private void Return_Product_Form_Load(object sender, EventArgs e)
        {
            label_return.Hide();
            textBox_product_name.Hide();
            Ok_button_Product_name.Hide();
            conn = new OracleConnection(ordb);
            conn.Open();

            OracleCommand cmd = new OracleCommand();

            cmd.Connection  = conn;
            cmd.CommandText = "select Bill_id from Bill";
            cmd.CommandType = CommandType.Text;

            OracleDataReader dr = cmd.ExecuteReader();

            while (dr.Read())
            {
                Select_Bill_ID_for_return_bill.Items.Add(dr[0]);
            }
            dr.Close();
        }