Practical tips for writing effective unit tests

Start Small and Test One Thing at a Time

Each unit test should focus on testing one specific piece of functionality. Keep your tests small, clear, and targeted. For example, if you’re testing a method that calculates the sum of two numbers, write a test that focuses only on verifying that calculation.
Use Descriptive Test Names

Name your tests in a way that clearly describes what they’re testing. This makes it easy to understand what each test does and why it might fail. A common approach is to use a naming convention like MethodName_StateUnderTest_ExpectedBehaviour (e.g., Add_TwoPositiveNumbers_ReturnsSum).
Focus on Testing Behaviour, Not Implementation

Write tests that verify what the method is supposed to do, not how it does it. Avoid testing private methods or internal workings; instead, focus on inputs and expected outputs. This makes your tests more resilient to changes in the underlying code.
Use Mocks and Stubs for Dependencies

When testing classes that depend on external services, databases, or complex objects, use mocking libraries (such as Moq in C#) to simulate these dependencies. This allows you to test the behaviour of your class in isolation without setting up external systems.
Write Tests Before Fixing Bugs or Adding Features

Following Test-Driven Development (TDD) principles, consider writing a failing test that represents the bug or new feature you want to address. This helps confirm that the bug is fixed or the feature works as expected once you make the changes.
Validate Edge Cases

Don’t just test the “happy path.” Consider edge cases, such as empty inputs, null values, or extremely large numbers. Handling edge cases helps make your code more robust and reduces the likelihood of unexpected failures.
Use Assertions Effectively

Use assertions to validate the results of your tests. Assertions should clearly define what you expect to happen in the test. For instance, Assert.AreEqual(expected, actual) is a common way to check if the result matches your expectations.
Aim for Independence

Each unit test should be independent of other tests. A test should not depend on the result of a previous test, nor should it alter any shared state that could affect subsequent tests. If you need shared setup, consider using setup methods that reset the state before each test.
Run Tests Frequently

Run your tests often, especially as you make changes to the code. Regularly running tests helps catch issues early, making them easier to debug and fix. Integrate tests into your CI/CD pipeline to ensure they’re run automatically on code pushes.
Keep Tests Simple and Readable

Write tests that are easy to read and understand, even at a glance. Avoid complex logic in your tests, as this can make them harder to maintain and debug. Simple, clear tests make it easier to identify why a test failed.
Use AAA (Arrange, Act, Assert) Structure

Follow the AAA pattern to organise your tests: Arrange (set up necessary data and objects), Act (execute the code you’re testing), and Assert (verify the results). This structure helps keep tests organised and easy to follow.
Don’t Aim for 100% Coverage as an Absolute Goal

Code coverage is useful, but high coverage alone doesn’t guarantee quality. Focus on covering important functionality and edge cases. Coverage tools can help you identify untested code, but prioritise meaningful tests over arbitrary coverage percentages.