KataFizzBuzz ala Reactive Framework

class Program
{
    static void Main(string[] args)
    {
        FizzBuzzer(1, 100).Subscribe(Console.WriteLine);
        Console.ReadLine();
    }
 
 
    internal static IObservable<string> FizzBuzzer(int start, int stop)
    {
        return Observable.Generate(start, x => x <= stop, x =>
                                    {
                                        if (IsFizz(x) && IsBuzz(x))
                                            return "FizzBuzz";
                                        if (IsFizz(x))
                                            return "Fizz";
                                        if (IsBuzz(x))
                                            return "Buzz";
                                        return x.ToString();
                                    }, x => x + 1);
    }
 
    internal static bool IsFizz(int x)
    {
        return (x % 3 == 0);
    }
 
    internal static bool IsBuzz(int x)
    {
        return (x % 5 == 0);
    }
}
 
[TestFixture]
class FizzBuzzTest
{
    [TestCase(1, 1, "1")]
    [TestCase(3, 3, "Fizz")]
    [TestCase(5, 5, "Buzz")]
    [TestCase(15, 15, "FizzBuzz")]
    public void FizzBuzzAlgoShouldNotFail(int from, int to, string result)
    {
        Program.FizzBuzzer(from, to).Subscribe(x => x.Should().Equal(result));
    }
}