public invoice search()
        {
            string        q   = "select * from table_invoice where in_id=" + in_id;
            SqlCommand    cmd = new SqlCommand(q, connection.get());
            SqlDataReader sdr = cmd.ExecuteReader();
            invoice       i   = new invoice();

            while (sdr.Read())
            {
                i.in_id       = (int)sdr[0];
                i.in_date     = (string)sdr[1];
                i.in_fk_us_id = (int)sdr[2];
            }
            sdr.Close();
            return(i);
        }
        public List <invoice> showall()
        {
            string         q   = "select * from table_invoice";
            SqlCommand     cmd = new SqlCommand(q, connection.get());
            SqlDataReader  sdr = cmd.ExecuteReader();
            List <invoice> lst = new List <invoice>();

            while (sdr.Read())
            {
                invoice i = new invoice();
                i.in_id       = (int)sdr[0];
                i.in_date     = (string)sdr[1];
                i.in_fk_us_id = (int)sdr[2];
                lst.Add(i);
            }
            sdr.Close();
            return(lst);
        }