Question

I have the following code in a very simple Silverlight 4 app.

    void MainPage_Loaded(object sender, RoutedEventArgs e)
    {
        var result = string.Empty;
        var mutex = new AutoResetEvent(false);
        var wc = new WebClient();
        wc.DownloadStringCompleted += (s, args) => {
            result = args.Result;
            mutex.Set();
        };
        wc.DownloadStringAsync(new Uri("http://localhost/SilverlightApplication2.Web/SilverlightApplication2TestPage.aspx", UriKind.Absolute));
        mutex.WaitOne();
        MessageBox.Show(result);
    }

For whatever reason the mutex.Set() is never called so the mutex.WaitOne() just locks up. What am I missing? I have also tried ManualResetEvent.

Thanks,
Randall

UPDATE1:

If I do the following it works as expected.

    void MainPage_Loaded(object sender, RoutedEventArgs e)
    {
        ThreadPool.QueueUserWorkItem(GetIt);
    }

    private void GetIt(object state)
    {
        var result = string.Empty;
        var mutex = new AutoResetEvent(false);

        var wc = new WebClient();
        wc.DownloadStringCompleted += (s, args) => {
            result = args.Result;
            mutex.Set();
        };
        wc.DownloadStringAsync(new Uri("http://localhost/SilverlightApplication2.Web/SilverlightApplication2TestPage.aspx", UriKind.Absolute));

        mutex.WaitOne();
        Dispatcher.BeginInvoke(() => MessageBox.Show(result));
    }

It almost seems like there is an issue with the UI thread that causes the callback to not be executed.

Answer 1

Here is the answer: http://www.codeproject.com/KB/silverlight/SynchronousSilverlight.aspx

You cannot block the UI thread in silverlight.