Question

i’m trying to learn multithreading and i’m stuck on this sample

Imports System.Threading

Public Class Form1

   Dim myThread As System.Threading.Thread = New Thread(AddressOf Me.AddItems)


   Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
      myThread.Start()
   End Sub

   Public Sub AddItems()
      Dim i As Int32
      Dim n As Int32 = 1
      Dim s As String = ""

      For i = 0 To 100
         s = n.ToString
         ListBox1.Items.Add(s)
         n *= 2
         Thread.CurrentThread.Sleep(100)
      Next
   End Sub

End Class

i get the following error:

“Cross-thread operation not valid: Control ‘ListBox1’ accessed from a
thread other than the thread it was created on”

Answer 1

ListBox1.Items.Add(s) is the problem.

You cannot update the UI from a non-UI thread. ListBox1 was created by, and belongs to the main UI thread.

You should be using Control.Invoke to perform the UI update.

Use something like:

Private Sub MyMethodOnANonUIThread()

        Dim s As String = "Hello, World!"
        Me.Invoke(New MethodInvoker(Sub() UpdateTheControlOnUIThread(s)))
  End Sub

  Private Sub UpdateTheControlOnUIThread(ByVal updateValue As String)

    ListBox1.Items.Add(updateValue)

  End Sub