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The beauty of tests is in the speed at which you can check whether your changes caused regression. At its core, a test is a program that invokes a software module that passes edge values to prove whether or not a particular behavior is working correctly. Note that not all code is inherently testable.

You have to keep three fundamental aspects in mind when writing a test: visibility, control, and simplicity.

Visibility indicates the degree at which the code under test allows you to observe changes in the state of the code. If changes are not observable, how can you determine whether the code works or fails?

Control indicates the degree at which the behavior of code under test can be influenced by external input. To be effective, a test must pass in selected input values. If input is hard-coded, running an effective test is much harder.

Finally, simplicity is an aspect of code that is never out of place. The simpler the code is, the simpler it is to test and the more reliable any results will be.

Unit Testing

Unit testing verifies that individual units of code are working properly according to their software contract. A unit is the smallest part of an application that is testable—typically, a method.

Unit testing consists of writing and running a small program (referred to as a test harness) that instantiates classes and invokes methods in an automatic way. In the end, running a battery of tests is much like compiling. You click a button, you run the test harness and, at the end of it, you know what went wrong, if anything.

In its simplest form, a test harness is a manually written program that reads test-case input values and the corresponding expected results from some external files. Then the test harness calls methods using input values and compares results with expected values. Obviously, writing such a test harness entirely from scratch is, at a minimum, time consuming and error prone. But, more importantly, it is restrictive in terms of the testing capabilities you can take advantage of.

A very effective way to conduct unit testing is to use an automated test framework. An automated test framework is a developer tool that normally includes a runtime engine and a framework of classes for simplifying the creation of test programs. One of these frameworks—MSUnit—is integrated in Visual Studio. All you have to do is create a new project of type Test. (Note that other tools, both open-source and commercial, are available for unit testing, some of which are also integrated with Visual Studio.)

Test-Driven Development

You can write tests at any time—before or after the method you intend to test. This is mostly a matter of preference and methodology. It can become a religious matter sometimes, but frankly nobody can claim that one approach or the other is absolutely and objectively better.

Test-driven development (TDD) is a methodology that naturally gets you to think about the expected interface and behavior of methods well before you actually start writing the code. TDD is an approach that might appear radical at first and that certainly takes time to fully digest. However, its goal is quite simple in the end: help to quickly write clean code that works.

In the traditional approach to coding, you develop a method according to the idea you have of the features the method must support. You start with a relatively simple body, and then you increase its capabilities until you reach the original goal. Along the way, you use the debugger to see how things are going and whether data is being moved around correctly. When you’ve determined that all is OK, if you’re a scrupulous developer you consider writing a bunch of unit tests to verify the behavior of the method from a few other angles to make it easier to catch regression failures later.

If you proceed this way, you eventually decide that tests are way too boring and hard to write and don’t really give you any concrete benefits. It’s your code, after all, and it works. A test won’t make the code richer and more appealing to users. So