Sample for ReactiveProperty

MainWindowViewModel.cs

public class MainWindowViewModel : BindableBase
{
    public ObservableCollection<MemberViewModel> Members { get; } = new ObservableCollection<MemberViewModel>();

    public ReactiveProperty<bool> IsAllLong { get; }

    public MainWindowViewModel()
    {
        // Observe if all MemberViewModels pass filter delegate for CLR property.
        IsAllLong = Members
            .ObserveElementProperty(x => x.IsLong) // Select CLR property to be observed.
            .Select(_ => Members.All(x => x.IsLong)) // Go through MemberViewModels by filter delegate.
            .ToReactiveProperty();

        // Alternative
        IFilteredReadOnlyObservableCollection<MemberViewModel> membersNotLong = Members
            .ToFilteredReadOnlyObservableCollection(x => !x.IsLong); // Provide filter delegate.

        IsAllLong = membersNotLong
            .CollectionChangedAsObservable()
            .Select(_ => Members.Any() && (0 == membersNotLong.Count))
            .ToReactiveProperty();
    }
}