Connect Timeout expired. All pooled connections are in use.

Hi @shengyan cheng ，welcome to Microsoft Q&A. Read about the use of using: when declared in a declaration, a local variable is released at the end of the scope in which it was declared. So you don't need to manually close resources in finally.

finally
{
    if (cn.State != ConnectionState.Open)
        cn.Close();
}

The syntax introduced since C# 8.0 should correctly use async and await when using asynchronous statements. You can modify your code like this:

public async Task ExecuteAsync()
{
    await using var cn = new MySqlConnection(_options.Value.Worker.DB);
    try
    {
        await cn.OpenAsync();
        await cn.QueryAsync("sp_demo", commandType: CommandType.StoredProcedure);
    }
    catch (Exception ex)
    {
        // handle exception
    }
}

Using await on cn.OpenAsync() ensures completion before proceeding to the next line of code.

For Task.Factory.StartNew and Task.run. After carefully reading this article What is the difference between Task.Run() and Task.Factory.StartNew(). (In your scenario, I prefer to use Task.run.)

For the code in your comment: you could put the connection into the whole statement below, which can properly release resources under multithreading.

for (int i = 0; i <= xxxx; i++)
{
    // others condition

    Task.Factory.StartNew(async () =>
    {
        using var cn = new MySqlConnection(_options.Value.Worker.DB);
        await cn.OpenAsync();

        try
        {
            await _db.ExecuteAsync(cn);
        }
        catch (Exception ex)
        {
            // handle exception
        }
    }, TaskCreationOptions.LongRunning).Unwrap();
}

If using Task.Run:

public async Task MainAsync(){
for (int i = 0; i < xxxx; i++)
{
    await Task.Run(async () =>
    {
        using var cn = new MySqlConnection(_options.Value.Worker.DB);
        await cn.OpenAsync();
        try
        {
            await _db.ExecuteAsync(cn);
        }
        catch (Exception ex)
        {
            // handle exception
        }
    });
}
}

Of course this depends on your own actual scenario.

Best Regards,

Jiale

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