ShowDialog in Closing-Event

If the user closes the Application a Save-File-Message have to be shown (to be sure that he wants to discard the changes of edited files).

to implement this, i have a menuitem with a command-binding (without key-gesture):

private void Command_Exit(object sender, ExecutedRoutedEventArgs e)
 {
     Application.Current.Shutdown();
 }
 

the mainwindow has a closing-event. in this event i check if there unsaved files. if yes, the savedialog has to be opened (to choose, which files have to be saved):

private void Window_Closing(object sender, CancelEventArgs e)
     {
         if (sdl.Count() > 0)
         {
             SaveDialog sd = new SaveDialog();
             IEnumerable<Doc> close = sd.ShowDialog(this);
             if (close == null) 
                 e.Cancel = true;
             else
                 foreach (Doc document in close)
                     document.Save();
         }
 
     }
 

in this ShowDialog-Method (implemented in my SaveDialog-Class) i call

bool? ret = ShowDialog();
 if (!ret.HasValue)
      return null;
 if (!ret.Value)
      return null;
 

The problem is:

If i use the Alt+F4-Shortcut to close the Application (default-behaviour of the mainwindow) it works and i get the savedialog if there are unsaved files. but if i close the application by executing the Command_Exit-Method, the Method-Call

bool? ret = ShowDialog(); 
 

returns null and the dialog does not appear.

If i assign the Alt+F4 KeyGesture to the CommandBinding, the problem is switched: Executing Command_Exit works well but Alt+F4 Shortcut not.

What is the reason that the ShowDialog()-Method works not in both cases and how to fix it?

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