Multithreading is supported in Windows Phone 7 Silverlight applications, however the emulator has a bug (which I discovered and was confirmed to me by the dev lead of the emulator team): If you attempt to start a background thread in the MainPage constructor, the thread never starts. The reason is a problem with the emulator UI thread which doesn’t leave any time to the background thread to start. Thankfully there is a workaround (see code below). Also, the bug should be corrected in a future release, so it’s not a big deal, even though it is really confusing when you try to understand why the *%&^$£% thread is not &$%&%$£ starting (that was me in the plane the other day ;)

This code does not work:

public partial class MainPage : PhoneApplicationPage
{
    public MainPage()
    {
        InitializeComponent();

        SupportedOrientations = SupportedPageOrientation.Portrait
            | SupportedPageOrientation.Landscape;

        var counter = 0;

        ThreadPool.QueueUserWorkItem(o =>
        {
            while (true)
            {
                Dispatcher.BeginInvoke(() =>
                {
                    textBlockListTitle.Text = (counter++).ToString();
                });
            }
        });
    }
}

This code does work:

public MainPage()
{
    InitializeComponent();

    SupportedOrientations = SupportedPageOrientation.Portrait
        | SupportedPageOrientation.Landscape;

    var counter = 0;

    ThreadPool.QueueUserWorkItem(o =>
    {
        while (true)
        {
            Dispatcher.BeginInvoke(() =>
            {
                textBlockListTitle.Text = (counter++).ToString();
            });

            // NOTICE THIS LINE!!!
            Thread.Sleep(0);
        }
    });
}

Note that even if the thread is started in a later event (for example Click of a Button), the behavior without the Thread.Sleep(0) is not good in the emulator. As of now, i would recommend always sleeping when starting a new thread.

Happy coding:

Laurent

Laurent Bugnion (GalaSoft)
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