private void searchButton_Click(object sender, RoutedEventArgs e)
        {
            if (IsTextAllowed(txtSearch.Text))
            {
                int billNo = int.Parse(txtSearch.Text);

                {
                    DataTable dt = billData.Search(billNo);
                    dgvBills.ItemsSource = dt.DefaultView;
                }
            }

            else
            {
                MessageBox.Show("Enter Valid Bill Number!");
            }
        }