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.

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Example

Example of an xUnit.net test fixture:

using Xunit;
public class MyTests

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

C:\MyTests\bin\Debug>xunit.console MyTestLibrary.dll
xUnit.net console test runner
Copyright 2007-11 Microsoft Corporation.
xunit.dll: Version 1.9.1.0
Test assembly: C:\MyTests\bin\Debug\MyTestLibrary.dll
1 total, 0 failed, 0 skipped, took 0.302 seconds