public void Count_ArgumentChecking()
{
ReactiveAssert.Throws <ArgumentNullException>(() => Observable.Count(default(IObservable <int>)));
ReactiveAssert.Throws <ArgumentNullException>(() => Observable.Count(default(IObservable <int>), _ => true));
ReactiveAssert.Throws <ArgumentNullException>(() => Observable.Count(DummyObservable <int> .Instance, default(Func <int, bool>)));
}