private void district_no_KeyPress(object sender, KeyPressEventArgs e)
        {
            if (e.KeyChar == 13)
            {
                try
                {
                    District d = new District();
                    Thana t = new Thana();
                    districtbox.Text = d.getDistrictName(Convert.ToInt32(district_no.Text));
                    thanabox.DataSource = t.getThanaList(d.getDistrictNo(districtbox.Text.ToString()));
                    thana_no.Select();

                }
                catch (Exception ex) {MessageBox.Show(ex.ToString()); }
            }
        }
 private void districtbox_SelectedIndexChanged(object sender, EventArgs e)
 {
     thanabox.Text = null;
     Thana t = new Thana();
     District d = new District();
     try
     {
         district_no.Text = d.getDistrictNo(districtbox.Text.ToString()).ToString();
         thanabox.DataSource = t.getThanaList(Convert.ToInt32(district_no.Text.ToString()));
         thanabox.SelectedIndex = -1;
         thanabox.Select();
     }
     catch (Exception ex) { MessageBox.Show(ex.ToString()); }
 }