using System;

using System.Reactive.Linq;

using System.Reactive.Subjects;

public class Program

{

    public static void Main()

    {

        var subject = new Subject<int?>();

​

        var nulls = subject

            .Where(x => !x.HasValue)

            .DistinctUntilChanged()

            .Select(x => "Null Criterion")

            ;

        var nonNulls = subject

            .Where(x => x.HasValue)

            .DistinctUntilChanged()

            .Where(x => x.HasValue)

            .Select(x => "Status criterion for " + x)

            ;

        nulls//.Merge(nonNulls)

            .Subscribe(Console.WriteLine)

            ;

        var isValidNotifications = subject

            .Select(x => x.HasValue)

            .Select(x => x ? "Valid" : "Invalid");

        //isValidNotifications.Subscribe(Console.WriteLine);

        subject.OnNext(null);

        subject.OnNext(null);

        subject.OnNext(1);

        subject.OnNext(null);

        subject.OnNext(2);

        subject.OnNext(2);

        subject.OnNext(null);

​

    }

}