The Background Worker

What is a background worker?

A background worker allow the designer to offload some of processing on another thread, which can make the UI feel more responsive.

How to use a background worker.

For this demonstration you'll have to create a new windows application project, until you grasped the basics.
Add two buttons to the form, name them Start_Button & Stop_Button
Add a progress bar
Add a label, named lbl_Status
Add a Background worker.

Set then enable state of the stop button to False

In the properties of the background worker set the following properties to true.
WorkerSupportsCancellation & WorkerReportsProgress

Now view to code of the form and add the following code the top.


Double Click on the start button and enter.


Double Click the stop button


Then Next section of code is the code that is to be run in the background.


Note: That the My_BgWorker.ReportProgress only excepts integer values between 0 to 100 inclusive. So you'll have to scale your value to this range.

Now to add the code to update the Progress Bar


Now for the code that runs after the Background Worker is finished.



Now that all it takes to use a Background Worker.

For the more adventurous of you
Let's use that label and display a percentage of completion.


Add this extra line of code to the DoWork procedure

And Run.

You get the following error Cross-thread operation not valid: Control 'Lbl_Status' accessed from a thread other than the thread it was created on.
Why?

Because we are using a background worker and we're attempting to access controls on a different thread (The UI or form).
VB.Net doesn't like use talking to objects on another thread directly, so have to go Indirectly through a intermediary called a Delegate
It is possible to turn it off and allow them but that's just asking for trouble.
I'll show you the proper and correct way to achieve this, by adding a new subroutine and a delegate.
A delegate helps to communicate to objects of a different thread;- It delegates the talking between the different threads.

Go back to the code and enter the following code.

To use comment out the line code in Do Work routine that caused the error, then enter the following.



And run, now you have the label displaying the percentage.


Now you have the Background Worker in your Code Ninja arsenal.

Greetings,
Thanks for the post, very helpful. I want to run an event timer in the tool strip. Timer works if there is no event to time. Stops when the event starts. How would you exacute a timer event within the background manager. Tried this, no good. Thanks for any help you can give.

Public Class Form1
Dim startTime As DateTime
Dim span As TimeSpan = DateTime.Now.Subtract(startTime)
Dim clock As String

Private Sub Start_Button_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Start_Button.Click
Me.bgwTimer.RunWorkerAsync()
End Sub

Private Sub bgwTimer_DoWork(ByVal sender As Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles bgwTimer.DoWork
Me.Timer1.Start()
End Sub

Private Sub Timer1_Tick(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles Timer1.Tick
Me.bgwTimer.ReportProgress(span.Milliseconds)
End Sub

Private Sub bgwTimer_ProgressChanged(ByVal sender As Object, ByVal e As System.ComponentModel.ProgressChangedEventArgs) Handles bgwTimer.ProgressChanged
clock = e.ToString
Me.ToolStripLabel1.Text = span.Minutes.ToString & ":" & _
span.Seconds.ToString & "." & span.Milliseconds
End Sub
End Class

Noi errors, just terminates at the do work event. other foreground and background events uneffectived.

I just would like to say that this is EXACTLY the information that I was trying to figure out on my own with starting individual threads and then trying to keep track of them.

thanks a bunch for this very clear and helpful tutorial!

Kevin

Thanks so much! Exactly what I have been needing for ages!