FsCheck

FsCheck is a free property-based testing library for .NET, built for developers who want to check rules across generated inputs in F#, C# or VB. Its shrinking can turn a failing case into a smaller counterexample, making it a strong fit when edge cases matter and a less straightforward choice when the experimental stateful API or C# exception checks are central to the workflow.

Overview

FsCheck exercises properties against randomly generated cases, helping developers probe behavior beyond a fixed set of examples. The property still has to capture the rule that matters, so the library is most useful when a team can state expected behavior clearly and wants varied inputs to challenge it.

When a property fails, shrinking reduces the input and reports a minimal counterexample. That gives developers a more focused starting point for diagnosing a failure. Custom generators and shrinkers, bundled as Arbitrary instances, let teams tailor both the test data and how failing cases are simplified.

FsCheck includes a runner for F# or C# Interactive, command-line programs and test frameworks. It documents integrations with xUnit.net and NUnit, alongside user-created integrations for Expecto, Fuchu, AutoFixture and MSTest. That range makes it practical to add property checks to several .NET testing setups, though the user-created integrations are not presented as project-documented integrations.

Key features

• Properties across .NET languages: Checks can be expressed in F#, C# or VB, with parameterized cases and parallel execution supported. The restriction is that properties cannot have generic types; type annotations can resolve types where needed.
• Generated data and shrinking: Custom generators and shrinkers give developers control over input shape and failure reduction. This is valuable for domain-specific data, while still requiring work to define suitable generators.
• Framework flexibility: The built-in runner works from interactive sessions, command-line programs or test frameworks. NuGet packages are available for FsCheck and its NUnit and xUnit.NET plugins.
• Stateful model-based checks: FsCheck has an experimental API for testing stateful systems and objects. Its exemption from semantic-versioning guarantees means minor releases may break code using that API, so it is a riskier foundation for compatibility-sensitive suites.

Pricing

FsCheck — free: The plan is a BSD-licensed library distributed as NuGet packages. There is no paid tier or free-trial period in the plan structure; the license permits modification and redistribution for commercial and non-commercial use. This suits teams that need a no-cost property-testing library and want permission to adapt or redistribute it.

Platforms

FsCheck supports Linux, macOS and Windows. Its supported languages are F#, C# and VB, so it fits .NET teams working across those operating systems.

Who it's for

FsCheck is a good fit for .NET developers who can express important behavior as properties and want generated cases to probe them. It is especially useful when custom data generation and minimized failure examples can make tricky inputs easier to investigate. Teams relying on stable stateful API compatibility should be cautious, and C# users who need the documented function for expecting exceptions will need another approach.

Pros and cons

• Pros: Free BSD licensing permits commercial and non-commercial use, modification and redistribution.
• Pros: Shrinking produces minimal counterexamples, making generated failures more focused to debug.
• Pros: Custom generators, shrinkers, a built-in runner and multiple integrations support varied .NET workflows.
• Cons: The experimental stateful API carries no semantic-versioning guarantee, so minor releases may break dependent code.
• Cons: Generic properties are unsupported, and the documented exception-expectation function is unavailable in the C# API.
• Cons: The project says its 2.x documentation is out of date and incomplete while 3.x is maintained and receives features and fixes. The documentation gap can make adoption and troubleshooting harder.

Alternatives

For another free property-testing library, Racket QuickCheck is a fit for Racket users; it is distributed as a Racket package and requires Racket v6.0 or newer. ScalaCheck is the corresponding option to consider for Scala, with a free BSD-3-Clause open-source library.

fast-check is a free, MIT-licensed JavaScript/TypeScript option for projects in that ecosystem. Go teams can consider Gopter, a free MIT-licensed Go property-testing library. JSVerify is another free open-source JavaScript property-based testing library.

Ruby developers can look at PropCheck, a free MIT-licensed gem requiring Ruby 2.5.1 or newer. QCheck is a free BSD-2-Clause OCaml property-testing library. For Scala, Scala Hedgehog is a free open-source testing library with no paid plans.

Browse more options in Property-Based Testing Tools or compare the broader Unit Testing Frameworks category.

Verdict

Choose FsCheck when you want free, permissively licensed property checks in a .NET workflow, with control over generated data and minimized failure examples. Its practical reach across F#, C# and VB and its built-in runner make it a capable choice for teams extending beyond hand-picked cases. Look elsewhere if a stable experimental stateful API is essential, or if your C# tests depend on FsCheck’s documented exception-expectation function.