How to stop BackgroundWorker correctly

I have a form with 2 comboboxes on it. And I want to fill combobox2.DataSource based on combobox1.Text and combobox2.Text (I assume that the user has completed input in combobox1 and is in the middle of inputting in combobox2). So I have an event handler for combobox2 like this:

private void combobox2_TextChanged(object sender, EventArgs e)
{
    if (cmbDataSourceExtractor.IsBusy)
       cmbDataSourceExtractor.CancelAsync();

    var filledComboboxValues = new FilledComboboxValues{ V1 = combobox1.Text,
       V2 = combobox2.Text};
    cmbDataSourceExtractor.RunWorkerAsync(filledComboboxValues );
}

As far as building DataSource is time-consuming process (it creates a request to database and executes it) I decided that it's better to perform it in another process using BackgroundWorker. So there's a scenario when cmbDataSourceExtractor hasn't completed its work and the user types one more symbol. In this case I get an exception on this line
cmbDataSourceExtractor.RunWorkerAsync(filledComboboxValues ); about that BackgroundWorker is busy and cannot perform several actions in the same time.
How to get rid of this exception?
Thanks in advance!

CancelAsync doesn't actually abort your thread or anything like that. It sends a message to the worker thread that work should be cancelled via BackgroundWorker.CancellationPending. Your DoWork delegate that is being run in the background must periodically check this property and handle the cancellation itself.

The tricky part is that your DoWork delegate is probably blocking, meaning that the work you do on your DataSource must complete before you can do anything else (like check for CancellationPending). You may need to move your actual work to yet another async delegate (or maybe better yet, submit the work to the ThreadPool), and have your main worker thread poll until this inner worker thread triggers a wait state, OR it detects CancellationPending.

http://msdn.microsoft.com/en-us/library/system.componentmodel.backgroundworker.cancelasync.aspx

http://www.codeproject.com/KB/cpp/BackgroundWorker_Threads.aspx

If you add a loop between the CancelAsync() and the RunWorkerAsync() like so it will solve your problem

 private void combobox2_TextChanged(object sender, EventArgs e)
 {
     if (cmbDataSourceExtractor.IsBusy)
        cmbDataSourceExtractor.CancelAsync();

     while(cmbDataSourceExtractor.IsBusy)
        Application.DoEvents();

     var filledComboboxValues = new FilledComboboxValues{ V1 = combobox1.Text,
        V2 = combobox2.Text};
     cmbDataSourceExtractor.RunWorkerAsync(filledComboboxValues );
  }

The while loop with the call to Application.DoEvents() will hault the execution of your new worker thread until the current one has properly cancelled, keep in mind you still need to handle the cancellation of your worker thread. With something like:

 private void cmbDataSourceExtractor_DoWork(object sender, DoWorkEventArgs e)
 {
      if (this.cmbDataSourceExtractor.CancellationPending)
      {
          e.Cancel = true;
          return;
      }
      // do stuff...
 }

The Application.DoEvents() in the first code snippet will continue to process your GUI threads message queue so the even to cancel and update the cmbDataSourceExtractor.IsBusy property will still be processed (if you simply added a continue instead of Application.DoEvents() the loop would lock the GUI thread into a busy state and would not process the event to update the cmbDataSourceExtractor.IsBusy)

You will have to use a flag shared between the main thread and the BackgroundWorker, such as BackgroundWorker.CancellationPending. When you want the BackgroundWorker to exit, just set the flag using BackgroundWorker.CancelAsync().

MSDN has a sample: http://msdn.microsoft.com/en-us/library/system.componentmodel.backgroundworker.cancellationpending.aspx

MY example . DoWork is below:

    DoLengthyWork();

    //this is never executed
    if(bgWorker.CancellationPending)
    {
        MessageBox.Show("Up to here? ...");
        e.Cancel = true;
    }

inside DoLenghtyWork :

public void DoLenghtyWork()
{
    OtherStuff();
    for(int i=0 ; i<10000000; i++) 
    {  int j = i/3; }
}

inside OtherStuff() :

public void OtherStuff()
{
    for(int i=0 ; i<10000000; i++) 
    {  int j = i/3; }
}

What you want to do is modify both DoLenghtyWork and OtherStuff() so that they become:

public void DoLenghtyWork()
{
    if(!bgWorker.CancellationPending)
    {              
        OtherStuff();
        for(int i=0 ; i<10000000; i++) 
        {  
             int j = i/3; 
        }
    }
}

public void OtherStuff()
{
    if(!bgWorker.CancellationPending)
    {  
        for(int i=0 ; i<10000000; i++) 
        {  
            int j = i/3; 
        }
    }
}

The problem is caused by the fact that cmbDataSourceExtractor.CancelAsync() is an asynchronous method, the Cancel operation has not yet completed when cmdDataSourceExtractor.RunWorkerAsync(...) exitst. You should wait for cmdDataSourceExtractor to complete before calling RunWorkerAsync again. How to do this is explained in this SO question.

In my case, I had to pool database for payment confirmation to come in and then update WPF UI.

Mechanism that spins up all the processes:

public void Execute(object parameter)
        {
            try
            {
                var url = string.Format("{0}New?transactionReference={1}", Settings.Default.PaymentUrlWebsite, "transactionRef");
                Process.Start(new ProcessStartInfo(url));
                ViewModel.UpdateUiWhenDoneWithPayment = new BackgroundWorker {WorkerSupportsCancellation = true};
                ViewModel.UpdateUiWhenDoneWithPayment.DoWork += ViewModel.updateUiWhenDoneWithPayment_DoWork;
                ViewModel.UpdateUiWhenDoneWithPayment.RunWorkerCompleted += ViewModel.updateUiWhenDoneWithPayment_RunWorkerCompleted;
                ViewModel.UpdateUiWhenDoneWithPayment.RunWorkerAsync();
            }
            catch (Exception e)
            {
                ViewModel.Log.Error("Failed to navigate to payments", e);
                MessageBox.Show("Failed to navigate to payments");
            }
        }

Mechanism that does checking for completion:

 private void updateUiWhenDoneWithPayment_DoWork(object sender, DoWorkEventArgs e)
    {
        Thread.Sleep(30000);
        while (string.IsNullOrEmpty(GetAuthToken()) && !((BackgroundWorker)sender).CancellationPending)
        {
            Thread.Sleep(5000);
        }

        //Plug in pooling mechanism
        this.AuthCode = GetAuthToken();
    }

Mechanism that cancels if window gets closed:

private void PaymentView_OnUnloaded(object sender, RoutedEventArgs e)
    {
        var context = DataContext as PaymentViewModel;
        if (context.UpdateUiWhenDoneWithPayment != null && context.UpdateUiWhenDoneWithPayment.WorkerSupportsCancellation && context.UpdateUiWhenDoneWithPayment.IsBusy)
            context.UpdateUiWhenDoneWithPayment.CancelAsync();
    }

My answer is a bit different because I've tried these methods but they didn't work. My code uses an extra class that checks for a Boolean flag in a public static class as the database values are read or where I prefer it just before an object is added to a List object or something as such. See the change in the code below. I added the ThreadWatcher.StopThread property. for this explation I'm nog going to reinstate the current thread because it's not your issue but that's as easy as setting the property to false before accessing the next thread...

private void combobox2_TextChanged(object sender, EventArgs e)
 {
  //Stop the thread here with this
     ThreadWatcher.StopThread = true;//the rest of this thread will run normally after the database function has stopped.
     if (cmbDataSourceExtractor.IsBusy)
        cmbDataSourceExtractor.CancelAsync();

     while(cmbDataSourceExtractor.IsBusy)
        Application.DoEvents();

     var filledComboboxValues = new FilledComboboxValues{ V1 = combobox1.Text,
        V2 = combobox2.Text};
     cmbDataSourceExtractor.RunWorkerAsync(filledComboboxValues );
  }

all fine

private void cmbDataSourceExtractor_DoWork(object sender, DoWorkEventArgs e)
 {
      if (this.cmbDataSourceExtractor.CancellationPending)
      {
          e.Cancel = true;
          return;
      }
      // do stuff...
 }

Now add the following class

public static class ThreadWatcher
{
    public static bool StopThread { get; set; }
}

and in your class where you read the database

List<SomeObject>list = new List<SomeObject>();
...
if (!reader.IsDbNull(0))
    something = reader.getString(0);
someobject = new someobject(something);
if (ThreadWatcher.StopThread == true)
    break;
list.Add(something);
...

don't forget to use a finally block to properly close your database connection etc. Hope this helps! Please mark me up if you find it helpful.

I agree with guys. But sometimes you have to add more things.

IE

1. Add this worker.WorkerSupportsCancellation = true;

2. Add to you class some method to do the following things

   public void KillMe() { worker.CancelAsync(); worker.Dispose(); worker = null; GC.Collect(); }

So before close your application your have to call this method.

3. Probably you can Dispose, null all variables and timers which are inside of the BackgroundWorker.