问访问父ViewModel中的ViewModel属性

EN

class MainViewModel : INotifyPropertyChanged 
{
    public ICommand SomeViewCommand { get; set; }
    public ICommand OtherViewCommand { get; set; }
    private object selectedViewModel;
    public event PropertyChangedEventHandler PropertyChanged;

    public object SelectedViewModel
    {
        get { return selectedViewModel; }
        set { selectedViewModel = value; OnPropertyChanged("SelectedViewModel"); }
    }

    public MainViewModel()
    {
        SomeViewCommand = new RelayCommand<object, object>(null, (object o) => OpenSomeView());
        OtherViewCommand = new RelayCommand<object, object>(null, (object o) => OpenOtherView());
    }

    private void OpenSomeView()
    {
       SelectedViewModel = new SomeViewModel();
    }

    private void OpenOtherView(object obj)
    {
       if(SelectedViewModel != null && SelectedViewModel.GetType() == typeof(SomeViewModel))
       {
          SomeViewModel s = (SomeViewModel)SelectedViewModel;

          // always 0
          if (s.NumberOfChanges > 0)
          {
             MessageBox.Show("test", "Error");
          }

          // SelectedViewModel = new OtherViewModel(); after confirmation dialog
       }
       else
          SelectedViewModel = new OtherViewModel();
    }

    private void OnPropertyChanged(string propName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propName));
        }
    }

}