“But it’s auto-generated, so why does it need to be unit tested?”

I recently overheard a conversation in which one developer was explaining the legacy code he had inherited to a new starter on the team. The developer described the code generators and the accompanying unit tests for the generated artifacts. Naturally the new developer asked why they needed unit tests if they were automagically making this stuff. This is a valid question and one which does beg a decent answer. I will try to answer the question based on my experience as a test-infected developer.

My response is as follows:

1. Auto-generation of software artifacts does not guarantee correctness. This is a garbage-in-garbage-out view of code generation. Engineers need to be sure that the changes that will inevitably occur in the code generation templates do not introduce defects. By running unit tests against the generated code you are testing that the generators works as they should and that specified inputs produce predictable outputs. The unit tests enforce the implied contract which stipulates that my code generator should always produce artifacts that do X.
2. Unit tests provide a reliable and repeatable way of regression testing the outputs of the generated artifacts. A change to the code templates might introduce subtle bugs. How else would you be able to detect these in the absence of automated unit testing. You are covering your own hide by having a battery of tests.
3. Unit tests allow you to develop with courage. You get immediate feedback when defects are introduced into the codebase (this assumes you are continuously running your tests, which you should).
4. The quality of the unit tests also dictates the testing experience. Several heuristics exist for developing good tests.