class Program
    {
        static void Main(string[] args)
        {
            var vm = new VM();
            Console.WriteLine(vm.P2);
        }
    }

    public class VM : ReactiveObject
    {
        [ObservableAsProperty] public double P1 { get; }
        [ObservableAsProperty] public double P2 { get; }

        public VM()
        {
            Observable.Return(0.0).ToPropertyEx(this, vm => vm.P1);
            this.WhenAnyValue(vm => vm.P1).ToPropertyEx(this, vm => vm.P2);
        }
    }