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

FodyReactiveViewModel.cs

[AddINotifyPropertyChangedInterface]
public class FodyReactiveViewModel
{
    public ReactiveCommand<Unit, Unit> Clear { get; }
    public string Greeting { get; private set; }
    public string Name { get; set; } = string.Empty;

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