Is there a way to unit test against side effects?

Question

Any code can provide side effects. Most of the time, side effects can be a sign of bad design and/or need of refactorisation, but when unit testing I find it hard to test against. Consider the following example:

[Test]
public void TrimAll_Removes_All_Spaces()
{
    // Arrange
    var testSubject = "A    string  with     lots   of     space";
    var expectedResult = "Astringwithlotsofspace";

    // Act
    var result = testSubject.TrimAll();

    // Assert
    Assert.AreEqual(expectedResult, result);
}

that tests the following extension:

public static string TrimAll(this string str)
{
    PokeAround();

    return str.Replace(" ", "");
}

The test will pass, but there is no guard agains side effects. The effects of the call to PokeAround will go completely unnoticed.

Given that you don't know what PokeAround is - it could be anything! - how do you write a test that guards against it? Is it at all possible?

Clarification: There have been a couple of comments about the PokeAround as completely unknown being a very unlikely scenario, since we have the source when we write the test. The reason I asked this question, though, was to find a way to guard against side effects added later on. That is, when I write the test, I might have the exension method look like this:

public static string TrimAll(this string str)
{
    return str.Replace(" ", "");
}

The test passes, all is good. Then, a month later when I'm on vacation, a colleague add's the PokeAround call. I want the test I already wrote to fail because he did.

Answer 1

This is what is called sensing in Working Effectively With Legacy Code. That is, sensing the effects of calling the tested method.

Given that you don't know what PokeAround is - it could be anything!

Since we are talking about unit tests, this should hardly be true - unit testing is whitebox testing, and the code is (should be) there for you to check. Unless it is in some closed source 3rd party library, in which case you don't need to test it can't unit test it by definition (maybe you need functional/acceptance tests, but that is an entirely different matter...).

Update: so you want to make sure that future changes to your unit tested method will never have any unanticipated side effects? I think you

1. can't,
2. shouldn't.

You can't, because there is no sensible way to detect the lack of side effects from a method call in a real life (nontrivial) program. What you are looking for is some check that the state of the whole universe has not changed apart from this and this little thing. Even from the point of view of a humble program, that universe is vast. A method call can create/update/delete any number of local objects (many of which you can't even see from your unit test environment), touch files on available local/network file systems, execute DB requests, make remote procedure calls...

You shouldn't, because it is up to your colleague making that future change to take care of unit testing his/her change. If you don't trust that this is going to happen, you have a people or process problem, not a unit testing problem.