Dispatcher throws InvalidOperationException on Messagebox.Show in Textchanged event

Question

First of all this is the errorlog entry on my error

crash program @ 15-9-2011 15:01:30error:System.InvalidOperationException: Dispatcher processing has been suspended, but messages are still being processed. at System.Windows.Threading.Dispatcher.WndProcHook(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)

Anyway code:

private void TB_postcode_cijfers_TextChanged(object sender, TextChangedEventArgs e){
if (TB_postcode_cijfers.Text != string.Empty || TB_postcode_cijfers.Text.Length > 0)
{
    LBL_postcode.Content = Postcode_cijfers + Postcode_letters;
    if (TB_postcode_cijfers.Text.Length == 4 && TB_postcode_letters.Text.Length == 2)
    {
        if (!ZoekOpPostcode(Injectioncheck(TB_postcode_cijfers.Text + TB_postcode_letters.Text)))
        {
            //MessageBox.Show("Geen resultaat gevonden, " + errortext);
            if (MessageBox.Show("Geen resultaat gevonden, " + errortext + ".\n Wilt u overschakelen naar handmatig? ", "Handmatig?", MessageBoxButton.YesNo) == MessageBoxResult.Yes)
            {
                RB_handmatig.IsChecked = true;
            }
            else
            {
                //
            }
        }
    }
}}

So on the messagebox.show method. this only happens when the user switches read mode to edit mode on my form. this involves collapsing en showing some labels and ui controls.

if the event fires from userinput everything is fine. What I whant to know: Why does the textchanged event fire when hiding and showing a few controls. What can i do to prevent this error?

EDIT: the code above is in a custom wpf control. placed in a winforms project/form

Answer 1

See this thread it describes the same problem as yours:

The exception is done on purpose to prevent reentrancy bugs caused by weirdness resulting from altering the visual tree, while such an event (which itself has been triggered by the visual tree altering) is firing. If you really must confirm something when the state of a UI element changes, delaying with Dispatcher.BeginInvoke is probably the right thing to do.

To run code on the UI Thread do the following:

 Application.Current.Dispatcher.BeginInvoke(DispatcherPriority.Normal, (Action)(() =>
     {

        if (MessageBox.Show("Geen resultaat gevonden, " + errortext + ".\n Wilt u overschakelen naar handmatig? ", "Handmatig?", MessageBoxButton.YesNo) == MessageBoxResult.Yes)
        {
            RB_handmatig.IsChecked = true;
        }
        else
        {
            //
        }
    }));