How to stop BackgroundWorker on Form's Closing event?

user_93422 0
Viewed 52926

I have a form that spawns a BackgroundWorker, that should update form's own textbox (on main thread), hence Invoke((Action) (...)); call.
If in HandleClosingEvent I just do bgWorker.CancelAsync() then I get ObjectDisposedException on Invoke(...) call, understandably. But if I sit in HandleClosingEvent and wait for bgWorker to be done, than .Invoke(...) never returns, also understandably.

Any ideas how do I close this app without getting the exception, or the deadlock?

Following are 3 relevant methods of the simple Form1 class:

    public Form1() {
        InitializeComponent();
        Closing += HandleClosingEvent;
        this.bgWorker.RunWorkerAsync();
    }

    private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e) {
        while (!this.bgWorker.CancellationPending) {
            Invoke((Action) (() => { this.textBox1.Text = Environment.TickCount.ToString(); }));
        }
    }

    private void HandleClosingEvent(object sender, CancelEventArgs e) {
        this.bgWorker.CancelAsync();
        /////// while (this.bgWorker.CancellationPending) {} // deadlock
    }
backgroundworker multithreading winforms c#
12 Answers

I've found another way. If you have more backgroundWorkers you can make:

List<Thread> bgWorkersThreads  = new List<Thread>();

and in every backgroundWorker's DoWork method make:

bgWorkesThreads.Add(Thread.CurrentThread);

Arter that you can use:

foreach (Thread thread in this.bgWorkersThreads) 
{
     thread.Abort();    
}

I used this in Word Add-in in Control, which i use in CustomTaskPane. If someone close the document or application earlier then all my backgroundWorkes finishes their work, it raises some COM Exception(I don't remember exatly which).CancelAsync() doesn't work.

But with this, I can close all threads which are used by backgroundworkers Immediately in DocumentBeforeClose event and my problem is solved.

user_12591290

Firstly, the ObjectDisposedException is only one possible pitfall here. Running the OP's code has produced the following InvalidOperationException on a substantial number of occasions:

Invoke or BeginInvoke cannot be called on a control until the window handle has been created.

I suppose this could be amended by starting the worker on the 'Loaded' callback rather than the constructor, but this entire ordeal can be avoided altogether if BackgroundWorker's Progress reporting mechanism is used. The following works well: 

private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
    while (!this.bgWorker.CancellationPending)
    {
        this.bgWorker.ReportProgress(Environment.TickCount);
        Thread.Sleep(1);
    }
}

private void bgWorker_ProgressChanged(object sender, ProgressChangedEventArgs e)
{
    this.textBox1.Text = e.ProgressPercentage.ToString();
}

I kind of hijacked the percentage parameter but one can use the other overload to pass any parameter.

It is interesting to note that removing the above sleep call clogs the UI, consumes high CPU and continually increases the memory use. I guess it has something to do with the message queue of the GUI being overloaded. However, with the sleep call intact, the CPU usage is virtually 0 and the memory usage seems fine, too. To be prudent, perhaps a higher value than 1 ms should be used? An expert opinion here would be appreciated... Update: It appears that as long as the update isn't too frequent, it should be OK: Link

In any case, I can't foresee a scenario where the updating of the GUI has to be in intervals shorter than a couple of milliseconds (at least, in scenarios where a human is watching the GUI), so I think most of the time progress reporting would be the right choice

user_678240

I really dont see why DoEvents is regarded as such a bad choice in this case if you are using this.enabled = false. I think it would make it quite neat.

protected override void OnFormClosing(FormClosingEventArgs e) {

    this.Enabled = false;   // or this.Hide()
    e.Cancel = true;
    backgroundWorker1.CancelAsync();  

    while (backgroundWorker1.IsBusy) {

        Application.DoEvents();

    }

    e.cancel = false;
    base.OnFormClosing(e);

}
user_23739270

Another way:

if (backgroundWorker.IsBusy)
{
    backgroundWorker.CancelAsync();
    while (backgroundWorker.IsBusy)
    {
        Application.DoEvents();
    }
}
user_13135400
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