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Unit Testing DateTime.Now Without Isolation

My friend Paul pointed something out to me the other day regarding my post about TDD even when you have to discard tests. I believe that this trick was taken from the book Working Effectively With Legacy Code by Michael Feathers (though I haven’t yet read this one, so I can’t be positive.

I was writing some TDD test surrounding the following production method:

and the problem I was having is that any tests that I write and check in will be good through the end of 2012 and essentially have an expiration date of Jan 1st, 2013.

What Paul pointed out is that I could refactor this to the following:

And, once I’ve done that, I can introduce the following class into my test class:

With all that in place, I can write a test that no longer expires:

In this test, I use the new class that requires me to specify the current year. It overrides the base class, which uses DateTime.Now in favor of the “current” year I’ve passed it, which has nothing to do with the non-deterministic quantity “Now”. As a result, I can TDD ’til the cows come home and check everything in so that nobody accuses me of having a Canadian girlfriend. In other words, I get to have my cake and eat it too.