Assembly Weaving. ReactiveUI.Fody. See: https://medium.com/@worldbeater/reactive-mvvm-for-net-platform-175dc69cfc82

ReactiveFodyViewModel.cs

public class ReactiveFodyViewModel : ReactiveObject
{
    private readonly ObservableAsPropertyHelper<string> _greeting;
    public string Greeting => _greeting.Value;

    [Reactive]
    public string Name { get; set; } = string.Empty;

    public ReactiveCommand<Unit, Unit> Clear { get; }

    public ReactiveFodyViewModel()
    { 
        Clear = ReactiveCommand.Create(() => { Name = string.Empty; });
        _greeting = this
            .WhenAnyValue(x => x.Name)
            .Select(name => $"Hello, {name}!")
            .ToProperty(this, x => x.Greeting);
    }
}