Tuesday, 26 May 2015

Testing Private Code

A common problem many people ask is - should you test private code? In short, you shouldn't. You should always test the public api of your code where possible. This is not always easy. Based on the context of the code in question there are a few options available.

Don't Test

Either don't test the private code or rely on manual testing. This will not be ideal in many cases, but if the code is covered in higher level tests you may be able to get away with it. If the code will be stable, short lived or low risk you can default to this option.

Test via Public Tests

Simply test the private code by adding assertions or verifications to exisiting public behaviour tests. If the setup requires a lot of work, many edge cases or much duplication you may want to avoid this technique.

Make the Code Public

Once public, the code is easily testable. Are we making this code public just for the sake of an automated test? Yes, but there are valid times to do this. Providing the behaviour is logically part of the object in question there is no harm, the single responsibility principle is not violated.

Interfaces can be used to control visibility here. For testing you always use a concrete instance, while your production code should hold references to interfaces only. To simply hide the method, don't add it to the interface. For dynamic langauges this is as simple as "don't invoke it" or relying on naming conventions to denoate implementation details.

Make a Public Class

When single responsibility principle would be violated in the technique above, this is your other option. Beware the power of just adding a new class and making it public. While it will allow testing in one place, each public dependency you introduce further increases coupling.

If the code that needs testing is a service, the act to introduce a public object should be considered thoughtfully. Once the class is pubic, you simply need to verify the use of the class, rather than what it does. However Value Objects can help limit the tests you need to write entirely and should be used whenever possible.


  1. If you are testing at the "right" level, whether the code is private or not is irrelevant.

    I've started to look at test as I am testing a behaviour not a method or class. I find it helps to reduce the coupling between tests and production code, a test for every method on every class is a killer for maintainability.

    Personally I will always try and remove code that is made public just for testing purposes, at the very least I try and make it private and move the test higher up. Don't get me wrong testing higher up has it's disadvantages as well.

    1. It's very relevant in my opinion, if it's private developers resort to reflection, magical tools or other trickery to wrap something in a test. Unfortunately this tends to be the default and wrong answer to this question, just check out the likes of StackOverflow on this very subject.

      Check out a post I did from a while back - it should solve the maintenance problem you have.

    2. Thanks for the link to the previous article, I agree with everything you stated in that article, however I feel that if you are saying most automated tests should fall into "Sociable Tests" then as long as the object does what it is expected to, then surely that's all that matters and you have no need to test any private methods within?

    3. Yeah, that's right. But that's only true if you're dealing with code you write yourself or greenfield development.

      If you've just inherited some legacy code or are simply working without tests then testing private code becomes an issue. At this point I stand by the advice in this post. If you're doing TDD or writing tests after the fact then the only way to test that behavior should be by the public API of the subject under test.

      Hope that clears it up.