xUnit.net is a free, open source, community-focused unit testing tool for the .NET Framework.
Fluent API for asserting the results of unit tests that targets .NET Framework 4.5, 4.7, .NET Standard 1.3, 1.6 and 2.0. Supports the unit test frameworks MSTest, MSTest2, Gallio, NUnit, XUnit, MBunit, MSpec, and NSpec.
Should testing for .NET - the way Asserting *Should* be!
Modern Testing for .NET
List of Automated Testing (TDD/BDD/ATDD/SBE) Tools and Frameworks for .NET
Easy integration testing helper for ASP.Net Core applications
Samples for xUnit.net (please open issues in https://github.com/xunit/xunit)