Most professional software developers these days understand the importance and value of writing and using unit tests. A nice summary of some of the oft-touted and oft-realized benefits of unit testing can be found in the StackOverflow.com thread Is Unit Testing worth the effort? [my only very minor criticism is the mixing of more specialized Test-Driven Development (TDD) with the more general unit test concept]. As with most good things, however, even unit testing enthusiasm can go too far. The benefits of unit testing can lead to overly enthusiastic unit testing developers forgetting that unit tests are not the end themselves, but rather are a means to the real end.
The "end" that most software developers are striving for is delivery of software solutions that make their users’ lives easier and more productive. Unit tests can be extremely valuable in obtaining this end and certainly add to software quality, but the overly zealous unit tester must beware of allowing the unit tests themselves to displace this end goal. It’s all too easy to allow oneself to get so bound up in writing exhaustive and "perfect" unit tests that one puts the true end goal at risk. In the remainder of this post, I look at ways in which developers can allow unit testing to move from helping achieve the desired end to unintentionally displacing the real end and putting it at risk.
Overreaching Unit Testing
Steven Sanderson has written "the benefit of unit testing is correlated with the non-obviousness of the code under test." I largely agree with this sentiment as a general guideline. I see little value in unit testing trivial "get" and "set" methods. Some methods are more readily evaluated via code review than via unit test.
The concept of code coverage can be a useful one as long as it’s not taken too far. Code coverage appears to provide high return for the effort for a while, but there comes a point of diminishing returns when gaining additional code coverage comes at much greater cost and may not be worth that cost. It’s also important to recognize that even the often highly expensive 100% code coverage typically means only all lines of code were executed and does not check all possible paths through the code.
All Code’s Unit Testability is Not Equal
The post Selective Unit Testing – Costs and Benefits clearly articulates well the differences in difficulty (cost) and advantages (benefits) of unit testing of different types of code. In cases where the advantages/benefits of a unit test are high and the cost/effort is low, the value of unit testing is obvious. On the opposite extreme, there are types of code that receive little benefit from unit testing.