How to stop BackgroundWorker correctly

Question

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

User · Answer

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

User · Answer

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 lt 10000000  i            int j   i 3        inside OtherStuff      public void OtherStuff         for int i 0   i lt 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 lt 10000000  i                              int j   i 3                      public void OtherStuff         if  bgWorker CancellationPending                  for int i 0   i lt 10000000  i                             int j   i 3

User · Answer

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

User · Answer

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     amp  amp     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  amp  amp  context UpdateUiWhenDoneWithPayment WorkerSupportsCancellation  amp  amp  context UpdateUiWhenDoneWithPayment IsBusy              context UpdateUiWhenDoneWithPayment CancelAsync

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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 lt SomeObject gt list   new List lt SomeObject gt         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

[c#] How to stop BackgroundWorker correctly

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