Whats wrong with my AutoResetEvent code?

Question

Hi, I have this code which seems pretty straightforward but the AutoResetEvent never gets signalled. Nothing seems to get returned from the web services and the WaitAll just times out after ten seconds. Everything works fine without the threading jiggerypokery so its not a web service issue. What am I doing wrong?

    AutoResetEvent[] autoEvents;
    ObservableCollection<Tx3.ResourceService.ResourceTime> resourceTime;
    ObservableCollection<Tx3.ResourceService.ResourceTimeDetail> resourceTimeDetail;

    private void UserControl_Loaded(object sender, RoutedEventArgs e)
    {
        autoEvents = new AutoResetEvent[]
        {
            new AutoResetEvent(false),
            new AutoResetEvent(false),
        };

        var resourceService = getResourceServiceClient();

        // Get ResourceTime data for this user
        resourceService.ListResourceTimeAsync(CategoryWorkItemId, ResourceId);
        resourceService.ListResourceTimeCompleted += new EventHandler<Tx3.ResourceService.ListResourceTimeCompletedEventArgs>(resourceService_ListResourceTimeCompleted);

        // Get ResourceTimeDetails
        resourceService.ListResourceTimeDetailAsync(CategoryWorkItemId, ResourceId);
        resourceService.ListResourceTimeDetailCompleted += new EventHandler<ListResourceTimeDetailCompletedEventArgs>(resourceService_ListResourceTimeDetailCompleted);

        WaitHandle.WaitAll(autoEvents, 10000);

        System.Diagnostics.Debug.WriteLine("do something with both datasets");
    }

    void resourceService_ListResourceTimeCompleted(object sender, Tx3.ResourceService.ListResourceTimeCompletedEventArgs e)
    {
        resourceTime = e.Result;
        autoEvents[0].Set();
    }

    void resourceService_ListResourceTimeDetailCompleted(object sender, ListResourceTimeDetailCompletedEventArgs e)
    {
        resourceTimeDetail = e.Result;
        autoEvents[1].Set();
    }

Answer 1

I can offer a naive first guess: it looks like you're adding the event handlers after calling the methods that start the asynchronous operations; it's possible there's a race condition in there or some other issue. Could you switch the order of operations so you attach the event handler, and then begin the operation?

Answer 2

These are AutoResetEvent objects -- looks like you want a ManualResetEvent -- the auto version triggers anything waiting, but immediately resets. Manual ones stay triggered so if the callback happens before you get to the WaitAll, it'll just fall through immediately.

Also, qid is correct -- you're attaching your event handlers too late too...so there's two different bugs going on here.

Answer 3

Are you using this code on a thread that is marked with the STA attribute, for example the main UI thread? If so, the WaitAll method is not supported on these threads.

Check here.