How to avoid starving the main thread?

Question

I'm doing a patch to solve a progress bar issue in an application that is a bit mess-up. The cancel on the progress bar used to do a Thread.Abort on the thread doing the heavy work. I changed that to raising a cancel flag that I can check at strategic place in the thread.

Most of the time it works fine but once in a while the cancellation doesn't work at all. I suppose I could do a Application.DoEvents before looking at the status of the flag (there is no risk of reentry) but I would like a more "clean" option.

I would appreciate if someone could provide me information to understand what exactly is going on and how this stuff works behind the scene. I would like to know how to deal with this issue without using the BackgroundWorker (like you would in .net 1.1) but I would also like to know if the BackgroundWorker solve that kind of problems and how it does it.

Edit: I'm taking notes of you suggestions and will try some tomorrow and report back. I used a volatile bool at first by I think I updated it to an automatic property and forgot about the volatile. Could the worker thread keeps looking for the cached value again and again? I don't see how I could have a deathlock. The worker to check for the flag since I managed to break there with by placing a breakpoint on-the-fly. I always test with the same set of data and most of the time it cancels just fine. The only thing that change between tests is the moment I press cancel. So far, I only tested in debug, started from VS.

Edit 2: It turns out that my issue isn't related to my flag or anything I added. It's more of a WinForm issue. The program get to call a ShowDialog (and there is already another ShowDialog blocked). I cannot drag the form and it doesn't refresh by itself. The cancel button on it doesn't even works. Here is the call stack when I pause everything.

[Code externe]  
    Mrnf.Son.Commun.dll!Mrnf.Son.Commun.Messages.BarreProgressionBase.ShowDialog(System.Windows.Forms.IWin32Window fenetre = {Mrnf.Son.Presentation.Windows.UI.Echanges.AssistantForm}) Ligne 274 + 0xb octets  C#
    Mrnf.Son.Commun.dll!Mrnf.Son.Commun.Controleurs.Utils.AttendreFinTraitement(System.Windows.Forms.Form parent = {Mrnf.Son.Presentation.Windows.UI.Echanges.AssistantForm}, Mrnf.Son.Commun.Messages.BarreProgressionBase progressionBase = {Mrnf.Son.Commun.Messages.BarreProgressionMessage}, System.Threading.Thread thread = {System.Threading.Thread}) Ligne 302 + 0xd octets    C#
    Mrnf.Son.Affaires.dll!Mrnf.Son.Affaires.Persisteurs.Echanges.LecteurDBFGeneriqueCollection.Importer(System.Windows.Forms.Form parent = {Mrnf.Son.Presentation.Windows.UI.Echanges.AssistantForm}) Ligne 95 + 0x1d octets    C#
    Mrnf.Son.Affaires.dll!Mrnf.Son.Affaires.Persisteurs.Echanges.PersisteurModeleEchanges.Importer(Mrnf.Son.Affaires.Entites.Echanges.ModeleEchanges unModele = {Mrnf.Son.Presentation.Windows.Controleurs.Echanges.ModeleEchanges.ModeleEchangesGenerique}, System.Windows.Forms.Form formParent = {Mrnf.Son.Presentation.Windows.UI.Echanges.AssistantForm}) Ligne 1880 + 0xd octets  C#
    Mrnf.Son.Affaires.dll!Mrnf.Son.Affaires.Entites.Echanges.ModeleEchanges.Importer(System.Windows.Forms.Form formParent = {Mrnf.Son.Presentation.Windows.UI.Echanges.AssistantForm}) Ligne 875 + 0x18 octets  C#
    Mrnf.Son.Presentation.Windows.exe!Mrnf.Son.Presentation.Windows.UI.Echanges.AssistantForm.EffectuerImport(Mrnf.Son.Affaires.Entites.Echanges.IModeleEchanges modele = {Mrnf.Son.Presentation.Windows.Controleurs.Echanges.ModeleEchanges.ModeleEchangesGenerique}) Ligne 1429 + 0xc octets  C#
    Mrnf.Son.Presentation.Windows.exe!Mrnf.Son.Presentation.Windows.UI.Echanges.AssistantForm._terminerBtn_Click(object sender = {Text = Impossible d'évaluer l'expression, car un frame natif se trouve en haut de la pile des appels.}, System.EventArgs e = {System.EventArgs}) Ligne 1334 + 0x1d octets C#
[Code externe]  
    Mrnf.Son.Presentation.Windows.exe!Mrnf.Son.Presentation.Windows.UI.Echanges.AssistantForm.WndProc(ref System.Windows.Forms.Message m = {System.Windows.Forms.Message}) Ligne 1133 + 0xb octets C#
[Code externe]  
    Mrnf.Son.Presentation.Windows.exe!Mrnf.Son.Presentation.Windows.Controleurs.Sondages.ActionsSondages.OnImporterSysExt() Ligne 1362 + 0x1f octets    C#
    Mrnf.Son.Presentation.Windows.exe!Mrnf.Son.Presentation.Windows.UI.Sondages.UEExploitationVue._mniImporterSysExt_Click(object sender = {System.Windows.Forms.ToolStripMenuItem}, System.EventArgs e = {System.EventArgs}) Ligne 820 + 0x12 octets   C#
[Code externe]  
    Mrnf.Son.Presentation.Windows.exe!Mrnf.Son.Presentation.Windows.Program.Main() Ligne 148 + 0x8 octets   C#
[Code externe]  

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Edit 3: If I pass null to the ShowDialog it works fine (the UI doesn't freeze, the cancel button works, it cancels fine). I don't really understand the magic behind all this.

Answer 1

BackgroundWorker doesn't do anything extra here, except to provide that flag in an easy to check location. So there are a few possibilities:

• your code is getting to check the flag, but not noticing that it has changed (theoretically possible if the flag isn't synchronized or volatile, but very unlikely in non-trivial code)
• your code isn't getting to check the flag

We'll assume the latter; a few common causes of that:

• you are accidentally deadlocking yourself (perhaps trying to Invoke back to the UI thread which is already waiting for a background thread, or getting into a loop around a lock or similar)
• you are calling out over COM (or similar), and that call is never completing - a particularly hard scenario to escape from in some cases

Can you narrow down which of these it is? Perhaps inject some code to track what your thread is doing at intervals, and ensure it is doing something useful - or if not, track where it is getting stuck.

Answer 2

Application.DoEvents is a means to allow pending events in the message pump to be processed. Normally should have absolutely nothing to do with your background thread.

If the the cancel 'doesn't work at all', the solution will largely depend on what 'doesn't work at all' mean. Are you unable to change the flag? Is the UI stuck? Does the background thread not respond to the flag chane? Is it something else? The solution depends primarily on what exactly the problem is. It could be that you aren't checking the flag from the background, it could be that you deadlock the two threads. Showing the code, or elaborating the problem details would help.

Answer 3

This is almost always easy to debug. When you see the worker thread ignoring the cancel request, use Debug + Break All. Then Debug + Windows + Threads and double-click the worker thread. Then look at the call stack to see what the thread is doing and why it doesn't pass through the code that checks the flag.

Beware that you have to declare the flag member with the volatile keyword. This prevents the JIT compiler from generating machine code that loads the member value in a register and never checking the actual variable value in memory. Which is liable to happen when you run the Release version of your program without a debugger. In that case, be sure to use Tools + Attach to Process to attach the debugger before you use the Break All command.

A ManualResetEvent, checked with a WaitOne(0) call is better.