private void ShellView_Closing(object sender, System.ComponentModel.CancelEventArgs e)
{
var askSave = MessageBox.Show("Do you want save ?", "Exit Application", MessageBoxButton.YesNoCancel, MessageBoxImage.Question);
if (askSave == MessageBoxResult.Cancel)
{
e.Cancel = true;
return;
}
if (askSave == MessageBoxResult.Yes)
((ShellViewModel)DataContext).Save();
}