public ContactViewModel(Contact contact, ContactRepository contactRepository)
        {
            if (contact == null)
                throw new ArgumentNullException("contact");

            if (contactRepository == null)
                throw new ArgumentNullException("contactRepository");

            _contact = contact;
            _contactRepository = contactRepository;
        }
        public SearchContactsViewModel(Contact contact,ref ContactViewModel cvm, ContactRepository contactRepository)
        {
            base.DisplayName = Resources.SearchContactsViewModel_DisplayName;
            if (contact == null)
                throw new ArgumentNullException("contact");

            if (contactRepository == null)
                throw new ArgumentNullException("contactRepository");

            _contact = contact;
            _contactRepository = contactRepository;
        }
        public MainWindowViewModel(string ContactsDataFile)
        {
            base.DisplayName = Resources.MainWindowViewModel_DisplayName;

            _contactRepository = new ContactRepository(ContactsDataFile);
        }