Why am I getting this error:“Cross-thread operation not valid: Control lbFolders accessed from a thread other than the thread it was created on.”?

Question

This is baffling me, maybe somebody can shine the light of education on my ignorance. This is in a C# windows app. I am accessing the contents of a listbox from a thread.

Answer 1

You're trying to write to a control from a thread other than the main thread. Use Invoke or BeginInvoke.

void SetMax()
{
    if (prgAll.InvokeRequired)
    {
        prgAll.BeginInvoke(new MethodInvoker(SetMax));
        return;
    }

    prgAll.Maximum = lbFolders.SelectedItems.Count;
}