Free and open-source unit testing framework for .NET languages.
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Pros
Pro Better support for integration and system test
Pro Excellent availability of learning resources
Due to it's popularity and active development, plenty of learning resources (such as detailed documentation and various tutorials) exist for learning NUnit.
Pro Widely used
NUnit is one of the most popular testing frameworks for .NET. Other than giving a certain sense of security in the continuation of the project, it also means that there are a lot of third-party resources, guides and tutorials available for NUnit.
Pro Built-in fluent Assertions
It has more readable Assertions out of the box like Assert.That(myClass.MyMethod(null), Throws.ArgumentNullException.With.Message.Contains("param");
NUnit also has good tutorials in using the variants for parameterized tests: e.g. its easy to find an example how to correctly use [TestCaseSource(typeof(myTesCaseEnumerator))].
Cons
Con Slow to adapt
NUnit is developed slowly; some developers complain that it is no longer able to keep up with new testing approaches.