Background worker problem

The answers you already have are not 100% complete.
You cannot call anything related to UI from a non UI thread. You need to use either Invoke or BeginInvoke methods of System.Threading.Dispatcher (works with both Forms and WPF), or System.Windows.Forms.Control (Forms only).

Unlike Invoke, BegingInvoke returns immediately. Now, you may not use InvokeRequired if you're sure that invocation method is called always from a non-UI thread because InvokeRequired will always return true. It does not matter which control is used to call Invoke or BeginInvoke; it only should be the control participating in currently running Application.

Please see my past answers for detailed explanation on how it works and usage samples:
Control.Invoke() vs. Control.BeginInvoke()[^],
Problem with Treeview Scanner And MD5[^].

See also more references on threading:
How to get a keydown event to operate on a different thread in vb.net[^],
Control events not firing after enable disable + multithreading[^].

—SA

hi,

Look at this article:

How to solve "Cross thread operation not valid"[^]

or this question...

Need a help for cross thread operation in C# Threads[^]

Valery.

You have to use a delegate to do it:

Updating the UI from a thread - The simplest way[^]

You cannot can access the UI controls in the DoWork event handler by creating a delegate and use the Control.Invoke method(here Me.Invoke) which is explained in Wayne's answer. You can also access them in the RunWorkerCompleted event as shown below.

I will create a field of type List<int> to store values of a in the DoWork like this:

Private lst as List(Of Integer);
Private bgwTest as BackgroundWorker;
Public sub new()
{
 lst = new List(Of Integer);
 'Add the handlers
 AddHandler(bgwTest.DoWork, AddressOf(bgwTest_DoWork))
 AddHandler(bgwTest.RunWorkerCompleted, AddressOf(bgwTest_WorkCompleted))
} 


Private Sub bgwTest_DoWork(ByVal sender As System.Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles bgwTest.DoWork
       Dim a As Integer = 0
       Do While a < 10 'Infinite loop
           'ComboBox1.Items.Add(1)
           lst.Add(1);
       Loop
   End Sub

Private Sub bgwTest_WorkCompleted(sender as Object, e as RunWorkerCompletedEventArgs)
  'Now access your combobox and values in it using lst.
  For Each i as Integer in lst
     combobox1.Items.Add(i);
  End For
End Sub

Another way to do is explained in Wayne's answer by using delegates.

Hope it helped. :)