private void ClientList_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            Client selectedClient = ((Client)ClientList.SelectedItem);

            if (selectedClient == null)
            {
                return;
            }

            TaxBox.Text        = ((Client)ClientList.SelectedItem).FUEL.ToString();
            ServiceTaxBox.Text = selectedClient.STAX.ToString();
            DiscountBox.Text   = selectedClient.AMTDISC.ToString();
            BillingDataDataContext db = new BillingDataDataContext();
            double prevdue            = db.GetPreviousDue(selectedClient.CLCODE) ?? 0;

            PreviousDueTextBox.Text = prevdue.ToString();
            if (SubClientList != null)
            {
                SubClientListSource.Source = SubClientList.ContainsKey(selectedClient.CLCODE) ? SubClientList[selectedClient.CLCODE] : new List <string>();
            }
            RefreshDataGridSource();
        }