What's the best way to prevent losing TextBox focus when there is a validation error?

Question

I\'ve messed around with PreviewLostKeyboardFocus which almost gets you there. I\'ve seen a couple of implementations using LostFocus, but that just f

Answer 1

If you attempt to focus an element inside its own LostFocus handler you will face a StackOverflowException, I'm not sure about the root cause (I suspect the focus kind of bounces around) but there is an easy workaround: dispatch it.

private void TextBox_LostFocus(object sender, RoutedEventArgs e)
{
    var element = (sender as TextBox);
    if (!theTextBoxWasValidated())
    {
        // doing this would cause a StackOverflowException
        // element.Focus();
        var restoreFocus = (System.Threading.ThreadStart)delegate { element.Focus(); };
        Dispatcher.BeginInvoke(restoreFocus);
    }
}

Through Dispatcher.BeginInvoke you make sure that restoring the focus doesn't get in the way of the in-progress loss of focus (and avoid the nasty exception you'd face otherwise)