NSubstitute

NSubstitute is a popular open-source .NET library that allows developers to create mock objects for use in unit testing, it is a powerful and flexible substitute for traditional mocking frameworks such as Moq and Rhino Mocks. In the context of ASP.NET, NSubstitute can be used to create mock objects for testing web applications built using the ASP.NET framework, it provides a simple and intuitive syntax for creating mock objects, setting up mock behavior, and making assertions on calls made to the mock objects. NSubstitute supports a wide range of testing frameworks and test runners, including MSTest, NUnit, and xUnit and it also supports various platforms including .NET Framework, .NET Core, Xamarin and Unity.