VB.NET BackgroundWorker

BackgroundWorker handles long-running tasks. It does not freeze the entire program as this task executes. The BackgroundWorker type provides an excellent solution. It enables a simple multithreaded architecture for VB.NET programs.

To begin, you will need a VB.NET Windows Forms project open. In the Toolbox pane, please double-click on the BackgroundWorker icon. This will add a BackgroundWorker1 instance to the tray at the bottom of the screen.

Tip: You can change some properties of the BackgroundWorker by right clicking on it and selecting Properties.

Also: You can click on the lightning bolt and add event handlers to your C# code file.

DoWork. Let's first add the DoWork event handler into the VB.NET code. In this handler, you can execute any VB.NET code you want on the background thread. In Properties, click on the lightning bolt and then double-click on the DoWork row.

And: At this point, a BackgroundWorker1_DoWork event handler is created. A Sleep method call can simulate some processor-intensive work.

Class that uses DoWork event handler: VB.NET Public Class Form1 Private Sub BackgroundWorker1_DoWork(ByVal sender As System.Object, _ ByVal e As System.ComponentModel.DoWorkEventArgs) _ Handles BackgroundWorker1.DoWork ' Do some time-consuming work on this thread. System.Threading.Thread.Sleep(1000) End Sub End Class

RunWorkerAsync. So now you have the DoWork event handler in your VB.NET code. But how can you specify the BackgroundWorker begins to do its work? You can use the RunWorkerAsync method on the BackgroundWorker instance.

So: Let's look at the complete program so far. The program starts the BackgroundWorker when the enclosing Form1 loads.

Class that uses Load and RunWorkerAsync: VB.NET Public Class Form1 Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load ' Start up the BackgroundWorker1. BackgroundWorker1.RunWorkerAsync() End Sub Private Sub BackgroundWorker1_DoWork(ByVal sender As System.Object, _ ByVal e As System.ComponentModel.DoWorkEventArgs) _ Handles BackgroundWorker1.DoWork ' Do some time-consuming work on this thread. System.Threading.Thread.Sleep(1000) End Sub End Class

RunWorkerCompleted. Now, we are going to tie the example together by adding another type to the program. We are adding the ArgumentType, which encapsulates two integer fields. These are the parameters to the BackgroundWorker.

In Form1_Load, we create the ArgumentType, and then pass it to the RunWorkerAsync method. This begins the DoWork event handler. In this handler, we call Sleep and then return the two fields multiplied together.

Finally: RunWorkerCompleted is invoked after about one second, and we show the value on the screen (30).

Class that introduces ArgumentType, RunWorkerCompleted: VB.NET Public Class Form1 Public Class ArgumentType Public _a As Int32 Public _b As Int32 End Class Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load ' Create the argument object. Dim args As ArgumentType = New ArgumentType() args._a = 5 args._b = 6 ' Start up the BackgroundWorker1. ' ... Pass argument object to it. BackgroundWorker1.RunWorkerAsync(args) End Sub Private Sub BackgroundWorker1_DoWork(ByVal sender As System.Object, _ ByVal e As System.ComponentModel.DoWorkEventArgs) _ Handles BackgroundWorker1.DoWork ' Do some time-consuming work on this thread. System.Threading.Thread.Sleep(1000) ' Get argument. Dim args As ArgumentType = e.Argument ' Return value based on the argument. e.Result = args._a * args._b End Sub Private Sub BackgroundWorker1_RunWorkerCompleted(ByVal sender As System.Object, _ ByVal e As System.ComponentModel.RunWorkerCompletedEventArgs) _ Handles BackgroundWorker1.RunWorkerCompleted ' Called when the BackgroundWorker is completed. MessageBox.Show(e.Result.ToString()) End Sub End Class

Summary. BackgroundWorker is useful for creating a background process, one which won't block the user interface if it takes a long time to run. Often, disk or network accesses are the slowest and should be put on a background thread if possible.

Review: The BackgroundWorker is both effective and relatively easy to use once you understand its concept.

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