public ShellViewModel(AViewModel a, BViewModel b, CViewModel c)
        {
            aViewModel = a;
            bViewModel = b;
            cViewModel = c;

            ActivateItem(aViewModel);
        }
 public ShellViewModel(AViewModel a, BViewModel b, CViewModel c)
 {
     aViewModel = a;
     bViewModel = b;
     cViewModel = c;
 }