VB.Net Asynchronous background worker inside a loop - vb.net

Question

Im using Vb.net visual studio 2008. i have got a process(system.diagnostics.process) to be run in background and it updates the Ui thread progressbar and a label and i have worked it using backgroundworker.runworkerAsync. Now the problem is i have to work the same process for a number of time with different inputs.
The code block is :
Private Sub fnStartEvent(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnStart.click
Dim inputFolder = Directory.GetFiles(textboxSourceFolder.text)
Dim currentNumberOfFile As Integer=1
For each Files in inputFolder
Dim arguments As Object = Files
updateProgressStatus(0, currentNumberOfFile)
BackgroundWorker1.WorkerReportsProgress = True
BackgroundWorker1.RunWorkerAsync(New Object() {arguments})
currentNumberOfFile += 1
Next
End Sub
Private Sub BackgroundWorker1_DoWork(ByVal sender As System.Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork
'uses the arguments
Dim Bg_process As System.Diagnostics.Process = New System.Diagnostics.Process
With Bg_process.StartInfo
.Arguments = str_arguments
.FileName = ffmpegPath
.CreateNoWindow = True
.UseShellExecute = False
.RedirectStandardOutput = True
.RedirectStandardError = True
End With
Bg_process.Start()
Dim outputReader As StreamReader = Bg_process.StandardError
Dim output As String
While Not Bg_process.HasExited
output = outputReader.ReadLine()
BackgroundWorker1.ReportProgress(0, output)
Threading.Thread.Sleep(500)
End While
End Sub
Private Sub BackgroundWorker1_ProgressChanged(ByVal sender As Object, ByVal e As System.ComponentModel.ProgressChangedEventArgs) Handles BackgroundWorker1.ProgressChanged
' process args
updateProgressStatus(args(0), args(1) )
End Sub
Private Function BackgroundWorker1_RunWorkerCompleted(ByVal sender As Object, ByVal e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles BackgroundWorker1.RunWorkerCompleted
Messagebox.Show(e.error.ToString)
End Try
Sub updateProgressStatus(ByVal progressValue As Integer, ByVal progressStatus As String)
progressBar.value = progressValue
lblprogressStatus.Text = progressStatus
End Sub
The problem in here is the fnStartEvent method once gets started, it calls BackgroundWorker1.runWorkerAsync process and that runs in a seperate thread and does not wait for the thread to complete and it moves to the next line of code i.e loops to the next item and it returns to same BackgroundWorker1.runWorkerAsync line and throws exception that it is already running.
Tried
1.
Private Sub fnStartEvent(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnStart.click
Dim inputFolder = Directory.GetFiles(textboxSourceFolder.text)
Dim currentNumberOfFile As Integer=1
For each Files in inputFolder
Dim arguments As Object = Files
updateProgressStatus(0, currentNumberOfFile)
BackgroundWorker1.WorkerReportsProgress = True
BackgroundWorker1.RunWorkerAsync(New Object() {arguments})
''
Do Until BackgroundWorker1.IsBusy
Thread.Sleep(500)
Loop
currentNumberOfFile += 1
Next
End Sub
But this does not updates the Ui thread used to denote progress.
2.Put the whole process inside the DoWork thread and loop for each file in there itself, but that to returns the cross thread error in progress bar updation.
what is the standard procedure in executing a loop for the backgroundworker to wait for finishing as well as update the UI without blocking it.

Answer 1

Whenever you use Thread.Sleep, you are actually sleeping the current thread that is hooked to your startup class (ie. Form). So, if you sleep your main thread, no UI updates will happen.
Per the MSDN article regarding BackgroundWorker.IsBusy, you need to throw an Application.DoEvents. Code below is copied from the article linked above.
Private Sub downloadButton_Click( _
ByVal sender As Object, _
ByVal e As EventArgs) _
Handles downloadButton.Click
' Start the download operation in the background.
Me.backgroundWorker1.RunWorkerAsync()
' Disable the button for the duration of the download.
Me.downloadButton.Enabled = False
' Once you have started the background thread you
' can exit the handler and the application will
' wait until the RunWorkerCompleted event is raised.
' If you want to do something else in the main thread,
' such as update a progress bar, you can do so in a loop
' while checking IsBusy to see if the background task is
' still running.
While Me.backgroundWorker1.IsBusy
progressBar1.Increment(1)
' Keep UI messages moving, so the form remains
' responsive during the asynchronous operation.
Application.DoEvents()
End While
End Sub