private void OnSelectedTheoryPartChanged(object sender, RoutedPropertyChangedEventArgs <object> e) { _currentModel.ClearSelection(); if (e.NewValue is TestViewModel theoryPart) { _currentModel.ShowTest(theoryPart); } else if (e.NewValue is DocumentViewModel theoryDoc) { _currentModel.ShowDocument(theoryDoc); } else if (e.NewValue is PracticeViewModel practiceViewModel) { _currentModel.ShowPractice(practiceViewModel); } }