Question

I’m using .NET Connector to connect to MySQL.
In my application there are few threads using the same connection, so if a MySQLDataReader is not closed yet and some thread is trying execute a query it gives that error:

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

Will there ever be support in MySQL for multiple result sets or however it’s called?

My database management class:

public class DatabaseConnection
{
    private MySqlConnection conn;

    public void Connect(string server, string user, string password, string database, int port = 3306)
    {
        string connStr = String.Format("server={0};user={1};database={2};port={3};password={4};charset=utf8",
            server, user, database, port, password);
        conn = new MySqlConnection(connStr);
        conn.Open();
    }

    private MySqlCommand PrepareQuery(string query, object[] args)
    {
        MySqlCommand cmd = new MySqlCommand();
        cmd.Connection = conn;
        for (int i = 0; i < args.Length; i++)
        {
            string param = "{" + i + "}";
            string paramName = "@DBVar_" + i;
            query = query.Replace(param, paramName);
            cmd.Parameters.AddWithValue(paramName, args[i]);
        }
        cmd.CommandText = query;
        return cmd;
    }

    public List<Dictionary<string, object>> Query(string query, params object[] args)
    {
        MySqlCommand cmd = PrepareQuery(query, args);
        MySqlDataReader reader = cmd.ExecuteReader();
        List<Dictionary<string, object>> rows = new List<Dictionary<string, object>>();
        while (reader.Read())
        {
            Dictionary<string, object> row = new Dictionary<string, object>();
            for (int i = 0; i < reader.FieldCount; i++)
            {
                row.Add(reader.GetName(i), reader.GetValue(i));
            }
            rows.Add(row);
        }
        reader.Close();
        return rows;
    }

    public object ScalarQuery(string query, params object[] args)
    {
        MySqlCommand cmd = PrepareQuery(query, args);
        return cmd.ExecuteScalar();
    }

    public void Execute(string query, params object[] args)
    {
        MySqlCommand cmd = PrepareQuery(query, args);
        cmd.ExecuteNonQuery();
    }

    public void Close()
    {
        conn.Close();
    }
}

An example of how I use this:

DatabaseConnection Conn = new DatabaseConnection();
Conn.Connect("localhost", "root", "", "foogle");
var rows = conn.Query("SELECT * FROM `posts` WHERE `id` = {0}", postID);
foreach (var row in rows)
{
   Console.WriteLine(row["title"]); // Writes the post's title (example)
}

Answer 1

Multiple result sets refers to a single query or query batch returning multiple row sets. Those results are accessed through the one and only DataReader for that connection.

What you’re asking for is something quite different. You need the ability to performing multiple simultaneous queries of a single connection. Afaik .NET does not support that, not for SQL Server or any other driver.

Sharing a connection between multiple threads is a bad idea and totally unnecessary. .NET will use a connection pool to limit the total number of connections so It’s perfectly safe to get a new connection for each (set of) queries you want to execute. Limit the scope of a connection to a thread and your problem will go away.