Connection Reuse

A MySqlConnection object may only be used for one operation at a time. It may not be shared across multiple threads and used simultaneously, nor reused on the same thread while there is an open MySqlDataReader.

Examples of Prohibited Use

Multiple Threads

You may not execute multiple operations in parallel, for example:

using (var connection = new MySqlConnection("..."))
    await connection.OpenAsync();
    await Task.WhenAll( // don't do this
        connection.ExecuteAsync("SELECT 1;"),
        connection.ExecuteAsync("SELECT 2;"),
        connection.ExecuteAsync("SELECT 3;"));

Nested Access on Single Thread

You may not reuse the connection when there is an open MySqlDataReader:

using (var connection = CreateOpenConnection())
using (var command = new MySqlCommand("SELECT id FROM ...", connection))
using (var reader = command.ExecuteReader())
    while (reader.Read())
        var idToUpdate = reader.GetValue(0);
        connection.Execute("UPDATE ... SET ..."); // don't do this

How to Fix

For the multithreaded scenario, if concurrent access to the database is truly necessary, create and open a new MySqlConnection on each thread. But in most cases, you should just write code that sequentially awaits each asychronous operation (without performing them in parallel).

For the nested access, read all the values from the MySqlDataReader into memory, close the reader, then process the values. (If the data set is large, you may need to use a batching approach where you read a limited number of rows in each batch.)