Question

Context: I am playing music through a media element, and using a slider to display the point in the song that it is at. That updating is done in a backgroundworker, for obvious reasons.

private void bgPlay_DoWork(object sender,DoWorkEventArgs e)
    {

        while (isMediaPlaying)
        {
            this.Dispatcher.Invoke((Action)(() =>
            {
                timelineSlider.Value = mediaElement1.Position.TotalMilliseconds;
            }));

        }
    }

    private void Library_MouseDoubleClick(object sender, MouseButtonEventArgs e)
    {
        try
        {
            mediaElement1.Stop();
            isMediaPlaying = false;
            mediaElement1.Source = new Uri(songData[Library.SelectedIndex].Location);
            mediaElement1.Volume = (double)volumeSlider.Value;

            mediaElement1.Play();

            isMediaPlaying = true;

            bgPlay.RunWorkerAsync();
        }
        catch(Exception ex) {
            F.MessageBox.Show(ex.ToString());
        }
    }

When I play a song, then double click on a different one, the background worker is still looping and throws an exception because it reaches bgPlay.RunWorkerAsync(); before the previous instance has finished. I tried to use the isMediaPlaying bool to tell the backgroundworker when to exit the loop, but the main thread reaches bgPlay.RunWorkerAsync(); before it finishes.

Answer 1

You are suffering of a common mistake when one is barely starting to program with threading, a race condition

I’d advise rewriting the code like this:

private static String threadingLock = "";

private void bgPlay_DoWork(object sender,DoWorkEventArgs e)
    {

        while (true)
        {
            lock(threadingLock) {
                if(!isMediaPlaying)
                    break;
            }
            this.Dispatcher.Invoke((Action)(() =>
            {
                timelineSlider.Value = mediaElement1.Position.TotalMilliseconds;
            }));

        }
    }

    private void Library_MouseDoubleClick(object sender, MouseButtonEventArgs e)
    {
        try
        {
            lock(threadingLock) {
                isMediaPlaying = false;
            }
            mediaElement1.Stop();

            mediaElement1.Source = new Uri(songData[Library.SelectedIndex].Location);
            mediaElement1.Volume = (double)volumeSlider.Value;

            mediaElement1.Play();

            isMediaPlaying = true;

            bgPlay.RunWorkerAsync();
        }
        catch(Exception ex) {
            F.MessageBox.Show(ex.ToString());
        }
    }

As a friendly tip, add a Thread.sleep(200) before invoking the update on the slider. It will reduce cpu usage without affecting the functionality of your application.