Question Access Listbox from ThreadPool

[sollniss]

I have two List(Of String) binded with two ListBoxes in the Form.Load event.
Then I have a ThreadPool wich fills these Lists, but the ListBoxes didn't show the items from the Lists.

Can someone halp me to fix this?

 

[JohnH]

You can add the items from the thread by invoking a method on the thread the UI control belongs to, invoking is done through a delegate pointing to the method, for example:

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
    Threading.ThreadPool.QueueUserWorkItem(New Threading.WaitCallback(AddressOf FillByThreadPool))
End Sub

Private Delegate Sub AddCallback(ByVal items() As Object)

Private Sub FillByThreadPool(ByVal state As Object)
    Dim l As New List(Of String)
    For i As Integer = 1 To 10
        l.Add(i.ToString)
    Next
    Me.ListBox1.Invoke(New AddCallback(AddressOf Me.ListBox1.Items.AddRange), New Object() {l.ToArray})
End Sub

 

[sollniss]

Thanks, but my Lists becoming bigger and bigger and if I use this code example I need to clear the ListBox everytime before I fill it.

Is there any possibility to do this without clearing, adding, clearing, adding... ?

BTW. The ListBox will be appended several thousand times.

 

[JohnH]

You have to use the same technique either way to modify the UI control from a different thread.

Private SafeAdd As Threading.WaitCallback

Private Sub AddItems(ByVal state As Object)
    For i As Integer = 1 To 10
        Me.ListBox1.Invoke(SafeAdd, i.ToString)
    Next
End Sub

Private Sub testForm1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
    SafeAdd = New Threading.WaitCallback(AddressOf Me.ListBox1.Items.Add)
    Threading.ThreadPool.QueueUserWorkItem(AddressOf AddItems)
End Sub