You're mixing definitions - integration tests concern testing the "integration" between all parts of a solution. It has nothing to do with reading a JSON file,

You're mixing definitions - integration tests concern testing the "integration" between all parts of a solution. It has nothing to do with reading a JSON file, its perfectly acceptable to read from a JSON file and use its data in a unit test. Also reading / parsing a JSON file is fast enough for hot reloads / auto rerunning unless you have multiple GB files - so the argument for speed makes no sense. I'd argue it's slower as a whole to have to code up mocks and fill in the data than copy paste some json. I do agree with the second being neater, however past a certain point of enterprise coding it's a negligible difference compared to the overall complexity of the code - so taking a shortcut and making your tests simpler through JSON files actually ends up being the cleaner / neater solution. >While unit testing, I'm going to test "given that System.IO.File can read a file", and write tests accordingly. I don't want a test sometimes fails because "read errors can happen IRL". That doesn't help test my business logic. Since you're given that - use it. If your test fails because a "low level" dependency is failing it's indicating something is seriously fucked up on your machine.