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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

Dispose While in Use

You may not Dispose any MySqlConnector objects while they are in use:

var connection = new MySqlConnection("...");
await connection.OpenAsync();
var command = new MySqlCommand("SELECT SLEEP(1)", connection);
var task = command.ExecuteScalarAsync();
connection.Dispose(); // don't do this
await task;

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.)

For lifetime management, prefer to use using (or await using) instead of explicitly calling Dispose (or DisposeAsync). Ensure that all outstanding operations have completed before disposing objects that are in use.