C# MySQL second DataReader in DataReader while loo

问题:

As you might have guessed from the title am I trying to do this:

    #region check new nations
    private void checknewnations()
    {
        addtolog("server","Checking for new nations");
        string sql = "SELECT * FROM " + variables.tbl_nations + " WHERE nations_new=0";
        MySqlCommand cmd = new MySqlCommand(sql, connection);
        MySqlDataReader reader = cmd.ExecuteReader();

        while (reader.Read())
        {
            addtolog("mysql",reader["nations_name"].ToString());

            int nation_ID = int.Parse(reader["nations_ID"].ToString());
            string nation_name = reader["nations_name"].ToString();
            string user_ID = reader["nations_user"].ToString();


            addnation(nation_ID, nation_name, user_ID);
        }
        addtolog("server","Finished checking for new nations.");
        //connection.Close();
        reader.Dispose();
    }
    #endregion

which calls this in the while loop:

    #region addnation
    private void addnation(int nationIDnot, string nationName, string userID)
    {
        string nationID = makesixdigits(nationIDnot);
        string userName = "";

        string sql = "SELECT * FROM " + variables.tbl_users + " WHERE users_ID=" + userID;
        MySqlCommand cmd = new MySqlCommand(sql, connection);
        MySqlDataReader reader = cmd.ExecuteReader();


        while (reader.Read())
        {
            userName = reader["users_name"].ToString();
        }
        reader.Dispose();
        addtolog("add", "[" + nationID.ToString() + "] " + nationName + " [" + userID + "] " + userName);
    }
    #endregion

This gives me an error in the second block of code (the one that is called upon in the while loop) saying that there already is a datareader associated with the connection. How am I to get this to work, because I am sure there is a way.

回答1:

In 2008 connection strings you can have multiple active results sets via MultipleActiveResultSets=true; I've not used mySQL so I'm not sure, but maybe if you are using an ADO provider.

Otherwise, just make another connection -- don't re-use the same one.

回答2:

DataReader holds on to connection until it iterates through all its records. Try this

using(reader)
{
   System.Data.DataTable dt = new System.Data.DataTable();
   dt.Load(reader);

   foreach(DataRow row in dt.Rows)
   {
        addtolog("mysql",row["nations_name"].ToString());

        int nation_ID = int.Parse(row["nations_ID"].ToString());
        string nation_name = row["nations_name"].ToString();
        string user_ID = row["nations_user"].ToString();


        addnation(nation_ID, nation_name, user_ID);
   }
}

This way the two reader are not sharing the same connection

回答3:

Open another connection in the loop, and use it to obtain the second reader. Since the loop is tight, you might prefer to open both connections in the first method, pass the second through to the called method, and then close them both at the end of the first ("outer") method. It should be a short enough time between calls that the normal rule about closing connections as soon as possible won't matter.