XUnit.net

xUnit.net is a free and open-source unit testing tool for the .NET Framework, written by the original author of NUnit. The software can also be used with .NET Core and Mono.

It is licensed under Apache License 2.0, and the source code is available on GitHub. xUnit.net works with Xamarin, ReSharper, CodeRush, and TestDriven.NET. It is authored by James Newkirk and Brad Wilson.

Example
Example of an xUnit.net test fixture:

After you compile the test, run it in your console: