Difficulty comparing two DateTime instances

Question

I'm writing a Unit Test class in C# (.NET 4.5). In one of the tests I'm checking the values of various properties after an instance of our class FeedbackDao is constructed. On construction, the FeedbackDate property of FeedbackDao is set to DateTime.Now.

FeedbackDao feedbackDao = new FeedbackDao();
// a couple of lines go here then I set up  this test:
Assert.IsTrue(feedbackDao.FeedbackDate.CompareTo(DateTime.Now) < 0);

My assumption is that feedbackDao.FeedbackDate should always be just a little earlier than the current time returned by DateTime.Now, even if it's only by a millisecond, and my IsTrue test should always pass, but sometimes it passes and sometimes it fails. When I add a message like this:

Assert.IsTrue(feedbackDao.FeedbackDate.CompareTo(DateTime.Now) < 0,
                feedbackDao.FeedbackDate.CompareTo(DateTime.Now).ToString());

the message sometimes reads -1 (meaning that FeedbackDate is earlier than Now) and sometimes reads 0 (meaning that the DateTime instances are equal).

Why is FeedbackDate not always earlier than Now? And, if I can't trust that comparison, how can I write a rigorous test to check the value of FeedbackDate when FeedbackDao is constructed?

Answer 1

My assumption is that feedbackDao.FeebackDate should always be just a little earlier than the current time returned by DateTime.Now, even if it's only by a millisecond.

What makes you think that? That would suggest that 1000 calls would have to take at least 1 second which seems unlikely.

Add to that the fact that DateTime.Now only has a practical granularity of about 10-15ms IIRC, and very often if you call DateTime.Now twice in quick succession you'll get the same value twice.

For the purpose of testability - and clean expression of dependencies - I like to use a "clock" interface (IClock) which is always used to extract the current system time. You can then write a fake implementation to control time however you see fit.

Additionally, this assertion is flawed:

Assert.IsTrue(feedbackDao.FeebackDate.CompareTo(DateTime.Now) < 0,
                feedbackDao.FeebackDate.CompareTo(DateTime.Now).ToString());

It's flawed because it evaluates DateTime.Now twice... so the value that it reports isn't necessarily the same one that it checks. It would be better as:

DateTime now = DateTime.Now;
Assert.IsTrue(feedbackDao.FeebackDate.CompareTo(now) < 0,
                feedbackDao.FeebackDate.CompareTo(now).ToString());

Or even better:

DateTime now = DateTime.Now;
DateTime feedbackDate = feedbackDao.FeebackDate;
Assert.IsTrue(now < feedbackDate,
              feedbackDate + " should be earlier than " + now);