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POEnsuring execution order between multiple threads
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copied!<p>I am having a kinda annoying problem mostly due to my low skill level/experience in C# multithreading.</p> <p>Here is the background. In my framework, I have a static class named <code>WaitFormHelper</code>, which has two static methods (well... actually more but we don't care here), <code>Start()</code> and <code>Close()</code> The <code>Start()</code> method initializes and starts a thread which will acquire a lock on the <code>locker</code> object and create a <code>WaitForm</code> (which is a small loading control with a custom message and a progress bar)</p> <p>In my current project, I have a method which starts a <code>WaitForm</code>, performs calculations, then closes the <code>WaitForm</code>. Nothing fancy at all. The method looks like the following, I simplified it as much as possible:</p> <pre><code>public void PerformCalculations() { try { WaitFormHelper.Start("Title", "message", false); if (this.CalculationsParameters.IsInvalid) { return; } // Perform all those lengthy calculations here } // catch whatever exception I may have to catch, we don't care here finally { WaitFormHelper.Close(); } } </code></pre> <p>Here are the <code>Start()</code> and <code>Close()</code> methods with related methods & attributes, simplified as well:</p> <pre><code>private static Thread instanceCaller; private static WaitForm instance; private static AutoResetEvent waitFormStarted = new AutoResetEvent(false); private static object locker = new object(); /// <summary> /// Initializes WaitForm to start a single task /// </summary> /// <param name="header">WaitForm header</param> /// <param name="message">Message displayed</param> /// <param name="showProgressBar">True if we want a progress bar, else false</param> public static void Start(string header, string message, bool showProgressBar) { InitializeCallerThread(showProgressBar, header, message); instanceCaller.Start(); } /// <summary> /// Initializes caller thread for executing a single command /// </summary> /// <param name="showProgressBar"></param> /// <param name="header"></param> /// <param name="message"></param> private static void InitializeCallerThread(bool showProgressBar, string header, string message) { waitFormStarted.Reset(); instanceCaller = new Thread(() => { lock (locker) { instance = new WaitForm() { Header = header, Message = message, IsProgressBarVisible = showProgressBar }; waitFormStarted.Set(); } instance.ShowDialog(); }); instanceCaller.Name = "WaitForm thread"; instanceCaller.SetApartmentState(ApartmentState.STA); instanceCaller.IsBackground = true; } /// <summary> /// Closes current form /// </summary> public static void Close() { lock (locker) { if (instance != null && !instance.IsClosed) { waitFormStarted.WaitOne(); instance.FinalizeWork(); instance.Dispatcher.Invoke( new Action(() => { instance.Close(); })); } } } </code></pre> <p><strong>Now let's get to the problem</strong></p> <p>This usually works fine, except in this case: If <code>this.CalculationsParameters.IsInvalid</code> is true (ie. as you probably already understood, I can't perform calculations for some reason, such as "user entering crap in my forms"), the execution directly closes my <code>WaitForm</code>. However in this case, the main thread will reach the <code>Close</code> method and acquire a lock on the <code>locker</code> object <strong>before</strong> the thread fired by the <code>Start()</code> method.</p> <p>What happens is that: <code>Close</code> acquires lock, tries to close the form but <code>instance</code> is still null because the <code>Thread</code> created in <code>InitializeCallerThread</code> is still waiting for the lock to actually create it. <code>Close</code> releases lock, <code>InitializeCallerThread</code> acquires it and... shows a <code>WaitForm</code> which will not close.</p> <p>Now I know that I can simply fix this problem by testing if the calculation parameters are invalid before actually starting the WaitForm, but well, problem is this <code>WaitForm</code> is supposed to be used by all applications of our framework (which includes 40+ different apps used and maintained in 4 countries), so ideally I'd rather prefer seeing my <code>WaitForm</code> working at all cases.</p> <p>Do you have any idea on how could I synchronize this to make sure the starter thread is definitely called and executed first? As you can see I already use an <code>AutoResetEvent</code> for this matter, but if I listen to it before the test <code>if (instance != null)</code> I will end up stuck in my case.</p> <p>Hope this is clear enough! Thank you!</p>

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