ASP.NET应用程序中任务的奇怪行为

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());