C＃中的DataReader异常

Question

Let me first explain my code.

1. fetch fare for selected destination(reader1)
2. fetch * for selected user id(reader2)
3. insert data to process new balance(cmd3)
4. retrieve new balance as calculated field (reader3)
5. update travel account (cmd5)
6. clear calc_tb for next customer input(cmd6)

My problem is while running, I get this error:

There is already an open DataReader associated with this Command which must be closed first.

How can I handle more than one DataReader accessing different tables? Is DataReader similar to resultset in Java ?

I'm a beginner in coding.

        string id, 
        destin, num;
        id = txt_id.Text;
        destin = cb_destin.Text;
        num = cb_num.Text;



        string sql1 = "SELECT fare FROM route_info WHERE route_name='" +destin + "' ";
        string sql2 = "SELECT * FROM trav_acc WHERE user_id='" + id + "'";

        con.Open();

        cmd1 = new SqlCommand(sql1, con);
        reader1 = cmd1.ExecuteReader();

        while(reader1.Read())
        {

            string fare = (string)reader1["fare"];

            cmd2 = new SqlCommand(sql2, con);
            reader2 = cmd2.ExecuteReader();
            string cur_bal = (string)reader2["cur_bal"];
            reader2.Close();

            string calc1 = "INSERT INTO calc_tb VALUES('" + cur_bal + "','" + num + "','" + fare + "')";
            string calc2 = "SELECT cur_bal - (nof_tickets * fare) AS new_bal FROM calc_tb";

            cmd3 = new SqlCommand(calc1, con);
            cmd3.ExecuteNonQuery();

            cmd4 = new SqlCommand(calc2, con);
            reader3 = cmd4.ExecuteReader();

            while(reader3.Read())
            {
                string new_bal = (string)reader3["new_bal"];

                string update = "UPDATE trav_acc SET cur_bal='" + new_bal + "',last_bal='" + cur_bal + "' WHERE user_id='" + id + "'  ";
                cmd5 = new SqlCommand(update, con);
                cmd5.ExecuteNonQuery();

                string clear = "DELETE FROM calc_tb";
                cmd6 = new SqlCommand(clear, con);
                cmd6.ExecuteNonQuery();
            }

        }
        con.Close();
        MessageBox.Show("Thank you for using EasyTravel.Come again soon!");



    }

Answer 1

It seems that reader3 and cmd4 are defined outside of the code you are showing us, but at least they are defined outside the loop for reader1 . So if your reader1 contains more than one row, reader3 and cmd4 will be assigned again, but the "old" reader3 is never closed. Close reader3 when it's finished reading. Or use a using statement, which will take care of the closing automatically.

using (DataReader reader1 = cmd1.ExecuteReader()) {
    ....
    while (reader1.Read()) {
        ....
        using (DataReader reader3 = cmd4.ExecuteReader()) {
            while (reader3.Read()) {
            }
        }  //reader3 is closed here automatically
    }
} //reader1 is closed here automatically

Furthermore, I'm not sure if I remember correctly, but I think it's not possible to have two open readers on the same connection. I may be wrong with this, though.