Backgroundworker doesn't work... VB.Net - vb.net

Question

this is my code:
Private Sub BackgroundWorker1_DoWork(ByVal sender As System.Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork
For i = 0 To 1000
Dim inum As String = i & "0"
Dim request As System.Net.HttpWebRequest = System.Net.HttpWebRequest.Create("http://www.google.nl/search?q=site:" & combobox1.Text & "&hl=nl&start=" & inum)
Dim response As System.Net.HttpWebResponse = request.GetResponse
Dim sr As System.IO.StreamReader = New System.IO.StreamReader(response.GetResponseStream())
Dim sourcecode As String = sr.ReadToEnd
Dim search As String = combobox1.Text
Dim r As New System.Text.RegularExpressions.Regex("http://" & search & "/\w*")
Dim matches As MatchCollection = r.Matches(sourcecode)
For Each itemcode As Match In matches
Dim item As String = (itemcode.ToString.Split("""").GetValue(0))
Dim url As New Net.WebClient
Dim str As String = url.DownloadString("http://www.prcheck.nl/results.php?url=" & item)
If str.Contains(">0/10") Then
ListBox1.Items.Add("(0/10) " & item)
ElseIf str.Contains("1/10") Then
ListBox1.Items.Add("(1/10) " & item)
ElseIf str.Contains("2/10") Then
ListBox1.Items.Add("(2/10) " & item)
ElseIf str.Contains("3/10") Then
ListBox1.Items.Add("(3/10) " & item)
ElseIf str.Contains("4/10") Then
ListBox1.Items.Add("(4/10) " & item)
ElseIf str.Contains("5/10") Then
ListBox1.Items.Add("(5/10) " & item)
ElseIf str.Contains("6/10") Then
ListBox1.Items.Add("(6/10) " & item)
ElseIf str.Contains("7/10") Then
ListBox1.Items.Add("(7/10) " & item)
ElseIf str.Contains("8/10") Then
ListBox1.Items.Add("(8/10) " & item)
ElseIf str.Contains("9/10") Then
ListBox1.Items.Add("(9/10) " & item)
ElseIf str.Contains("10/10") Then
ListBox1.Items.Add("(10/10) " & item)
Else
ListBox1.Items.Add("(0/10) " & item)
End If
Label2.Text = ListBox1.Items.Count
Next
If Not sourcecode.Contains("<b>Volgende</b>") Then
MsgBox("")
Exit For
End If
Next
End Sub
and combobox1.text = www.google.nl ( example )
at button 1 the code is:
BackgroundWorker1.RunWorkerAsync()
and if backgroundworker is done:
Private Sub BackgroundWorker1_RunWorkerCompleted(ByVal sender As Object, ByVal e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles BackgroundWorker1.RunWorkerCompleted
MsgBox("Done")
End Sub
if i click button 1, i get within a half second the message: Done
what's wrong with the code??
if i put the code inside backgroundworker1 just in button1 it works but goes really slow..

Answer 1

You can only update the UI from within the main application thread, in this case you're attempting to do it via a background thread that has been created by the background worker which will throw an exception as you've found.
What you'll need to do it run the code which adds to the ListBox on the main thread which you can do via BeginInvoke and a custom delegate which takes the item you want to add as a parameter, the delegate can then add the item to list box - there's an example of how to do this in the docs for BeginInvoke.

Answer 2

I would return a list or array of items to be added from the background worker and then fill the ListBox in the RunWorkerCompleted event handler.