FsCheck

FsCheck is a property-based testing framework for the .NET platform, inspired by Haskell's QuickCheck. It automatically generates random test data and checks that specified properties hold for many inputs, rather than testing a single example. The approach helps reveal edge cases and incorrect assumptions by exploring a broad space of inputs.

Core concepts include generators (Gen<'T>) for producing values, Arbitrary<'T> definitions that supply both a generator and a shrinker, and properties (Prop) that describe invariants to verify. When a property fails, FsCheck uses shrinking to try to simplify the counterexample to the smallest input that still fails, making debugging easier. Properties can be written in a declarative style and composed from simpler components to model complex behavior.

FsCheck can be used from F#, C#, and other .NET languages. It integrates with common test frameworks through adapters such as FsCheck.Xunit, FsCheck.NUnit, and FsCheck.MSTest, enabling tests to be discovered by standard runners. In F#, properties are typically written with Prop.forAll and composed generators; in C#, tests can be decorated with [Property] or invoked via Check. The library supports custom and composite generators, function values, and recursive data types, with control over data size and randomness.

Features include automatic shrinking, configurable random seeds for reproducibility, sized generation to control data complexity, and support for user-defined types through Arbitrary instances. It provides a family of combinators for building complex data generators and properties, along with tooling to integrate into existing test suites and continuous integration workflows.

FsCheck is open source under the MIT license and maintained on GitHub. The core library is complemented by adapters for popular test frameworks and by examples and guides to help developers adopt property-based testing in .NET projects.