Show how to use a builder with unit tests.
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You can use this sample as a starting point to practice using the Builder Pattern for your unit test data.
Source - Using the Builder Pattern for Your Test Data
It includes a small working test data builder for the Order class, but requires you to create additional builders for Customer and Address. You can go through the comments in OrderService to build out the PlaceOrder method's functionality as a kata to practice applying this pattern.
Fork or clone the repository locally so you can work on it. You should be able to run the tests using xunit either from the command line or using Visual Studio. Run the tests. The included test(s) should pass.
Open OrderService.cs. Read through the TODO comments. Each one represents a new piece of functionality to be added and tested. Write a test in OrderServicePlaceOrder for each new TODO item. Be sure to use a Builder to create the order and its associated customer and address. Verify the test fails when you first run it, since you haven't yet implemented the code that will make it pass. Then add the code. Once you see the test pass (and all other tests still pass), you should remove the TODO comment (and optionally commit your code to your local repository).
Continue until no more TODO comments remain and all tests are passing. If you're using Visual Studio, you can use the Task List window to see a list of your remaining TODO items:
There are generally two approaches to the Builder pattern. The simpler one is shown in the OrderBuilder that is provided. In this approach, a new instance of the thing-to-be-built is created as a private field when the Builder is created. It is manipulated by the Builder methods and then simply returned by Build. Something to watch out for with this approach is reuse of the Builder, since any calls to Builder methods after Build has been called will still modify the instance that was previously returned. You can correct this by re-instantiating the private field when Build is called.
The second approach requires a bit more code but is useful when you can't just instantiate the object initially. You can try out this approach when you build a CustomerBuilder as part of this exercise. Customer has a parameterized constructor that requires an id, and its Id property is private. Thus, you can't just instantiate a private Customer instance when you create the CustomerBuilder. Instead, you should store each property value in its own private field, and then create the Customer instance in the Build method. This is the "more correct" way to implement the pattern, but it does require more code than the "shortcut" approach of just creating the instance when you create the builder.