Crash from Invoke when MainForm is Disposed in FormClosing

Question

I have an issue that relates to threading, cleaning up unmanaged resources and shutting down my app.

In the main UI thread I have a method that creates a new instance of class Worker. In Worker's constructor I start a new thread that has a while(Scanning) loop that updates some controls in my UI using Invoke() continuously (until Scanning bool is set to false). In the UI thread I raise the event FormClosing() whenever the application is closing down (through X button or Application.Exit() etc.). In FormClosing() I set Scanning to false and do some cleanup of unmanaged resources (that can only be done after the worker thread is done, because it uses those resources. The problem is that when I close the app down the MainForm apparently gets instantly disposed, so the app crashes at the Invoke (because it is trying to make a delegate run from UI thread, but that thread is disposed).

In an attempt to make the worker finish before the UI closes I tried to create a method StopWorker() in the worker class where I put Scanning = false, and then Thread.Join. As you can imagine the Join caused a deadlock as it makes the UI thread sleep but the Invoke needs the UI thread to move on.

In summary I need to cleanup unmanaged resources in FormClosing. I need the worker thread to be done before I do that though, as it uses these resources. The worker thread cannot finish (it uses Invoke) if the MainForm is disposed, therefore creating a tricky situation.

Answer 1

Based on Hans Passant's answer here, I created the below solution. It seems to be working very well.

In UI class/thread:

private void Form1_FormClosing(object sender, FormClosingEventArgs e)
{
    var button = sender as Button;
    if (button != null && string.Equals(button.Name, @"CloseButton"))
    {
        //FormClosing event raised by a user-created button action
    }
    else
    {
        //FormClosing event raised by program or the X in top right corner
        //Do cleanup work (stop threads and clean up unmanaged resources)
        if (_bw.Scanning)
        {
            _bw.Scanning = false;
            ClosePending = true;
            e.Cancel = true;
            return;
        }

        //Code to clean up unmanaged resources goes here (dummy code below)
        ApplicationLogger.Get.Log("Doing final cleanup work and exiting application...");
        MemoryHandler.Get.Dispose();
        ApplicationLogger.Get.Dispose();
    }
}

My worker thread is in another class that has a public bool property called Scanning. It also has this while loop (notice the line at the bottom):

private void Worker()
{
    while (Scanning)
    {
        Thread.Sleep(50);

        _sendBackValue[0] = "lbOne";
        _sendBackValue[1] = "blaBla";
        _synch.Invoke(_valueDelegate, _sendBackValue);

        _sendBackValue[0] = "lbTwo";
        _sendBackValue[1] = "blaBla";
        _synch.Invoke(_valueDelegate, _sendBackValue);

        _sendBackValue[0] = "lbThree";
        _sendBackValue[1] = "blaBla";
        _synch.Invoke(_valueDelegate, _sendBackValue);
    }

    MainForm.Get.Invoke((Action)(() => MainForm.Get.StopScanning()));
}

Finally, back in the UI class/thread I have this method:

public void StopScanning()
{
    if (!ClosePending) return;
    ApplicationLogger.Get.Log("Worker thread is closing the application...");
    Close();
}