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Editorial Team
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Asked: 2026-05-23T02:11:20+00:00

I understand this is a subjective question and, as such, may be closed but

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I understand this is a subjective question and, as such, may be closed but I think it is worth asking.

Let’s say, when building an application using TDD and going through a refactor, a library appears. If you yank the code out of your main application and place it into an separate assembly, do you take the time to write tests that cover the code, even though your main application is already testing it? (It’s just a refactor.)

For example, in the NerdDinner application, we see wrappers for FormsAuthentication and MembershipProvider. These objects would be very handy across multiple applications and so they could be extracted out of the NerdDinner application and placed into their own assembly and reused.

If you were writing NerdDinner today, from scratch, and you noticed that you had a grab-bag of really useful wrappers and services and you bring them into a new assembly, do you create new tests that fully cover your new assembly–possibly having repeat tests? Is it enough to say that, if your main application runs green on all its tests, your new assembly is effectively covered?

While my example with NerdDinner may be too simplistic to really worry about, I am more thinking about larger APIs or libraries. So, do you write tests to re-cover what you tested before (may be a problem because you will probably start with all your tests passing) or do you just write tests as the new assembly evolves?

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1 Answer

  1. Editorial Team

    In general, yes, I’d write tests for the new library; BUT it’s very dependent upon the time constraints. At the least, I’d go through and refactor the unit tests that exist to properly refer to the refactored components; that alone might resolve the question.

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