This essay says that inheritance is harmful and if possible you should “ban inheritance completely”. You see these arguments a lot, as well as things like “prefer composition to inheritance”. A lot of these arguments argue that in practice inheritance has problems. But they don’t preclude inheritance working in another context, maybe with a better language syntax. And it doesn’t explain why inheritance became so popular in the first place. I want to explore what’s fundamentally challenging about inheritance and why we all use it anyway.
The general idea is to not test things that need to be mocked, keep all logic out of those and keep them at the edges of the application. Then you test the remaining 90% of your code which would ideally be pure functions that don’t need mocks .
The remaining 5% can be integration tests which doesn’t care about DI.
In reality this doesn’t always work, because sometimes it’s better to write a complex query with logic and let your database do the processing.