# Exercise: Writing Unit Tests

Three finished classes each hide one bug. Plan scenarios from their requirements, write MSTest tests that find the bugs, and fix the code.

Apprentice · Unit Testing · C# · Updated October 30, 2026

This time the code is written, and the tests are yours. Three small classes are finished, but each one hides **one bug**. Your tests will find them.

## Get the exercise code

Open this lesson's GitHub repository, [github.com/ask-codey/exercise-writing-unit-tests](https://github.com/ask-codey/exercise-writing-unit-tests), also linked from the **Exercise** box beside the lesson. Click **Code**, then **Download ZIP**, and unzip it into your `bootcamp` folder. Or clone it with Git. Then open `TestingExercise.slnx` in Visual Studio.

| Class | What it does |
|---|---|
| `ParkingGarage` | Works out what a stay in a parking garage costs: 15 free minutes, $3.00 for the first hour, $2.00 for each hour after, $20.00 at most |
| `LibraryCard` | Keeps the books a library card holds: at most 5, never the same title twice |
| `WordTools` | Counts vowels, and spots palindromes ignoring spaces and upper or lower case |

The full requirements are in the comments at the top of each class and above each method. Each test class in `TestingExercise.Tests` has one example test, which passes.

## What to do

For each class:

1. **Plan** the scenarios from the requirements, as a table: the normal cases and the edges.
2. **Write** one test per scenario, named for it, in Arrange, Act, Assert. Use `[DataRow]` where several inputs share a scenario.
3. **Run** the tests. A failure is either a mistake in your test or a bug in the class: read the requirement again to decide which.
4. **Fix** the bug in the class, and run the tests until they all pass.

There are three bugs in all, one per class.

## Hints

- **Edges find bugs.** For "at most 5 books", test the 4th, 5th and 6th books. For "the first hour", test 59, 60 and 61 minutes.
- **Test what the requirements say**, not what the code does. Work out each expected value by hand, from the requirement.
- **Give each Assert a message** that states the requirement, so a failure explains itself.
- **Never change a correct test to make it pass.** If the test matches the requirement, the code is wrong.

## When you're done

Every test passes, and each class has a test that would have caught its bug. Put the bug back for a moment, and watch your test turn red: that's the proof your test works.

## Submit your exercise

The exercise is how you finish this lesson. When your work is done, put it on GitHub in a repo of your own, then submit that repo's address here.

1. On GitHub, click **+**, then **New repository**. Name it (for example `exercise-writing-unit-tests`), choose **Public**, and click **Create repository**. Leave out the README: your folder already has one.
2. In a terminal in your exercise folder (Git Bash on Windows), send your work to your new repo. If you downloaded the ZIP:

   ```bash
   git init -b main
   git add .
   git commit -m "My solution"
   git remote add origin https://github.com/your-name/exercise-writing-unit-tests.git
   git push -u origin main
   ```

   If you cloned the starter instead, point your copy at your own repo, then commit and push:

   ```bash
   git remote set-url origin https://github.com/your-name/exercise-writing-unit-tests.git
   git add .
   git commit -m "My solution"
   git push -u origin main
   ```

3. Paste your repo's address, such as `https://github.com/your-name/exercise-writing-unit-tests`, into **Submit your exercise** on this page. The site checks that the repo is public and holds your commits, and then the lesson is complete.

Changed something after you submitted? Push it, and submit again: your latest submission is the one that counts.

## So what

You've just done what testers and developers do every day: turned requirements into scenarios, scenarios into tests, and failing tests into fixed code.

## Key terms

- [Unit Test](https://www.ask-codey.com/glossary/unit-test-unit-testing)
- [Test Case](https://www.ask-codey.com/glossary/test-case)
- [Edge Case](https://www.ask-codey.com/glossary/edge-case)
- [Arrange, Act, Assert](https://www.ask-codey.com/glossary/arrange-act-assert)

## Related

- [Planning Unit Tests](https://www.ask-codey.com/lessons/unit-testing-with-mstest-planning-unit-tests)
- [Writing Tests](https://www.ask-codey.com/lessons/unit-testing-with-mstest-writing-tests)

Source: https://www.ask-codey.com/lessons/exercise-writing-unit-tests