Unit testing and how do you do it?


Exact duplicate of many posts:

What is unit testing?
What Makes a Good Unit Test?
New to Unit Testing
Unit Testing – definitions
Learning Unit Testing
How to properly mock and unit test
Unit Testing: Beginner Questions
And many more …
Also, Google for site:stackoverflow.com "how do you" unit-test

I have read some questions on unit testing, but I don't exactly know WHAT it is or how you do it. I was hoping if someone can tell me the following:

  • What exactly IS unit testing? Is it built into code or run as separate programs? Or something else?
  • How do you do it?
  • When should it be done? Are there times or projects not to do it? Is everything unit-testable?

Thanks a lot for the help.

Best Solution

What is unit testing?

Unit testing simply verifies that individual units of code (mostly functions) work as expected. Usually you write the test cases yourself, but some can be automatically generated.

The output from a test can be as simple as a console output, to a "green light" in a GUI such as NUnit, or a different language-specific framework.

Performing unit tests is designed to be simple, generally the tests are written in the form of functions that will determine whether a returned value equals the value you were expecting when you wrote the function (or the value you will expect when you eventually write it - this is called Test Driven Development when you write the tests first).

How do you perform unit tests?

Imagine a very simple function that you would like to test:

int CombineNumbers(int a, int b) {
    return a+b;

The unit test code would look something like this:

void TestCombineNumbers() {
    Assert.IsEqual(CombineNumbers(5, 10), 15); // Assert is an object that is part of your test framework
    Assert.IsEqual(CombineNumbers(1000, -100), 900);

When you run the tests, you will be informed that these tests have passed. Now that you've built and run the tests, you know that this particular function, or unit, will perform as you expect.

Now imagine another developer comes along and changes the CombineNumbers() function for performance, or some other reason:

int CombineNumbers(int a, int b) {
    return a * b;

When the developer runs the tests that you have created for this very simple function, they will see that the first Assert fails, and they now know that the build is broken.

When should you perform unit tests?

They should be done as often as possible. When you are performing tests as part of the development process, your code is automatically going to be designed better than if you just wrote the functions and then moved on. Also, concepts such as Dependency Injection are going to evolve naturally into your code.

The most obvious benefit is knowing down the road that when a change is made, no other individual units of code were affected by it if they all pass the tests.