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UNIT TEST – THE CLASSIC DEFINITION
A unit test is a piece of a code (usually a method) that invokes another piece of code and checks the correctness
of some assumptions afterward. If the assumptions turn out to be wrong, the unit test has failed. A “unit” is a
method or function.
The piece of code being tested is often called SUT (System Under Test). The piece of code that tests the SUT
will usually reside in a Test Method. This classic definition, while technically correct, is hardly enough to get us
started down a path where we can better ourselves as developers. Chances are you already know this and are
getting bored even reading this definition again, as it appears practically in any web site or book that discusses unit
testing.
Don’t worry; in this book, I’ll take you beyond the classic definition of unit testing by addressing issues not
mentioned in it: Maintainability, Readability, Correctness and more. However, precisely because you probably
might already be familiar with the classic definition, it gives us a shared knowledge base from which to extend the
idea of a unit test into something with more value than is currently defined.
No matter what programming language you are using, one of the hardest aspects of defining a unit test is
defining what is meant by a “good” one.
1.1.1 Defining a “good” unit test
I firmly believe that there’s no point in writing a bad unit test. If you’re going to write a unit test badly, you may as
well not write it all and save yourself the trouble it will cause down the road with maintainability and time
schedules. Defining what a good unit test is, is the first step we can do to make sure we don’t start off with the
wrong notion of what we’re trying to write.
Most people who try to unit test their code either give up at some point or don’t actually perform unit tests.
Instead, they either rely on system and integration tests to be performed much later in the lifecycle of the product
they are developing or resort to manually testing the code via custom test applications or actually using the end
product they are developing to invoke their code.
To succeed, it is essential that you not only have a technical definition of a unit test, but that you describe
the properties of a good unit test. To understand what a good unit test is, we’ll need look at what we do today to
understand what developers have been doing so far in a software project.
If you haven’t been doing unit tests, how did you make sure that the code works?
1.1.2 We’ve all written unit tests
You may be surprised to learn this, but you’ve already implemented some types of unit testing on your own. Have
you ever met a developer who has not tested the code they wrote before letting it out from under their hands?
Well, neither have I.
It might have been a console application that called the various methods of a class or component, some specially
created Winform or Webform UI that checks the functionality of that class or component, or even manual tests that
were run by performing various actions within the real application’s UI to test the end functionality. The end result
is that the developer is certain, to a degree, that the code works well enough to give it away to someone else.
Figure 1.1 shows how most developers test their code.
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