private void btnBillSearch_Click(object sender, EventArgs e)
{
string searchString = txtBillSearchString.Text;
DateTime?fromDate = dtpBillSearchFromDate.Checked ? (DateTime?)dtpBillSearchFromDate.Value : null;
DateTime?toDate = dtpBillSearchToDate.Checked ? (DateTime?)dtpBillSearchToDate.Value : null;
decimal fromTotal = nmrBillFromTotal.Value;
decimal toTotal = nmrBillToTotal.Value;
List <Bill> bills = billBUS.Search(searchString, fromDate, toDate, fromTotal, toTotal);
ShowBills(bills);
}