# Exercise: Collections

Solve 14 problems with List<T> and Dictionary<TKey, TValue> (filtering, searching, counting, looking up and combining) and check each one with tests.

Apprentice · C# · Updated October 30, 2026

Lists and dictionaries are the collections you'll use most in C#. In this exercise you'll use each of them the way real programs do: building new lists from old ones, finding things, counting things, and looking values up by name.

## Get the exercise code

Open this lesson's GitHub repository, [github.com/ask-codey/exercise-collections](https://github.com/ask-codey/exercise-collections), 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.

| File | Methods |
|---|---|
| `ListProblems.cs` | `ArrayToList`, `ListToArray`, `LongWords`, `Largest`, `OddOnly`, `AppearsTwice`, `FizzBuzz` |
| `DictionaryProblems.cs` | `Mascot`, `WordCount`, `CartTotal`, `CombineStock`, `LetterGrades`, `MostWins`, `FirstLetters` |

Open `CollectionsExercise.slnx` in Visual Studio, replace each method's `return` line with your own code, and run the tests with **Test > Run All Tests**.

## Reading the examples

The comments show lists in square brackets and dictionaries in curly braces, with each key and its value:

```text
LongWords(["a", "train", "is", "late"], 4) -> ["train", "late"]
WordCount(["pizza", "tacos", "pizza"]) -> { "pizza": 2, "tacos": 1 }
```

## Hints

- **A new list:** start with `new List<string>()`, loop through the input, and `Add` what you keep. Don't change the list you were given.
- **Largest:** start with the first number, not 0: the numbers might all be negative.
- **A lookup table:** build the dictionary inside the method, then look the key up. To ignore capital letters, store lowercase keys and look up `school.ToLower()`.
- **Counting:** use the thing as the key. If the key's already there, add 1; if not, set it to 1.
- **Combining:** `new Dictionary<string, int>(firstStore)` makes a copy you can change safely.
- **The keys of `FirstLetters`** are `char`s: `word[0]` is a word's first letter, and `char.ToLower(letter)` lowercases one.

## When you're done

Every test is green. Look at the methods that build a list or dictionary: do they all leave their inputs unchanged? A method that quietly changes what it was given is a common source of bugs.

## 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-collections`), 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-collections.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-collections.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-collections`, 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

Filter, find, count, look up, combine: with these patterns and two collections, you can handle most of the data a program will ever give you.

## Key terms

- [List&lt;T>](https://www.ask-codey.com/glossary/list-t)
- [Dictionary&lt;TKey, TValue>](https://www.ask-codey.com/glossary/dictionary-tkey-tvalue)

## Related

- [Working with List&lt;T>](https://www.ask-codey.com/lessons/lists-in-csharp-working-with-list-t)
- [Dictionaries in C#](https://www.ask-codey.com/lessons/dictionaries-in-csharp)

Source: https://www.ask-codey.com/lessons/exercise-collections