public void Append_ArgumentChecking() { var scheduler = new TestScheduler(); var someObservable = Observable.Empty <int>(); ReactiveAssert.Throws <ArgumentNullException>(() => Observable.Append(default(IObservable <int>), 1)); ReactiveAssert.Throws <ArgumentNullException>(() => Observable.Append(default(IObservable <int>), 1, scheduler)); ReactiveAssert.Throws <ArgumentNullException>(() => Observable.Append(someObservable, 1, default(IScheduler))); }