There is already an open datareader associated with this connection which must be closed first

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Hello,

First of all, sorry for my english (im french).

I want to select all my passwords from my database (who are encrypted) and store them into a datareader, decrypt them, and then update the decrypted password in the database.

I've got an error by running this code ("There is already an open DataReader associated with this Connection which must be closed first").

This is my code :

try
     {
          connexion.Open();
          MySqlCommand sql = new MySqlCommand("SELECT * FROM station", connexion);
          MySqlDataReader dr = sql.ExecuteReader();

          while (dr.Read())
          {

                int id = (int)dr["idStation"];
                string pwd = (string)dr["pwdStation"];
                string pwdDecrypt = AesCryp.Decrypt(pwd);
                
                //error here        
                MySqlCommand cmdUpdate = new MySqlCommand("UPDATE station SET 
                pwdStation='" + pwdDecrypt + "' WHERE idStation='" + id + "';", 
                connexion);

                cmdUpdate.ExecuteNonQuery();


           }
                dr.Close();


           }
     catch (Exception ex)
     {
         MessageBox.Show(ex.Message);
     }

What I have tried:

I am new in programming so i did not try anything cause i'm stuck :/

Thanks for your help.

Posted 18-Sep-19 21:29pm

Nico Bellic

Updated 18-Sep-19 21:42pm

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OriginalGriff · Accepted Answer · 2019-09-18T21:42:00

You can only have one DataReader open on a connection - until that is closed, you can;t do anything else. And that code only closes the DataReader once all rows have been processed - so you can't do an UPDATE on the the same connection until after the loop, you would need a second Connection object to do that within the loop. And that's assuming that there is no error - if there is, the connection and the DataReader associated with it remain open forever!

But ... that's a minor problem compared to the actual code!
First off: Never concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Always use Parameterized queries instead.

When you concatenate strings, you cause problems because SQL receives commands like:

SQL

SELECT * FROM MyTable WHERE StreetAddress = 'Baker's Wood'

The quote the user added terminates the string as far as SQL is concerned and you get problems. But it could be worse. If I come along and type this instead: "x';DROP TABLE MyTable;--" Then SQL receives a very different command:

SQL

SELECT * FROM MyTable WHERE StreetAddress = 'x';DROP TABLE MyTable;--'

Which SQL sees as three separate commands:

SQL

SELECT * FROM MyTable WHERE StreetAddress = 'x';

A perfectly valid SELECT

SQL

DROP TABLE MyTable;

A perfectly valid "delete the table" command

SQL

--'

And everything else is a comment.
So it does: selects any matching rows, deletes the table from the DB, and ignores anything else.

So ALWAYS use parameterized queries! Or be prepared to restore your DB from backup frequently. You do take backups regularly, don't you?

Secondly, never store passwords in clear text, never encrypt passwords - they are both a major security risk. There is some information on how to do it here: Password Storage: How to do it.[^]

And remember: if this is web based and you have any European Union users then GDPR applies and that means you need to handle passwords as sensitive data and stored them in a safe and secure manner. Text or encryption is neither of those and the fines can be .... um ... outstanding. In December 2018 a German company received a relatively low fine of €20,000 for just that.

Thirdly, I'd strongly suggest that you create your SqlConnection and use your SqlCommand and DataReader objects in a using block - that way, they are automatically closed and disposed when you are finished with the, regardless of how the code exits:

C#

using (SqlConnection con = new SqlConnection(strConnect))
    {
    con.Open();
    using (SqlCommand cmd = new SqlCommand("SELECT Id, description FROM myTable", con))
        {
        using (SqlDataReader reader = cmd.ExecuteReader())
            {
            while (reader.Read())
                {
                int id = (int) reader["Id"];
                string desc = (string) reader["description"];
                Console.WriteLine("ID: {0}\n    {1}", id, desc);
                }
            }
        }
    }