Unit testing constructor injection

Question

Suppose that my Foo class has the following:

readonly IService service;

public Foo(IService service) 
{
    if (service == null)
        throw new ArgumentNullException("service");

    this.service = service;
}

public void Start()
{
    service.DoStuff();
}

So far, I have one unit test for the constructor where I pass in null to verify that an ArgumentNullException gets thrown. Do I need a second unit test for my constructor where I pass in a valid IService and verify that this.service gets set (which would require a public accessor)?

Or should I just rely on my unit test for the Start method to test this code path?

Answer 1

Setting this.service is an implementation detail, so you should just be testing that it is used where expected and just test this via the Start method. Lest your tests become brittle.

You only want to test that your service is used appropriately. You shouldn't care how it is stored.