Dependency Injection – Keeping Your Tests Independent

In this post I’m going to look at how to resolve the problem of how tight coupling in code causes problems for unit tests. What we have at the moment is a View Model that is tightly coupled to a repository, which makes it hard to impossible to test the View Model’s behaviour for cases like the repository throwing an exception, or being unavailable.

The way around this is by using a technique called Dependency Injection. There are many, many articles written on this subject, so I won’t go into too much depth here. I do encourage you to read up on this, because there are a few different ways of using DI.

In this case I’m going to use the most common one, which is constructor injection. Constructor injection simply means that we’re going to inject an instance of the repository through the constructor, like this:

As you can see, it’s very simple. Instead of creating the repository inside the constructor like before, we pass it as an argument. It still gets stored in the same private field, and if it’s null, we throw an exception. The exception is not necessary, of course, but in this case it makes sense because the View Model rather depends on having a valid repo to do its job.

Changing the constructor means that now the project will no longer compile, of course, so we need to go in and change a few more things.

First off, since we’re looking at unit testing, let’s change the test setup:

While this is still using the old mock repository, it’s now possible to use different repos in further tests. I’m also going to create a second test to test that passing a null argument will throw the correct exception:

The final step is to change the construction of the OverviewVM in the constructor of the OverviewPage class. For now let’s just inject null. It means that the app won’t run because of the exception that will get thrown, but it will compile so that you can run the tests.

Well, that’s it for now. In the next post I’m going to create more tests and introduce you to mocking frameworks – fun times ahead!

Unit Testing – Keeping Your Code Running

Now that we have code that’s starting to resemble something usable, it’s high time to put in some Unit Tests. I’m using NUnit as my testing framework, but feel free to choose something else. First off, I like to keep my tests organised, so I usually create a folder tree in my solution. In this case I start by adding the following folder structure: Testing -> GoalBuddy for the tests of my common code. Inside that folder I then create a new NUnit3 unit test project.

Two things to mention here: after you install the NUnit test runner and templates, you can create four different kinds of test projects: Android, iOS and UWP, located under “Cross-Platform”, and a platform independent one, located under “Test”. Choose the last one for unit testing your common code.

The other thing to do is to make sure your unit test project is compatible with .NET Standard. Out of the box, your common code will be in a .NET Standard 2.0 library, so your test project needs to reference .NET 4.6 or higher.

Now add a reference to the project that holds your common code to your unit test project, build your solution and run the default test as a sanity check.

Next, let’s add a test that checks that data actually gets loaded into the ViewModel:

Now you can see that, although the test should run without problems, this is already a bit weird. I assert that the ViewModel should have 3 ToDoItems, but I never arrange for that. Right now that’s hard coded into the constructor of my ViewModel, which is really bad for testing. What if I want to add another test that tests for the correct behaviour if my repository throws an exception?

Sure, I could change my repository to do that, but then it will always do that, of course, but that would then cause the test I just wrote to fail. Not great. And that’s where Dependency Injection comes in – more on that in my next post!