Quote:
NUnit is an open-source unit testing framework for the .NET Framework and Mono.
NUnit - Wikipedia[
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Although you might get the impression that you must use Unit tests when you hear some "experts" talking, I think it depends on the situation and how your software is composed.
I'm in working in a very small development team that has to maintain hundreds of projects and Unit testing is simply too time consuming for us, when a project changes usually the unit tests have to be updated too which is problematic too say the least.