public async Task Should_call_creator_once_when_context_exists()
        {
            using (sut.StartContext())
            {
                var value1 = sut.GetOrCreate("Key", () => ++ called);

                await Task.Delay(5);

                var value2 = sut.GetOrCreate("Key", () => ++ called);

                Assert.Equal(1, called);
                Assert.Equal(1, value1);
                Assert.Equal(1, value2);
            }
        }