C# MySQL Parameterized Query Problem

Question

I have the following method which is used to populate a DAO from the database. It performs 3 reads - one for the primary object, and 2 for some translations.

public bool read(string id, MySqlConnection c)
{
    MySqlCommand m = new MySqlCommand(readCommand);
    m.Parameters.Add(new MySqlParameter("@param1", id));
    m.Connection = c;
    MySqlDataReader r = m.ExecuteReader();
    r.Read();
    accountID = Convert.ToInt32(r.GetValue(0).ToString());
    ...
    comment = r.GetValue(8).ToString();
    r.Close();
    m = new MySqlCommand(getAccountName);
    m.Parameters.Add(new MySqlParameter("@param1", accountID));
    m.Connection = c;
    r = m.ExecuteReader();
    r.Read();
    account1Name = r.GetValue(0).ToString();
    r.Close();
    m = new MySqlCommand(getAccountName);
    m.Parameters.Add(new MySqlParameter("@param1", secondAccountID));
    m.Connection = c;
    r = m.ExecuteReader();
    r.Read();
    account2Name = r.GetValue(0).ToString();
    r.Close();
    return true;
}

On the line account2Name = r.GetValue(0).ToString(); I get the following error:

Invalid attempt to access a field before calling Read()

I don't understand what the problem is - the previous line calls read!

JerSchneid · Accepted Answer · 2009-05-07 04:46:50Z

3

That seems to be working earlier in your code. Are you sure there is a value to read there (i.e. is your query returning 0 rows or null or something else that can't be converted to a string?)

answered May 7, 2009 at 4:46

JerSchneid

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lc. · Accepted Answer · 2009-05-07 04:55:25Z

1

Looks like that procedure works fine on the line account1Name = r.GetValue(0).ToString();. Maybe you want to check the return value of r.Read() to see if there are any rows first. The error could be interpreted as "Attempted to access a field without data"...
It looks like you're trying to get a single value out of a query. You might want to look into the ExecuteScalar() method instead of ExecuteReader().

answered May 7, 2009 at 4:55

lc.

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