Retain focus in a UiElement(TextBox) on its Lost focus

Question

Set focus of an element from its lost focus

With the continuation of above query, still its not able to retain the focus.

Is any other way to achieve this?

void OnPreviewLostKeyboardFocus(object sender, KeyboardFocusChangedEventArgs e)
{
    if (!Valid())
    {
        e.Handled = true;
        (sender as Control).Focus();
    }
}

With this code am not able to retain the focus..