Changes in Property that implements INotifyPropertyChanged not being reflected in UI

Question

I am trying to implement the MVVM Pattern but unfortunately is taking longer than expected.

I have a ListView populated by an ObservableCollection of ContactsVm, Adding or Removing Contacts works perfectly, the problem comes when trying to change only one Item from this collection by selecting it.

The Xaml where I am setting my bindings:

<ListView ItemsSource="{Binding ContactsToDisplay}" 
                  SelectedItem="{Binding SelectedContact, Mode=TwoWay}"
                  SeparatorColor="Black" 
                  ItemSelected="OnItemSelected">
            <ListView.ItemTemplate>
                <DataTemplate>
                    <TextCell Text="{Binding FirstName}" 
                              Detail="{Binding Id}">
                        <TextCell.ContextActions>
                            <MenuItem 
                                Text="Delete" 
                                IsDestructive="true" 
                                Clicked="Delete_OnClicked"
                                CommandParameter="{Binding .}" />
                        </TextCell.ContextActions>
                    </TextCell>
                </DataTemplate>
            </ListView.ItemTemplate>
        </ListView> 

Its cs:

public ContactBookApp()
        {
            InitializeComponent();
            MapperConfiguration config = new MapperConfiguration(cfg => {
                cfg.CreateMap<Contact, ContactVm>();
                cfg.CreateMap<ContactVm, Contact>();
            });

            BindingContext = new ContactBookViewModel(new ContactService(), new PageService(), new Mapper(config));

        }

        private void AddButton_OnClicked(object sender, EventArgs e)
        {
            (BindingContext as ContactBookViewModel)?.AddContact();
        }

        private void OnItemSelected(object sender, SelectedItemChangedEventArgs e)
        {
            (BindingContext as ContactBookViewModel)?.SelectContact(e.SelectedItem  as ContactVm);
        }


        private void Delete_OnClicked(object sender, EventArgs e)
        {
            (BindingContext as ContactBookViewModel)?.DeleteContact((sender as MenuItem)?.CommandParameter as ContactVm);
        }
    }

My ViewModel, here the "problematic" part is the SelectContact method, I am posting the rest in case it helps:

public class ContactBookViewModel : BaseViewModel
    {
        private readonly IContactService _contactService;
        private readonly IPageService _pageService;
        private readonly IMapper _mapper;
        private ContactVm _selectedContact;
        public ObservableCollection<ContactVm> ContactsToDisplay { get; set; }
        public ContactVm SelectedContact
        {
            get => _selectedContact;
            set => SetValue(ref _selectedContact, value);
        }
        public ContactBookViewModel(IContactService contactService, IPageService pageService, IMapper mapper)
        {
            _contactService = contactService;
            _pageService = pageService;
            _mapper = mapper;
            LoadContacts();
        }
        private void LoadContacts()
        {
            List<Contact> contactsFromService = _contactService.GetContacts();
            List<ContactVm> contactsToDisplay = _mapper.Map<List<Contact>, List<ContactVm>>(contactsFromService);
            ContactsToDisplay = new ObservableCollection<ContactVm>(contactsToDisplay);

        }

        public void SelectContact(ContactVm contact)
        {
            if (contact == null)
                return;
            //None of this approaches works:
            //SelectedContact.FirstName = "Test";
            //contact.FirstName = "Test;
        }
    }
}

My ContactVm class:

public class ContactVm : BaseViewModel
    {
        private string _firstName;

        public int Id { get; set; }

        public string FirstName
        {
            get => _firstName;
            set => SetValue(ref _firstName, value);
        }   
    }

The BaseViewModel:

public class BaseViewModel
    {
        public event PropertyChangedEventHandler PropertyChanged;

        [NotifyPropertyChangedInvocator]
        protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }

        protected void SetValue<T>(ref T backingField, T value, [CallerMemberName]string propertyName = null)
        {
            if (EqualityComparer<T>.Default.Equals(backingField, value))
                return;
            backingField = value;
            OnPropertyChanged(propertyName);
        }
    }

As you can see, I am trying to update each selected contact setting its FirstName = "Test", the changed are updated but unfortunately they are not getting reflected in the UI, hope you can help me to find what I am doing wrong.

Thanks in advance!

Answer 1

Your BaseViewModel does not implement the INotifyPropertyChanged interface.