Establishing the Pattern in C#.NET

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Establishing the Pattern
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The process of hooking up the background worker pattern is as follows
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1 Register the long-running method with the BackgroundWorkerDoWork event In this example, the long-running task is the call to CalculatePi() 2 To receive progress or status notifications, hook up a listener to BackgroundWorkerProgressChanged and set BackgroundWorkerWorkerReportsProgress to true In Listing 168, the UpdateDisplayWithMoreDigits() method takes care of updating the display as more digits become available 3 Register a method (Complete()) with the BackgroundWorkerRunWorkerCompleted event 4 Assign the WorkerSupportsCancellation property to support cancellation Once this property is assigned the value true, a call to BackgroundWorkerCancelAsync will set the DoWorkEventArgsCancellationPending flag 5 Within the DoWork-provided method (CalculatePi() ), check the DoWorkEventArgsCancellationPending property and exit the method when it is true 6 Once everything is set up, you can start the work by calling BackgroundWorkerRunWorkerAsync() and providing a state parameter that is passed to the specified DoWork() method When it is broken down into steps, the background worker pattern is relatively easy to follow and provides the advantage over the asynchronous results pattern of a mechanism for cancellation and progress notification The drawback is that you cannot use it arbitrarily on any method Instead, the DoWork() method has to conform to a SystemComponentModel DoWorkEventHandler delegate, which takes arguments of type object and DoWorkEventArgs If this isn't the case, then a wrapper function is required The cancellation- and progress-related methods also require specific signatures, but these are in control of the programmer setting up the background worker pattern
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Exception Handling
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If an unhandled exception occurs while the background worker thread is executing, then the RunWorkerCompletedEventArgs parameter of the RunWorkerCompleted ' delegate (Completed's eventArgs) will have an Error property set with the exception As a result, checking the Error property within the RunWorkerCompleted callback in Listing 169 provides a means of handling the exception
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Listing 169 Handling Unhandled Exceptions from the Worker Thread
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// static void Complete( object sender, RunWorkerCompletedEventArgs eventArgs) { ConsoleWriteLine(); if (eventArgsCancelled) { ConsoleWriteLine("Cancelled"); } else if (eventArgsError!= null) { // IMPORTANT: check error to retrieve any exceptions ConsoleWriteLine( "ERROR: {0}", eventArgsErrorMessage); } else { ConsoleWriteLine("Finished"); } resetEventSet(); } //
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It is important that the code check eventArgsError inside the RunWorkerCompleted callback Otherwise, the exception will go undetected; it won't even be reported to AppDomain
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One more important threading concept relates to user interface development using the SystemWindowsForms namespace The Microsoft Windows suite of operating systems uses a singlethreaded, message-processing-based user interface This means that only one thread at a time should access the user interface, and any alternate thread interaction should be marshaled via the Windows message pump The process involves calling a component's InvokeRequired property to determine if marshaling is necessary Internally, Invoke() will check InvokeRequired anyway, but it can be more efficient to do so beforehand explicitly Listing 1610 demonstrates this pattern
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Listing 1610 Accessing the User Interface via Invoke()
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System; SystemDrawing; SystemThreading; SystemWindowsForms;
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class Program : Form { private SystemWindowsFormsProgressBar _ProgressBar; [STAThread] static void Main() { ApplicationRun(new Program()); } public Program() { InitializeComponent(); ThreadStart threadStart = Increment; threadStartBeginInvoke(null, null); } void UpdateProgressBar() { if (_ProgressBarInvokeRequired) { MethodInvoker updateProgressBar = UpdateProgressBar; _ProgressBarInvoke(updateProgressBar); } else { _ProgressBarIncrement(1); } } private void Increment() { for (int i = 0; i < 100; i++) { UpdateProgressBar();
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ThreadSleep(100); } if (InvokeRequired) { // Close cannot be called directly from // a non-UI thread Invoke(new MethodInvoker(Close)); } else { Close(); } } private void InitializeComponent() { _ProgressBar = new ProgressBar(); SuspendLayout(); _ProgressBarLocation = new Point(13, 17); _ProgressBarSize = new Size(267, 19); ClientSize = new Size(292, 53); ControlsAdd(this_ProgressBar); Text = "Multithreading in Windows Forms"; ResumeLayout(false); } }
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This program displays a window that contains a progress bar that automatically starts incrementing Once the progress bar reaches 100 percent, the dialog box closes Notice from Listing 1610 that you have to check InvokeRequired twice, and then the marshal calls across to the user interface thread if it returns true In both cases, the marshaling involves instantiating a MethodInvoker delegate that is then passed to Invoke() Since marshaling across to another thread could be relatively slow, an asynchronous invocation of the call is also available via BeginInvoke() and EndInvoke()
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Invoke() , BeginInvoke() , EndInvoke(), and InvokeRequired comprise the members of the SystemComponentModelISynchronizeInvoke interface which is implemented by SystemWindowsFormsControl , from which Windows Forms controls derive
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