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Multithreading The Easy Way: The BackgroundWorker
by Ged Mead | Published  06/01/2008 | Framework 2.0 Framework 3.0 .NET Intermediate Visual Studio 2005 Visual Studio 2008 Visual Basic 2005 Windows Presentation Foundation | Rating:
Getting Started - DoWork

  To follow along with this example, start a new Windows Forms project and drag a BackgroundWorker component from the Toolbox on to the form.

  Copy and paste the code from the previous page into the form's code file.  Note that you will also need to add an Imports statement at the top of the file:

 Code Copy

Imports System.IO

   I have named my demo BackgroundWorker:  bWkrFileCheck.

   In the Properties Window for this BackgroundWorker you will see that there are only a few properties.  Two of them - WorkerReportsProgress and WorkerReportsCancellation -  have a default setting of False.

  In order for you to be able to write working code that gives user feedback and allows the background process to be aborted, you need to set both these properties to True.

Properties Window

   For this first stage you will also need to add a button to the form.   I have named this button:   btnStart.

   When you click on this button, code will be fired which instructs the BackgroundWorker to begin carrying out whatever task(s) it has been allotted.

   So the code to place in the button's click event is:

Code Copy
Private Sub btnStart_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnStart.Click


End Sub

     The RunWorkerAsync method is a fairly self-explanatory one which means that it goes off and finds the task that bWkrFileCheck has been assigned and carries it out asynchronously.  That is, it works separately.  And in this context working separately means that the task is undertaken on a separate thread.

   If you add the button and enter the above code and run the project, you should see .... well, nothing, actually!   Hopefully, not an Exception message, but also no sign of any activity either.  

   And in fact there is no activity to monitor, because we haven't yet told the BackgroundWorker specifically what its task is.   That task of course will be to run the file search routine we entered previously, the "FileFinder" procedure.

  The place where we specifically tell the BackgroundWorker what task it has been assigned is in its DoWork event.  As with the list of properties you saw above, the list of events of a BackgroundWorker is fairly sparse; DoWork being the most fundamental one.


Properties Window


  In this first stage, the code for the DoWork event will be:


Code Copy
  Private Sub bWkrFileCheck_DoWork(ByVal sender As Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles bWkrFileCheck.DoWork

        FileFinder("C:\Program Files\")

  End Sub
      which is basic code that calls the FileFinder procedure and passes in a target folder as its argument.  
  If you want to  check that this works so far, then change the code of the FileFinder sub so that it writes the file names to the output window:
Code Copy
For Each fname As String In Directory.GetFiles(dir)
       If fname.EndsWith("txt") Then
        End If
     If you run this, you will see that the task has been properly handed off to the BackgroundWorker and while that task is visibly running you can still do other things with your form, such as move it or resize it.   We'll improve the demo in the next few steps.
   So, even at this early stage we now have a second process running on a separate thread, so allowing the user to continue doing other things.  But of course it would be much better if we were to give some kind of feedback on how much progress the task has made.
   That will be our next step.
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