using System;

using System.Linq; 

using System.Collections.Generic;

using ReactiveUI;

using System.Reactive.Linq;

using System.Reactive;

​

public static class DeclaredProperties<T> {

    public static List<string> Names = typeof(T).GetProperties

            ( System.Reflection.BindingFlags.Public

              | System.Reflection.BindingFlags.Instance

              | System.Reflection.BindingFlags.DeclaredOnly ).Select(p=>p.Name).ToList();

}

​

public static class RxUIExtensions {

  public static IObservable<T> WhenAnyChanged<T>(T source) where T : ReactiveObject 

        => source.Changed.Where(ev => DeclaredProperties<T>.Names.Contains(ev.PropertyName));

}

​

​

// If we list all the natural numbers below 10 that are multiples of 

// 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23.

// Find the sum of all the multiples of 3 or 5 below 1000.

public static class  Program

{

    public static IEnumerable<int> Infinite(){

        var i = 0;

        while(true){

            yield return i;

            i++;

        }

    }

    public static IEnumerable<int> FilterFactors(this IEnumerable<int> source, IList<int> factors){

        return

            source.Where(i=>factors.Any(f=>i%f==0));

    }

    public static void Main()

    {

        var sum = Infinite()

            .TakeWhile(i=>i<1000)

            .FilterFactors(new []{3,5})

            .Sum();

        Console.WriteLine(sum);

    }

}