Question

I have a WPF window with a button that spawns a BackgroundWorker thread to create and send an email. While this BackgroundWorker is running, I want to display a user control that displays some message followed by an animated “…”. That animation is run by a timer inside the user control.

Even though my mail sending code is on a BackgroundWorker, the timer in the user control never gets called (well, it does but only when the Backgroundworker is finished, which kinda defeats the purpose…).

Relevant code in the WPF window:

private void button_Send_Click(object sender, RoutedEventArgs e)
{
    busyLabel.Show(); // this should start the animation timer inside the user control

    BackgroundWorker worker = new BackgroundWorker();
    worker.RunWorkerCompleted += new RunWorkerCompletedEventHandler(worker_RunWorkerCompleted);
    worker.DoWork += new DoWorkEventHandler(worker_DoWork);
    worker.RunWorkerAsync();      
}

void worker_DoWork(object sender, DoWorkEventArgs e)
{
    this.Dispatcher.Invoke((Action)(() =>
    {      
        string body = textBox_Details.Text;
        body += "User-added addtional information:" + textBox_AdditionalInfo.Text; 

        var smtp = new SmtpClient
        {
            ...
        };

        using (var message = new MailMessage(fromAddress, toAddress)
        {
            Subject = subject,
            Body = body
        })
        {
            smtp.Send(message);
        }
    }));

}

Relevant code in the user control (“BusyLabel”):

public void Show()
{
    tb_Message.Text = Message;
    mTimer = new System.Timers.Timer();
    mTimer.Interval = Interval;
    mTimer.Elapsed += new ElapsedEventHandler(mTimer_Elapsed);
    mTimer.Start();
}

void mTimer_Elapsed(object sender, ElapsedEventArgs e)
{
    this.Dispatcher.Invoke((Action)(() =>
    { 

        int numPeriods = tb_Message.Text.Count(f => f == '.');
        if (numPeriods >= NumPeriods)
        {
            tb_Message.Text = Message;
        }
        else
        {
            tb_Message.Text += '.';
        }         
    }));
}

public void Hide()
{
    mTimer.Stop();
}

Any ideas why it’s locking up?

Answer 1

Using Dispatcher.Invoke in your worker_DoWork method is putting execution back on the UI thread, so you are not really doing the work asynchronously.

You should be able to just remove that, based on the code you are showing.

If there are result values that you need to show after the work is complete, put it in the DoWorkEventArgs and you will be able to access it (on the UI thread) in the worker_RunWorkerCompleted handler’s event args.

A primary reason for using BackgroundWorker is that the marshalling is handled under the covers, so you shouldn’t have to use Dispatcher.Invoke.