CODEWITHSUNDEEP
nsubstitute
SO19
SO19

Reputation: 115

Is NSubstitute a mock or stub library?

As per official website, NSubstitute is A friendly substitute for .NET mocking libraries. I did a search / reading around this & found good article for reference & this is it. Here are few lines from it

For Stub

Because this code only knows about abstractions (ie. the interfaces), it's easy to run this code without using the production implementation of those interfaces. I could just create another implementations just for the test that implements those interfaces, but doesn't call the database. These test implementations are known as 'stubs'.

& for Mock

A mocking library allows you to simulate an interface or abstract type's implementation. You instantiate a 'mock' object of the interface, and tell that mock object what it should return if a method/property is called against that mock. You can also assert that a method/property was or wasn't called.

So if we want to corelate / understand it better, such as in terms of mock / stub library, what it is ? Is it a Mock / Stub / both with simple way of doing things?

Upvotes: 1

Views: 1091

Answers (1)

David Tchepak
David Tchepak

Reputation: 10484

I think the definitive source on this is Gerard Meszaros' xUnit Test Patterns book. Martin Fowler has a good summary of Meszaros' types of test doubles. From these definitions, stubs are test doubles that return specific results, whereas mocks are set up with specific expectations on the calls that should be received before a test runs.

NSubstitute is designed for Arrange-Act-Assert (AAA) testing, which to my knowledge was first popularised by the wonderful Moq library. The terms mock and stub predate AAA, so I don't think they exactly fit in with these types of libraries. The terminology has blurred over time so that any test double tends to be called a "mock", even if we aren't setting explicit expectations.

If we are happy to be a bit loose with the definitions, in an NSubstitute context we can use "stubbing" to refer to responses we've set using Returns, and "mocking" as when we assert that an expected call was received using Received. This can be done on the same test double object. i.e. we don't create a mock OR a stub, we create a test double (or "substitute") that can kind of do both. NSubstitute deliberately blurs these lines. From the website:

Mock, stub, fake, spy, test double? Strict or loose? Nah, just substitute for the type you need!

NSubstitute is designed for Arrange-Act-Assert (AAA) testing, so you just need to arrange how it should work, then assert it received the calls you expected once you're done. Because you've got more important code to write than whether you need a mock or a stub.

In answer to your question, if we are being strict with definitions, NSubstitute is a library for creating test doubles, and supports stubs and an alternative to mocks (AAA rather than call expectations). In practice, everyone tends to be loose with the definitions and just call test doubles "mocks".

Upvotes: 3

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