Weird behaviour of task in ASP.NET application

Question

I'm trying to connect and recieve very small amount of data from Mysql servers from all branches asynchronously, without waiting one to finish.

I have my own event-driven .NET MySQL Connector Wrapper library and it works fine without asynchronously.

Looks like I'm missing something while multi-tasking but I could not figured why.

public void GetALL()
    {
        TaskList = new Task[RemSQL.Count];

        Response.Write("<h1>starting..........</h1>");

        Task t;
        BranchInfo b;
        Wrapper w;

        for (int tx = 0; tx < RemSQL.Count; tx++)
        {
            int txx = tx; //strongly suggested on MSDN while using tasks/threads in loops
            b = RemSQL[txx];
            w = b.Wrapper;
            Response.Write("<h2>TASK #" + txx.ToString() + " branch.id #" + w.id + " starts...</h2>");

            w.Connecting += Wrapper_Connecting;            
            w.Connected += Wrapper_Connected;
            w.ConnectionError += Wrapper_ConnectionError;

            //w.Connect() //disabling multitasking works just fine

            t = new Task(() =>
            {
                w.Connect();
            });

            TaskList[txx] = t;
            t.Start();

        }

        Task.WaitAll(TaskList);

        Response.Write("<h1>Tasks completed</h1>");

        foreach(BranchInfo bb in RemSQL)
        {
            bb.Wrapper.Dispose();
        }

        Response.Flush();
        Response.End();

    }

    private void Wrapper_Connected(object sender)
    {
        Wrapper w = (Wrapper)sender;
        WriteScript("Connected('" + w.id + "');");
    }

    private void Wrapper_Connecting(object sender)
    {
        Wrapper w = (Wrapper)sender;
        WriteScript("Connecting('" + w.id + "');");
    }

    private void Wrapper_ConnectionError(object sender, Exception ex)
    {
        Wrapper w = (Wrapper)sender;
        WriteScript("ConnectionFailed('" + w.id + "', '" + ex.Message + "');");
    }

private void WriteScript(string scr)
{
    Response.Write("<script>" + scr + "</script>\n");
    Response.Flush();
}

And here is the output:

<h1>starting..........</h1><h2>TASK #0 branch.id #2 starts...</h2>
<h2>TASK #1 branch.id #3 starts...</h2>
<h2>TASK #2 branch.id #4 starts...</h2>
<h2>TASK #3 branch.id #5 starts...</h2>
<h2>TASK #4 branch.id #6 starts...</h2>
<h2>TASK #5 branch.id #7 starts...</h2>
<h2>TASK #6 branch.id #8 starts...</h2>
<h2>TASK #7 branch.id #9 starts...</h2>
<h2>TASK #8 branch.id #10 starts...</h2>
<h2>TASK #9 branch.id #11 starts...</h2>
<h2>TASK #10 branch.id #13 starts...</h2>
<h2>TASK #11 branch.id #14 starts...</h2>
<h2>TASK #12 branch.id #15 starts...</h2>
<h2>TASK #13 branch.id #16 starts...</h2>
<h2>TASK #14 branch.id #17 starts...</h2>
<h2>TASK #15 branch.id #19 starts...</h2>
<h2>TASK #16 branch.id #20 starts...</h2>
<script>Connecting('20');</script>
<script>Connecting('20');</script>
<script>Connecting('20');</script>
<script>Connecting('20');</script>
<script>Connecting('20');</script>
<script>Connecting('20');</script>
<script>Connecting('20');</script>
<script>Connecting('20');</script>
<script>Connecting('20');</script>
<script>Connecting('20');</script>
<script>Connecting('20');</script>
<script>Connecting('20');</script>
<script>Connecting('20');</script>
<script>Connecting('20');</script>
<script>Connecting('20');</script>
<script>Connecting('20');</script>
<script>Connecting('20');</script>
<script>ConnectionFailed('20', 'There is already an open DataReader associated with this Connection which must be closed first.');</script>
<script>ConnectionFailed('20', 'There is already an open DataReader associated with this Connection which must be closed first.');</script>
<script>ConnectionFailed('20', 'There is already an open DataReader associated with this Connection which must be closed first.');</script>
<script>ConnectionFailed('20', 'There is already an open DataReader associated with this Connection which must be closed first.');</script>
<script>ConnectionFailed('20', 'There is already an open DataReader associated with this Connection which must be closed first.');</script>
<script>ConnectionFailed('20', 'There is already an open DataReader associated with this Connection which must be closed first.');</script>
<script>ConnectionFailed('20', 'There is already an open DataReader associated with this Connection which must be closed first.');</script>
<script>ConnectionFailed('20', 'There is already an open DataReader associated with this Connection which must be closed first.');</script>
<script>ConnectionFailed('20', 'There is already an open DataReader associated with this Connection which must be closed first.');</script>
<script>Connected('20');</script>
<script>ConnectionFailed('20', 'There is already an open DataReader associated with this Connection which must be closed first.');</script>
<script>ConnectionFailed('20', 'There is already an open DataReader associated with this Connection which must be closed first.');</script>
<script>ConnectionFailed('20', 'There is already an open DataReader associated with this Connection which must be closed first.');</script>
<script>ConnectionFailed('20', 'There is already an open DataReader associated with this Connection which must be closed first.');</script>
<script>ConnectionFailed('20', 'There is already an open DataReader associated with this Connection which must be closed first.');</script>
<script>ConnectionFailed('20', 'There is already an open DataReader associated with this Connection which must be closed first.');</script>
<script>Connected('20');</script>
<h1>Tasks completed</h1>

As you can see, I guess it looks like the Wrapper w always gets the reference of the last call in loop. But when I comment out the tasking part and use w.Connect() directly in the loop, it all fixes up.

Any ideas ?

Answer 1

Your problem is that you're closing over local variable w which you're changing later, so, by the time you got to the task it already contains other value that was at the time of running.

And solution is simple: declare t , b and w inside the loop. In fact, if I were you, I'd have rewritten it into simple Select :

Response.Write("<h1>starting..........</h1>");
Task.WaitAll(
  RemSQL.Select(b => b.Wrapper).Select(w => TaskFactory.Run(() => {
        Response.Write("<h2>TASK #" + txx.ToString() + " branch.id #" + w.id + " starts...</h2>");

        w.Connecting += Wrapper_Connecting;            
        w.Connected += Wrapper_Connected;
        w.ConnectionError += Wrapper_ConnectionError;

        w.Connect();
  })).ToArray());