NSubstitute
A friendly substitute for .NET mocking libraries
...It is an attempt to satisfy our craving for a mocking library with a succinct syntax that helps us keep the focus on the intention of our tests, rather than on the configuration of our test doubles. We've tried to make the most frequently required operations obvious and easy to use, keeping less usual scenarios discoverable and accessible, and all the while maintaining as much natural language as possible. There are already some great mocking libraries around for .NET, so why create another? We found that for all their great features, none of the existing libraries had the succinct syntax we were craving — the code required to configure test doubles quickly obscured the intention behind our tests.