Unit testing and checking private variable value

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The quick answer is that you should never, ever access non-public members from your unit tests. It totally defies the purpose of having a test suite, since it locks you into internal implementation details that you may not want to keep that way.

The longer answer relates to what to do then? In this case, it is important to understand why the implementation is as it is (this is why TDD is so powerful, because we use the tests to specify the expected behavior, but I get the feeling that you are not using TDD).

In your case, the first question that comes to mind is: “Why are the IAuditable objects added to the internal list?” or, put differently, “What is the expected externally visible outcome of this implementation?” Depending on the answer to those questions, that’s what you need to test.

If you add the IAuditable objects to your internal list because you later want to write them to an audit log (just a wild guess), then invoke the method that writes the log and verify that the expected data was written.

If you add the IAuditable object to your internal list because you want to amass evidence against some kind of later Constraint, then try to test that.

If you added the code for no measurable reason, then delete it again 🙂

The important part is that it is very beneficial to test behavior instead of implementation. It is also a more robust and maintainable form of testing.

Don’t be afraid to modify your System Under Test (SUT) to be more testable. As long as your additions make sense in your domain and follow object-oriented best practices, there are no problems – you would just be following the Open/Closed Principle.

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