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Fuchu (pronounced "foo choo" with the accent on the last syllable) is a test library for .NET, supporting C# and VB.NET but with a special focus on F#. It draws heavily from Haskell's test-framework and HUnit. You can read about the rationale and underlying concepts in this blog post, or TL;DR: tests should be first-class values so that you can move them around and execute them in any context that you want. Also, if they are first-class values, then you can take extra care with what the test methods return, making integrations with external libraries much cheaper.


Binaries are available on NuGet.


Add the following line to your paket.dependencies file

github mausch/Fuchu Fuchu/Fuchu.fs

Writing tests

Here's the simplest test possible:

open Fuchu

let simpleTest = 
    testCase "A simple test" <| 
        fun _ -> Assert.Equal("2+2", 4, 2+2)

Tests can be grouped (with arbitrary nesting):

let tests = 
    testList "A test group" [
        testCase "one test" <|
            fun _ -> Assert.Equal("2+2", 4, 2+2)
        testCase "another test" <|
            fun _ -> Assert.Equal("3+3", 3, 3+3)

In C#:

static Test ATest {
    get {
        return Test.List("A test group", new[] {
            Test.Case("one test", () => Assert.Equal("2+2", 4, 2+2)),
            Test.Case("another test", () => Assert.Equal("3+3", 3, 3+3)),

The first parameter in the assertions describes the assertion. This is usually an optional parameter in most test frameworks; in Fuchu it's required to foster descriptive failures, so you'll get a failure like "3+3 Expected value 3, actual 6" instead of just "Expected value 3, actual 6".

For more examples, including a few ways to do common things in other test frameworks like setup/teardown and parameterized tests, see the F# tests and the C# tests


Fuchu is mainly oriented to test organization. Although it does have a few basic assertions, you're encouraged to write your own specialized assertions for each project (they're only a couple of lines in F#), or use some other library for assertions, like Unquote, FsUnit, or even MbUnit or NUnit.

Running tests

The test runner is the test assembly itself. It's recommended to compile your test assembly as a console application. You can run a test directly like this:

run simpleTest // or runParallel

which returns 1 if any tests failed, otherwise 0. Useful for returning to the operating system as error code. Or you can mark the top-level test in each test file with the [<Tests>] attribute, then define your main like this:

open Fuchu

let main args = defaultMainThisAssembly args

This defaultMainThisAssembly function admits a "/m" parameter passed through the command-line to run tests in parallel. In order to get diagnostic messages you pass in a "/d".

You can single out tests by filtering them by name. For example:

|> Test.filter (fun s -> s.EndsWith "another test")
|> run

You can use the F# REPL to run tests this way.

Using Fuchu with C#

In C#:

static int Main(string[] args) {
    return ATest.Run(); // or RunParallel()

Or scanning for tests marked with the [Tests] attribute:

static int Main(string[] args) {
    return Tests.DefaultMainThisAssembly(args);

Using Fuchu with Fable

In order to be able to test your code both with .net and Fable you can adjust your main:

open Fuchu
let exitIfNonZero v =
    if v <> 0 then
        failwithf "expected a nonzero exitcode, but got %i" v

let main args =
    defaultMain Tests.tests args
    |> exitIfNonZero

Note that we don't use assembly scanning with Fable.

You will need to have esm installed, leaving you with the following package.json:

    "private": true,
    "dependencies": {
        "esm": "^3.2.25"
    "name": "tests"

In order to run the tests with Fable you then use dotnet fable to run your test project:

dotnet fable YourTestProject --outDir bin --runScript

FsCheck integration

Reference FsCheck and Fuchu.FsCheck to test properties:

let config = { FsCheck.Config.Default with MaxTest = 10000 }

let properties = 
    testList "FsCheck" [
        testProperty "Addition is commutative" <|
            fun a b -> 
                a + b = b + a
        // you can also override the FsCheck config
        testPropertyWithConfig config "Product is distributive over addition" <|
            fun a b c -> 
                a * (b + c) = a * b + a * c

run properties

In C# (can't override FsCheck config at the moment):

static Test Properties =
    Test.List("FsCheck", new[] {
        FsCheck.Property("Addition is commutative",
                            (int a, int b) => a + b == b + a),
        FsCheck.Property("Product is distributive over addition",
                            (int a, int b, int c) => a * (b + c) == a * b + a * c),

You can freely mix FsCheck properties with regular test cases and test lists.

PerfUtil integration

The integration with Eirik's PerfUtil project.

open global.PerfUtil

module Types =
    type Y = { a : string; b : int }

type Serialiser =
    inherit ITestable
    abstract member Serialise<'a> : 'a -> unit

type MySlowSerialiser() =
    interface ITestable with
        member x.Name = "Slow Serialiser"
    interface Serialiser with
        member x.Serialise _ =

type FastSerialiser() =
    interface ITestable with
        member x.Name = "Fast Serialiser"
    interface Serialiser with
        member x.Serialise _ =

type FastSerialiserAlt() =
    interface ITestable with
        member x.Name = "Fast Serialiser Alt"
    interface Serialiser with
        member x.Serialise _ =

let alts : Serialiser list = [ FastSerialiser(); FastSerialiserAlt() ]
let subj = MySlowSerialiser() :> Serialiser

open Types

let normal_serlialisation : PerfTest<Serialiser> list = [
    perfTest "serialising string" <| fun s ->
    perfTest "serialising record" <| fun s ->
        s.Serialise { a = "hello world"; b = 42 }

let tests =
    testList "performance comparison tests" [
        testPerfImpls "implementations of Serialiser" subj alts normal_serlialisation
        testPerfHistory "historical MySlowSerialiser" subj "v1.2.3" normal_serlialisation

This example shows both a comparison performance test between MySlowSerialiser, FastSerialiser and FastSerialiserAlt: testPerfImpls and a historical comparison of MySlowSerialiser alone which saves an xml file next to the dll on every run.

You can find detailed docs in the source code of PerfUtil.fs on all parameters and data structures. All things that can be configured with PerfUtil can be configured with the conf parameter to testPerfImplsWithConfig and testPerfHistoryWithConfig.

The functions are discoverable by starting with testPerf*.

Handle the results explicitly by giving a config with a value of handleResults. Use that if you want to export the data to e.g. CSV or TSV.

More examples

Some projects using Fuchu:


Functional test library for F# / C# / VB.NET







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