In the physical world, two workers are twice as productive as one. In the programming world, using additional threads safely can increase an application’s productivity and add a depth and richness that help the user to be more productive too. The drawback has been that multi-threaded applications typically have been harder to write and debug. This is still true to some extent; but with .NET, multithreading is getting easier to use.
Early editions of .NET introduced thread pools, and .NET 2.0 introduces the BackgroundWorker component. The BackgroundWorker component permits you to incorporate additional worker threads, and it isn’t much harder to use then the old standby, the Timer component.
This article demonstrates how to use the BackgroundWorker component safely, including how to marshal control from the worker thread back to the Windows Forms thread.
Implementing the DoWorkEventHandler
Because the BackgroundWorker class is a component, you can drag and drop it onto a Windows Form or UserControl and wire up event handlers visually. If you don’t have a form or UserControl, you can declare and create an instance of the BackgroundWorker and bind its event properties with code.
The BackgroundWorker component is defined in the System.ComponentModel namespace. To use the BackgroundWorker, you can add an Imports statement or use the namespace in the declaration and initialization statement. Here is an example:
Dim worker as System.ComponentModel.BackgroundWorker worker = new System.ComponentModel.BackgroundWorker
The worker thread is expressed as a delegate wired to the BackgroundWorker.DoWork event property. The following is the signature of the delegate (event handler) for the DoWork property:
Sub DoWork(Sender As Object, _ e As System.ComponentModel.DoWorkEventArgs)
Any subroutine with the two argument types in the order shown can be assigned to the DoWork event property. The BackgroundWorker component also has a ProgressChanged event property, and you can signal the relative progress of the background thread by wiring an event handler to the ProgressChanged event. The following is the signature for ProgressChanged:
Sub ProgressChanged(sender As Object, _ e As System.ComponentModel.ProgressChangedEventArgs)
Listing 1 shows how to create an instance of the BackgroundWorker component and wire up DoWork and ProgressChanged in the Form’s OnLoad event.
Listing 1: Create and Wire Up DoWork and ProgressChanged Events for BackgroundWorker Component
Private worker As BackgroundWorker Private Sub Form1_Load(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles MyBase.Load worker = New BackgroundWorker() worker.WorkerReportsProgress = True AddHandler worker.DoWork, New _ DoWorkEventHandler(AddressOf OnWork) AddHandler worker.ProgressChanged, _ New ProgressChangedEventHandler(AddressOf OnProgressChanged) End Sub
Running the Worker Thread
To start the worker thread, you invoke the BackgroundWorker.RunWorkerAsync method. This example adds ProgressBar to a form and uses a button to simulate an event that initiates the background worker process. The following snippet demonstrates how to start the background thread:
Private Sub Button1_Click(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles Button1.Click worker.RunWorkerAsync() End Sub
Figure 1 shows the simple example Form. The ProgressBar shown will be updated indirectly by an event the BackgroundWorker component raises.
Figure 1: Example Form with ProgressBar
Safe Multithreading in Windows Forms
At this point, you have enough code to use multithreading. To interact with Windows Forms controls and control properties without crashing the program, however, you have to use delegates a bit more.
Presently, .NET is not designed to safely and reliably manipulate Windows Forms controls across thread boundaries. To correct this deficiency, you have to marshal any data or interactions from the background thread to the same thread on which the Windows Forms and controls are. You do this with a delegate. For example, if you want to update the ProgressBar from the BackgroundWorker’s ProgressChanged event handler, you need to define a delegate with arguments mirroring the data you want to pass and use the Form’s Invoke method. Call Invoke with the delegate address and data—this is referred to as marshalling.
Listing 2 shows the implementation of BackgroundWorker’s ProgressChanged event handler, as well as the delegate definition and the additional event handler used to update the ProgressBar. OnProgressChanged responds to the ProgressChanged event and the Invoke method call uses the custom ChangeProgressBarHandler to marshal data to the Form’s thread.
Listing 2: Implementation of BackgroundWorker’s ProgressChanged Event Handler
Private Sub OnProgressChanged(ByVal sender As Object, _ ByVal e As ProgressChangedEventArgs) Invoke(New ChangeProgressBarHandler( _ AddressOf ChangeProgressbar), e.ProgressPercentage) End Sub Private Delegate Sub ChangeProgressBarHandler(ByVal percentage _ As Integer) Private Sub ChangeProgressBar(ByVal percentage As Integer) ProgressBar1.Value = percentage End Sub
Listing 3 provides the complete code set for the example, showing the OnWork event handler that represents background work.
Listing 3: All the Custom Code for the Sample Form Shown in Figure 1
Imports System.ComponentModel Public Class Form1 Private worker As BackgroundWorker Private Sub Form1_Load(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles MyBase.Load worker = New BackgroundWorker() worker.WorkerReportsProgress = True AddHandler worker.DoWork, New _ DoWorkEventHandler(AddressOf OnWork) AddHandler worker.ProgressChanged, _ New ProgressChangedEventHandler(AddressOf OnProgressChanged) End Sub Private Sub OnWork(ByVal sender As Object, _ ByVal e As DoWorkEventArgs) Dim I As Integer For I = 1 To 100 System.Threading.Thread.Sleep(10) worker.ReportProgress(I) Next End Sub Private Sub OnProgressChanged(ByVal sender As Object, _ ByVal e As ProgressChangedEventArgs) Invoke(New ChangeProgressBarHandler( _ AddressOf ChangeProgressbar), e.ProgressPercentage) End Sub Private Delegate Sub ChangeProgressBarHandler(ByVal _ percentage As Integer) Private Sub ChangeProgressBar(ByVal percentage As Integer) ProgressBar1.Value = percentage End Sub Private Sub Button1_Click(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles Button1.Click worker.RunWorkerAsync() End Sub End Class
OnWork really only shows that the ProgressBar is being updated, and interacting with the Form will clearly demonstrate that the Form’s thread is available—for example, the form is refreshed if you move it around. That said, the sample application does demonstrate all of the preparation and steps necessary to use additional threads. All that remains for you to do is find useful work for the background thread.
Tip: If you want to see the various threads the sample application in Listing 3 uses, set a breakpoint in the code. When the breakpoint is reached, select Debug|Windows|Threads.
Delegates Are Our Friends
I don’t recommend writing code that interacts with Windows Forms controls or the form itself in event handlers that are raised by asynchronous processes, threads from the thread pool, or the BackgroundWorker component. Sometimes, doing so seems to work, but cross-threaded control interaction will eventually crash your program. You can make these interactions inherently thread safe, though—I believe Delphi’s Visual Control Library (VCL, the equivalent of the .NET Framework) is thread safe, but Delphi’s VCL is a more mature framework.
Until the .NET Framework controls are thread safe, you will have to use Control.Invoke and delegates to marshal data from background worker threads to the Form thread. However, once you master delegates, using them and marshaling data across threads is safe and relatively easy.
About the Author
Paul Kimmel is the VB Today columnist for www.codeguru.com and has written several books on object oriented programming and .NET. Look for his upcoming book UML DeMystified from McGraw-Hill/Osborne (October 2005) and C# Express from Addison Wesley (Spring 2006). You may contact him for technology questions at email@example.com.
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Copyright © 2005 by Paul T. Kimmel. All Rights Reserved.