Exercise: Dependency Injection

A small cinema needs a box office for its movie nights. Prices change with the time of day, a seat can only be sold once, a full house needs a waiting list, and the cashier needs an undo for the sale they didn't mean to make. You'll write the BoxOffice class.

Get the exercise code

Open this lesson's GitHub repository, github.com/ask-codey/exercise-dependency-injection, 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 MovieNight.slnx in Visual Studio.

What to do

1. Fix the constructor. It's given a Clock and a PriceList, but it ignores them and makes its own with new. Keep the ones it's given.
2. TicketPrice() asks the price list for the price at the clock's current time.
3. Seats: Sell(seat), IsSold(seat) and SoldCount. A seat can only be sold once.
4. The waiting list: JoinWaitingList(name), WaitingCount, and ReleaseSeat(seat), which frees a seat and returns the first person waiting, or null.
5. Undo: UndoLastSale() cancels the most recent sale whose seat is still sold, and returns the seat, or null.

Run the program with Ctrl+F5, and the tests with Test > Run All Tests.

Hints

• One collection per job. Sold seats can't repeat; the waiting list is first come, first served; undo works newest first. Which collection is each?
• HashSet.Add and Remove return whether they changed anything. That answers "was it already sold?" and "was it sold at all?" for you.
• The fakes only work if you use what you're given. The tests hand the box office a clock that's always 2 PM or 7 PM, and a price list where every ticket is $5.00.
• TryDequeue and TryPop return false when there's nothing left, instead of throwing.
• Undo after a release: a seat on the undo stack may have been released since. Keep popping until you find one that's still sold.

When you're done

Every test passes. Notice that none of them depended on the time you ran them: that's the fake clock, and it only works because BoxOffice asks for its clock instead of making one.

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-dependency-injection), 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:

   git init -b main
   git add .
   git commit -m "My solution"
   git remote add origin https://github.com/your-name/exercise-dependency-injection.git
   git push -u origin main
   

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

   git remote set-url origin https://github.com/your-name/exercise-dependency-injection.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-dependency-injection, 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

Injected dependencies and the right collection for each job: a box office that's easy to test at any hour, where a seat can't be double-sold and nobody on the waiting list gets skipped.

Key terms

• Constructor Injection
• HashSet<T>
• Queue<T>
• Stack<T>

Related

• Constructor Injection
• HashSet
• Queues
• Stacks