OpenMoq

The most popular and friendly mocking library for .NET

  var mock = new Mock<ILoveThisLibrary>();

  // WOW! No record/replay weirdness?! :)
  mock.Setup(library => library.DownloadExists("2.0.0.0"))
      .Returns(true);

  // Use the Object property on the mock to get a reference to the object
  // implementing ILoveThisLibrary, and then exercise it by calling
  // methods on it
  ILoveThisLibrary lovable = mock.Object;
  bool download = lovable.DownloadExists("2.0.0.0");

  // Verify that the given method was indeed called with the expected value at most once
  mock.Verify(library => library.DownloadExists("2.0.0.0"), Times.AtMostOnce());

Moq also is the first and only library so far to provide Linq to Mocks, so that the same behavior above can be achieved much more succinctly:

  ILoveThisLibrary lovable = Mock.Of<ILoveThisLibrary>(l =>
    l.DownloadExists("2.0.0.0") == true);

  // Exercise the instance returned by Mock.Of by calling methods on it...
  bool download = lovable.DownloadExists("2.0.0.0");

  // Simply assert the returned state:
  Assert.True(download);
  
  // If you want to go beyond state testing and want to 
  // verify the mock interaction instead...
  Mock.Get(lovable).Verify(library => library.DownloadExists("2.0.0.0"));

You can think of Linq to Mocks as "from the universe of mocks, give me one whose behavior matches this expression".

Check out the Quickstart for more examples!