Foq is a mature open source .Net mocking library written in F#. It’s raison d'être is to enable F# as a unit testing language, for F#, C# or VB.Net code bases. Foq is part a rich set of testing tools available to F# developers. Use Foq when unit testing in combination with staple .Net testing frameworks like NUnit and xUnit through to F# specific fluent assertion libraries like FsUnit and Unquote. Combine Foq with AutoFixture to simplify test setup even further.