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.