Observable null vs. empty

observable_null_vs_empty.cs

    public class MyTests
    {
        public class Package { }

        public class Service
        {
            public IObservable<Package> DeliverRegular()
            {
                return DeliverRegularAsync().ToObservable();
            }

            private async Task<Package> DeliverNullAsync()
            {
                return null;
            }

            public IObservable<Package> DeliverNull()
            {
                return DeliverNullAsync().ToObservable();
            }

            public IObservable<Package> DeliverEmpty()
            {
                return Observable.Empty<Package>();
            }
            private Task<Package> DeliverRegularAsync()
            {
                return Task.FromResult(new Package());
            }
        }

        [Test]
        public async Task When_regular_delivery_is_requested_it_should_not_be_null_or_empty()
        {
            var service = new Service();
            var result = await service.DeliverRegular().FirstAsync();
            result.Should().NotBeNull();
        }

        [Test]
        public async Task When_null_delivery_is_requested_it_should_be_null()
        {
            var service = new Service();
            var result = await service.DeliverNull().FirstAsync();
            result.Should().BeNull();
        }

        [Test]
        public async Task When_empty_delivery_is_requested_it_should_throw()
        {
            var service = new Service();
            Assert.Throws<InvalidOperationException>(async () => { var result = await service.DeliverEmpty().FirstAsync(); }, "FirstASync() should complete but there is no item in the sequence!");
        }
    }