Exercise: an API Client

Your products API from Exercise: Northwind Web API needs a client: a C# class that other programs use to read and change products, without knowing anything about URLs, status codes or JSON. You'll write ProductClient.

Get the exercise code

Open this lesson's GitHub repository, github.com/ask-codey/exercise-an-api-client, 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 NorthwindClient.slnx in Visual Studio.

What to do

Any other unsuccessful answer, such as 400, 409 or 500, throws an HttpRequestException.

Run the tests with Test > Run All Tests. Then start your products API, check the address at the top of Program.cs, and run this program with Ctrl+F5.

Hints

• The tests need no API. They give ProductClient an HttpClient that talks to a FakeApi class, which records each request and answers with whatever status and JSON the test chose. That only works because ProductClient asks for its HttpClient instead of making one.
• GetFromJsonAsync suits GetAllAsync; it throws on any failure, which is what you want there.
• Where 404 is a normal answer, use GetAsync, PutAsJsonAsync or DeleteAsync, check for HttpStatusCode.NotFound first, then call response.EnsureSuccessStatusCode() for everything else.
• LowStockAsync calls your own GetAllAsync(null), then LINQ. p.UnitsInStock ?? 0 counts a missing stock figure as 0.

When you're done

Every test passes, and with the API running, the program prints:

Beverages:
  Chai $18.00
  Chang $19.00
  Chartreuse verte $18.00
  Côte de Blaye $263.50
  Guaraná Fantástica $4.50
  Ipoh Coffee $46.00
  Lakkalikööri $18.00
  Laughing Lumberjack Lager $14.00
  Outback Lager $15.00
  Rhönbräu Klosterbier $7.75
  Sasquatch Ale $14.00
  Steeleye Stout $18.00
Running low (fewer than 5 in stock):
  Alice Mutton: 0
  Chef Anton's Gumbo Mix: 0
  Gorgonzola Telino: 0
  Perth Pasties: 0
  Thüringer Rostbratwurst: 0
  Sir Rodney's Scones: 3
  Longlife Tofu: 4
  Louisiana Hot Spiced Okra: 4
There's no product 999

Stop the API and run the program again: it says the API had a problem, instead of crashing.

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-an-api-client), 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-an-api-client.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-an-api-client.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-an-api-client, 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

Two programs, talking over HTTP: the API you built, and a client that uses it like a set of ordinary methods. That's how most modern software fits together, from phone apps to the services behind big websites.

Key terms

• API Client
• HttpClient
• JSON Serialization
• Async/Await

Related

• A Simple API Client
• Exercise: Northwind Web API