"Development of a set of customized assertion methods is a key to
productive test-writing for NUnit. In many cases, a private method in one of
your test fixtures will be all that is needed. In others, you may wish to deploy
one or more assertions so that they can be reused by the entire development
team.
NUnit's new Assert extensibility model allows tailoring specific
assertions for your application in a way that is consistent with the basic NUnit
assertions. These extensions can provide messages that are consistent in
appearance with those provided by NUnit and use features of the NUnit framework
to avoid duplication."