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fsharp-hedgehog NuGet

Hedgehog will eat all your bugs.

Hedgehog is a modern property-based testing system, in the spirit of QuickCheck. Hedgehog uses integrated shrinking, so shrinks obey the invariants of generated values by construction.


  • Integrated shrinking, shrinks obey invariants by construction.
  • Convenient syntax for generators and properties with gen and property expressions.
  • Range combinators for full control over the scope of generated numbers and collections.


The root namespace, Hedgehog, includes almost everything you need to get started writing property tests with Hedgehog.

open Hedgehog

Once you have your import declaration set up, you can write a simple property:

let propReverse : Property<Unit> =
    property {
        let! xs = Gen.list (Range.linear 0 100) Gen.alpha
        return xs |> List.rev |> List.rev = xs

You can then load the module in F# Interactive, and run it:

> Property.render propReverse |> printfn "%s"
+++ OK, passed 100 tests.

More examples can be found in the tutorial.

👉 For auto-generators (à la AutoFixture) and other convenience generators, check out fsharp-hedgehog-experimental.

C#/LINQ Example

Hedgehog provides an alternative namespace, meant to provide compatibility with other .NET languages. To use this, simply import the Hedgehog.Linq namespace. The base Hedgehog namespace isn't necessary in this scenario.

Generators are then built with a more comfortable fluent API.

using Hegehog.Linq;

class Generators
    static void Example()
        // This creates a generator for 10 character strings, with only alphabetical characters.
        var stringLength = 10;
        var stringGen = Gen.String(Gen.Alpha, Range.FromValue(stringLength));

        // This creates a property that can be checked, rechecked or rendered.
        var property =
            from string in Property.ForAll(stringGen)
            select Assert.AreEqual(stringLength, string.Length);

        // Checking accepts an argument for the number of test cases to generate and check.
        property.Check(tests: 30);

Building from source

To build Hedgehog from source, you will need either the .NET Core SDK or Visual Studio.

Building & running tests

With Visual Studio you can build Hedgehog and run the tests from inside the IDE, otherwise with the dotnet command-line tool you can execute:

dotnet build

To run the tests, you can execute:

dotnet test tests/Hedgehog.Tests/Hedgehog.Tests.fsproj
dotnet test tests/Hedgehog.Linq.Tests/Hedgehog.Linq.Tests.csproj

Building the NuGet package

As of a NuGet package is created automatically during build.

If you want to manually create a NuGet package you can run:

dotnet pack src/Hedgehog/Hedgehog.fsproj -c Release

This will produce Hedgehog-x.y.z.nupkg in src/Hedgehog/bin/Release.

Development Environments

Visual Studio

Hedgehog can be developed in Visual Studio, once these requirements are met:

  • .NET Core (2.1+) and .NET Framework (4.5.1+) are installed.

Visual Studio Code

Hedgehog can be developed in Visual Studio Code, with a couple of scenarios supported.

Bare Metal

Developing on bare metal is essentially the same as developing in Visual Studio, install the appropriate .NET SDK then open the project folder.

Remote Containers

Development can also be done using Remote Development.

The steps to get this up and running are as follows:

  1. Install Docker.
  2. Install the Remote Containers extension.
  3. Open the project folder, then run the Remote-Containers: Reopen in Container command.