Why was FakeItEasy created?
There was a good question on Stack Overflow that asks what
distinguishes FakeItEasy from other libraries. Creator of FakeItEasy,
Patrik Hägne, answered the question there but we reproduce the
answer here. Note that the text has been preserved, and particular
constructs referenced (such as
DummyDefinitions) have changed or
been renamed in newer versions of FakeItEasy.
The question on Stack Overflow: "Are fakes better than Mocks?"
Patrik Hägne's answer
To be clear, I created FakeItEasy so I'll definitely not say whether one framework is better than the other, what I can do is point out some differences and motivate why I created FakeItEasy. Functionally there are no major differences between Moq and FakeItEasy.
FakeItEasy has no "Verifiable" or "Expectations" it has assertions however, these are always explicitly stated at the very end of a test, I believe this makes tests easier to read and understand. It also helps beginners to avoid multiple asserts (where they would set expectations on many calls or mock objects).
I used Rhino Mocks before and I quite liked it, especially after the AAA-syntax was introduced I did like the fluent API of Moq better though. What I didn't like with Moq was the "mock object" where you have to use mock.Object everywhere, I like the Rhino-approach with "natural" mocks better. Every instance looks and feels like a normal instance of the faked type. I wanted the best of both worlds and also I wanted to see what I could do with the syntax when I had absolutely free hands. Personally I (obviously) think I created something that is a good mix with the best from both world, but that's quite easy when you're standing on the shoulders of giants.
As has been mentioned here one of the main differences is in the terminology, FakeItEasy was first created to introduce TDD and mocking to beginners and having to worry about the differences between mocks and stubs up front is not very useful.
I've put a lot of focus into the exception messages, it should be very easy to tell what went wrong in a test just looking at an exception message.
FakeItEasy has some extensibility features that the other frameworks don't have but these aren't very well documented yet.
FakeItEasy is (hopefully) a little stronger in mocking classes that has constructor arguments since it has a mechanism for resolving dummy-values to use. You can even specify your own dummy value definitions by implementing a DummyDefinition(Of T) class within your test project, this will automatically be picked up by FakeItEasy.
The syntax is an obvious difference, which one is better is largely a matter of taste.
I'm sure there are lots of other differences that I forget about now (and to be fair I've never used Moq in production myself so my knowledge of it is limited), I do think these are the most important differences though.