If you have time consuming operations such as extensive mathematical calculations, file or database operations on a large amount of data, etc., then the GUI can go unresponsive until that operation is completed. The .Net Framework's solution to this problem is the BackgroundWorker class.
BackgroundWorker class executes the time consuming operation on a separate background thread thereby keeping the GUI responsive to the end user. It can also report the progress of the operation periodically, cancel the operation while it is running and indicate when the operation is completed.
The following VB sample code demonstrates how to use the BackgroundWorker class to run an operation in the background. The time-consuming operation in this example is to import the contents of a CSV file to a data grid in VB.
First of all, Open Microsoft Visual Studio and create a new project of type Windows Forms Application
Drag and drop the following controls to the form and set their properties as below using the Properties window
|Control||Property Name||Property Value|
The purpose of this program is to import the contents of a CSV file, perform a simple calculation and update the data onto a data grid view control.
The Form Load event (Form1_load) sets two key properties of the Backgroundworker control -
WorkerSupportsCancellation property is set to
True which allows the time consuming operation to be cancelled while it is running.
WorkerReportsProgress property is also set to
True which allows screen to report the progress of the operation. Various other properties of the Data grid, Button and Label controls are also set during the form load event.
'Allow background operation to be cancelled bgWorker.WorkerSupportsCancellation = True 'Allow background operation to report progress bgWorker.WorkerReportsProgress = True
When the user clicks the Start button, the RunWorkerAsync method of background worker object is called. This method raises the DoWork event which executes the time consuming operation in the background.
'Start time-consuming operation in background Call bgWorker.RunWorkerAsync()
If the user want to cancel the operation at anytime while it is running, he can do so by pressing the Stop button. In the Stop button click event you call the CancelAsync() property of background worker which sets the CancellationPending property to True.
'Stop background operation bgWorker.CancelAsync()
DoWork event you need check if the user pressed the cancel button if not execute the backgrouns opeartion and report the progress. A basic framewor of
DoWork event is shown below:
Private Sub bgWorker_DoWork(ByVal sender As Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles bgWorker.DoWork If bgWorker.CancellationPending Then e.Cancel = True Exit Sub Else 'Do some operation 'Report the progress bgWorker.ReportProgress(percent, userstate) End if End Sub
DoWork Event of this sample program we use a TextFieldParser object to read the contents of a csv file line by line. The progress of this background operation is reported after each line is read. This is achieved by calling the ReportProgress method of background worker object. In our program we have added some sleep after each line is read in order to simulate a time-consuming operation. The
ReportProgress method inside DoWork raises the ProgressChanged event.
ProgressChanged event execute the code that updates the rows that are read from the csv file to the datagrid. It also changes the value of the progress bar.
Private Sub bgWorker_ProgressChanged(ByVal sender As Object, ByVal e As System.ComponentModel.ProgressChangedEventArgs) Handles bgWorker.ProgressChanged 'Report the progress of background operation here End Sub
Finally when all the lines of the csv file are read and the
DoWork finish execution, the RunWorkerCompleted event is fired. This is where you do the clean-up such as disposing the TextFieldParser, enable/disable buttons, change messages etc.
Private Sub bgWorker_RunWorkerCompleted(ByVal sender As Object, ByVal e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles bgWorker.RunWorkerCompleted If e.Cancelled Or Not IsNothing(e.Error) Then 'If the background opeartion was cancelled 'or an error occured during execution then do something Else 'If Background execution completed normally then do something else End If End Sub