protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
        {
            if (DropDownList1.SelectedValue == "0")
            {
                this.ShowErrorNotification("Please select proper id");
            }
            else
           {
            Load_Data_Grid();
            lblCustName.Visible = true;
            gridProduct1.Visible = true;
            lblTotal1.Visible = true; 
            txtDiscount1.Visible = true;
            txtTax1.Visible = true;
            lblNetTotal1.Visible = true;
            SaleInvoice repository = new SaleInvoice();
            tblSaleInvoice saleInvoice = repository.Get(Convert.ToInt32(DropDownList1.SelectedValue));
            Customer c = new Customer();
            tblCustomer cust = c.Get(saleInvoice.CustomerId);
            lblCustName.Text = cust.Firstname + " " +cust.Lastname;
            lblTotal1.Text= saleInvoice.TotalSale.ToString();
            txtTax1.Text= saleInvoice.Tax.ToString();
            txtDiscount1.Text=saleInvoice.Discount.ToString();
            lblNetTotal1.Text=saleInvoice.NetSale.ToString();

            SaleInvoiceDetail repo = new SaleInvoiceDetail();
            tblSaleInvoiceDetail sid = new tblSaleInvoiceDetail();
            
IEnumerable<tblSaleInvoiceDetail> enume= repo.GetOnInvoiceId(Convert.ToInt32(saleInvoice.InvoiceId));
            var enumer= enume.GetEnumerator();
            while (enumer.MoveNext())
            {
                int rowindex = enumer.Current.ProductId - 1;
                CheckBox cb = (CheckBox)gridProduct1.Rows[rowindex].FindControl("chkbox1");
                cb.Checked = true;
                
                TextBox txt = (TextBox)gridProduct1.Rows[rowindex].FindControl("txtQuantity");
                txt.Visible = true;
                txt.Text = enumer.Current.Quantity.ToString();

            }
            total = Convert.ToDouble(lblTotal1.Text);
           }
        }