private void CommandBinding_Executed(object sender, ExecutedRoutedEventArgs e)
        {
            Window        activeWindow   = Application.Current.Windows.OfType <Window>().SingleOrDefault(x => x.IsActive);
            IInputElement focusedControl = FocusManager.GetFocusedElement(activeWindow);

            if (focusedControl is DependencyObject)
            {
                string str = HelpProvider.GetHelpKey((DependencyObject)focusedControl);
                if (str.Equals("index"))
                {
                    str = "prijava";
                }
                HelpProvider.ShowHelp(str);
            }
            else
            {
                string str = HelpProvider.GetHelpKey((DependencyObject)activeWindow);
                HelpProvider.ShowHelp(str);
            }
        }