaidBillList) { totalAmount += viewUnpaidBill.BillAmount; } totalAmountTextBox.Value = totalAmount.ToString(); }