Invoke Method never returns

Question

I have a thread running, which grabs images from a camera, saves them, and updates the pictureBox to display that bitmap. When I want to exit the application, I set my bool exitApplication to true, but it never returns on the join() call.

    public void GrabThread()
    {
        Bitmap copyBmp;
        while (!exitApplication)
        {
            mImage = m_camera.GrabImage();
            SaveImage();

            copyBmp = (Bitmap)mImage.bmp.Clone();
            if (pictureBox1.InvokeRequired)
            {
                pictureBox1.Invoke(new MethodInvoker(
                delegate ()
                {
                    pictureBox1.Image = copyBmp;
                }));
            }
            else
            {
                pictureBox1.Image = copyBmp;
            }
            m_camera.ReleaseImage();
        }
    }

My Exit code:

        exitApplication = true;
        threadGrab.Join();
        m_camera.Stop();
        Close();

When I call Break All from the Debug menu, the Thread hangs at

pictureBox1.Invoke(new MethodInvoker(
delegate ()
{
    pictureBox1.Image = copyBmp;
}));

Why? And how can I prevent this? Currently I have to call Abort() on the thread to exit it.

Answer 1

This code deadlocks because pictureBox1.Invoke is trying to execute code in the UI thread. That thread though is blocked by the call to threadGrab.Join();

This can be fixed using async/await . await awaits already executing asynchronous tasks without blocking the way Join() does. When it finishes, execution resumes in the original synchronization context. In a WinForms or WPF application, this means execution resumes on the UI thread.

I assume the only thing that needs to run in the background is m_camera.GrabImage(); . In that case, the code could look something like this :

public async void Button1_Click(object sender, EventArgs args)
{
    await Task.Run(()=>m_camera.GrabImage();
    //Back in the UI thread
    SaveImage();
    var copyBmp = (Bitmap)mImage.bmp.Clone();
    pictureBox1.Image = copyBmp;
    m_camera.ReleaseImage();
}

Task.Run will execute m_camera.GrabImage() in a threadpool thread. await will await for this to finish without blocking the UI thread. When it finishes, execution resumes in the UI thread which means there's no need for Invoke .

This could run in a loop too :

public async void Capture_Click(object sender, EventArgs args)
{
    while(someCondition)
    {
        await Task.Run(()=>m_camera.GrabImage();

        SaveImage();
        var copyBmp = (Bitmap)mImage.bmp.Clone();
        pictureBox1.Image = copyBmp;
        m_camera.ReleaseImage();
    }
}

Answer 2

It may not the right answer but you can using both OnClosing and OnClosed to Stop your camera then close your app, I don't know does it work in Winform but it worked with me in WPF

protected override void OnClosing(CancelEventArgs e)
{

            e.Cancel = false; //prevent your application to shutdown
            Camera.Stop(); //stop your camera
            base.OnClosing(e);

}

protected override void OnClosed(EventArgs e)
{
            base.OnClosed(e);
            Application.Current.Shutdown(); //then shutdown again
}

Via 2 step, your camera will stop without Join any thread. And note that this code is written in Wpf, not Winform but I thought you can find a similar way to do in Winform

Hope this answer may help you.