Using “friend”-declarations for unit testing. Bad idea?

Question

[Of course, the question is not restricted to a specific \"friend\" implementation, feel free though to point out implementation specifics if relevant]

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Answer 1

It's the only use I've ever personally applied to InternalsVisibleTo - and it's been very, very handy indeed.

I don't view unit tests as black box testing - they're already coupled to the implementation to some extent. Being able to test internal types and methods allows a much tighter focus (smaller units).