The best replacement for nunit is mstest.

The best alternatives to nunit are rated as following:
★★★★★  mstest
★★★★  xunit
★★★  mbunit
★  junit
★  vsts
★  xunit.net
  csunit
  visual studio test
  visual studio unit testing
  mstest 2010
  msunit

Your alternative for nunit is not listed? Suggest!
Speaking about nunit, one probably shouldn't forget about moq.

Other search terms related to nunit and their ratings are:
★★★★★ moq
★★★★★ selenium
★★ visual studio 2010 express
★★ rhino mocks
★★ resharper
★ msbuild
vs 2010
You have in mind a companion for nunit and it is not yet here? Propose!



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