# Exercise: Polymorphism

Write a café Order that holds any mix of menu items in one List<MenuItem>, and works out the total, a meal-deal discount and the receipt, letting each item price itself.

Journeyman · C# · OOP · Updated October 30, 2026

The café's menu from *Exercise: Inheritance* is finished: `MenuItem`, `Drink` and `Pastry`. Now customers need to order. You'll write the `Order` class, which holds any mix of items in one list and adds them up.

## Get the exercise code

Open this lesson's GitHub repository, [github.com/ask-codey/exercise-polymorphism](https://github.com/ask-codey/exercise-polymorphism), 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 `CafeMenu.slnx` in Visual Studio.

| File | What's in it |
|---|---|
| `CafeMenu/MenuItem.cs`, `Drink.cs`, `Pastry.cs` | The menu, finished |
| `CafeMenu/Order.cs` | The order, for you to finish. Its requirements are in the comment at the top. |
| `CafeMenu.Tests/OrderTests.cs` | Tests for `Order` |

## What to do

Finish each member of `Order`:

| Member | What it does |
|---|---|
| `Add(item)` | Puts an item on the order |
| `Total` | Every item's price, added up |
| `DrinkCount()`, `PastryCount()` | How many drinks, and how many pastries |
| `Discount` | $0.50 off for each pastry that comes with a drink |
| `AmountDue` | The total, less the discount |
| `ReceiptLines()` | Each item's menu line, then the discount if there is one, then the amount due |

Run the program with **Ctrl+F5** to see a receipt, and the tests with **Test > Run All Tests**.

## Hints

- **Let each item price itself.** Loop over the items as `MenuItem`s: `item.Price` and `item.ToString()` run each item's own override, so a large latte comes to $5.00 without a single `if`.
- **Counting is the one place you need the kind:** `if (item is Drink)`.
- **The meal deal** pairs drinks with pastries: two drinks and three pastries make two deals. `Math.Min` finds the smaller count.
- **One test brings a stranger:** a `Soup` class that `Order` has never heard of. If your `Total` uses `Price`, it passes with no change to `Order`.

## When you're done

Every test passes, and the program prints:

```text
Large Latte, $5.00
Croissant, $3.75 (warmed)
Blueberry Muffin, $2.50
Meal deal: -$0.50
Amount due: $10.75
```

Look at your `Total`: one loop, no `if`, and it prices every kind of item correctly, including ones that don't exist yet.

## 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-polymorphism`), 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-polymorphism.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-polymorphism.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-polymorphism`, 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

That's polymorphism earning its keep. The order works with "menu items", and each item knows its own price, so a new kind of item on the menu needs no change to the order at all.

## Key terms

- [Polymorphism](https://www.ask-codey.com/glossary/polymorphism-c-sharp)
- [Upcasting](https://www.ask-codey.com/glossary/upcasting)
- [Method Overriding](https://www.ask-codey.com/glossary/method-overriding-c-sharp)
- [is Pattern](https://www.ask-codey.com/glossary/is-pattern)

## Related

- [Polymorphism with Base Classes](https://www.ask-codey.com/lessons/polymorphism-in-csharp-polymorphism-with-base-classes)
- [Exercise: Inheritance](https://www.ask-codey.com/lessons/exercise-inheritance)

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