Moq vs NSubstitute: A Practical Guide to Mocking in .NET Unit Tests

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A detailed technical guide explores mocking frameworks in .NET, focusing on the two most widely used libraries — Moq and NSubstitute — and their contrasting design philosophies. The guide introduces the precise vocabulary of test doubles, distinguishing between dummies, stubs, spies, mocks, and fakes, each serving a different purpose in unit testing. Both libraries are compared side by side, covering core usage, argument matching, verifying interactions, and handling return sequences. The guide also addresses what can and cannot be mocked, auto-mocking containers, and common pitfalls developers encounter. Importantly, it highlights when heavy reliance on mocking signals genuine test isolation versus an underlying design problem that needs rethinking.

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