XUnit with F#

10 April 2021

Updated: 03 September 2023

An important part of writing any software is testing. Unit testing is an automated testing method in which we test individual components of our software to verify that their behaviour aligns with our expectations

This post will take a look at the process of setting up a new F# library and two methods of configuring XUnit to test your project’s code

Create a Project

Before we can start testing we need a project that we can run tests on

First, we’re going to create a folder that we can work in:

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mkdir MyProject
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cd MyProject

We can use the following command to create a new project in our MyProject directory:

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dotnet new classlib -lang=f# -o MyProject.Lib

The project that we created will contain the following Library.ts file, this is the file that we’ll write tests for. First, we want to update the hello function so that it returns a formatted string:

MyProject.Lib/Library.fs

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namespace MyProject.Lib
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module Say =
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    let hello name =
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        sprintf "Hello %s" name

Adding Tests

Depending on our preferred project structure we can either:

Add tests in a separate project
Add test files alongside lib files

Method 1: Create Tests in Separate Project

The standard method of .NET unit testing with XUnit is to make use of separate Project and Test solutions, so a normal test setup would look something like:

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MyProject.Lib
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MyProject.Lib.Tests

To add an new XUnit test project you can run:

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dotnet new xunit -lang=f# -o MyProject.Tests

Then, so we’re able to test the code from MyProject.Lib, we need to add a reference to it from the Test project we just created:

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dotnet add MyProject.Tests reference MyProject.Lib

Then we can create a file in our test project called LibraryTests.fs which will contain our test code which we will cover in the last section, as well as adding a reference to this file in the MyProject.Tests.fsproj

MyProject.Tests.fsproj

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...
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  <ItemGroup>
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    <Compile Include="Tests.fs" />
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    <!-- Add the next line -->
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    <Compile Include="LibraryTests.fs" />
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    <Compile Include="Program.fs" />
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  </ItemGroup>
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...

Method 2: Create Tests Alongside Lib Files

The second method I’m going to discuss is keeping our test.fs files alongside the code that the file is testing. The structure of our project is something more like this:

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MyProject.Lib
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|- Library.fs
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|- Library.test.fs

Overall I find this more manageable is the way I keep my test code in other languages and frameworks as well

To implement this method we need to add some dependencies to our project

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cd MyProject.Lib
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dotnet add package Microsoft.NET.Test.Sdk
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dotnet add package xunit
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dotnet add package xunit.runner.visualstudio

Then we can create a file in our project called Library.test.fs which will contain our test code which we will cover next, as well as a reference to this file in the MyProject.Lib.fsproj

MyProject.Lib.fsproj

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...
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  <ItemGroup>
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    <Compile Include="Library.fs" />
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    <!-- Add the next line -->
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    <Compile Include="Library.test.fs" />
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  </ItemGroup>
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...

It’s important to note that this file must be added below the Library.fs file as it will reference it for tests to run

Test Files

More detailed information on XUnit can be found in Unit Testing notes

Since we’ve configured XUnit it may be useful to understand how these tests work in the context of F#. XUnit tests are organized into modules. Regardless of which of the two methods above you’re using the test files work the same

Generally, a test file will contain:

A top-level module definition
open statements to import XUnit
Test functions annotated with Fact or Theory

XUnit tests can be broken into 2 types:

Single-case tests without input parameters inputs are labelled Fact
Multi-case tests which make use of input parameters are labelled Theory and use InlineData

Fact

Let’s add the following content into our test file into one that tests the hello function from our Lib code with the input "Name"

LibraryTests.fs/Library.test.fs

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module LibraryTests
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open Xunit
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open MyProject.Lib.Say
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[<Fact>]
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let ``Say.hello -> "Hello name" `` () =
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    let name = "name"
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    let expected = "Hello name"
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    let result = hello name
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    Assert.Equal(expected, result)

F# allows us to name our functions using special characters provided they’re enclosed in backticks as seen above. Naming test functions this way allows them to be more discriptive than more traditional variable names

Additionally, there’s the normal XUnit test setup which includes calling our test function with some input and asserting something about it using Assert.Equal from XUnit

Theory

We can add a Theory to test our function with multiple different inputs:

LibraryTests.fs/Library.test.fs

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[<Theory>]
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[<InlineData("name", "Hello name")>]
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[<InlineData("World", "Hello World")>]
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let ``Say.hello -> concantenated string`` (name:string, expected: string) =
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    let result = hello name
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    Assert.Equal(expected, result)

In the above we add the InlineData attribute which allows us to provide inputs to our test, as well as specifying a name and expected argument for our function. The test framework will then call our test using the arguments as specified in InlineData

When we’re done our test file should have the following content:

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module LibraryTests
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open Xunit
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open MyProject.Lib.Say
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[<Fact>]
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let ``Say.hello -> "Hello name" `` () =
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    let name = "name"
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    let expected = "Hello name"
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    let result = hello name
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    Assert.Equal(expected, result)
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[<Theory>]
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[<InlineData("name", "Hello name")>]
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[<InlineData("World", "Hello World")>]
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let ``Say.hello -> concantenated string`` (name:string, expected: string) =
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    let result = hello name
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    Assert.Equal(expected, result)

Running Tests

In order to run tests we can use the dotnet-cli. Depending on the method used you can run your test from the project’s root directory using the following command:

Method 1 - dotnet test MyProject.Tests
Method 2 - dotnet test MyProject.Lib

Alternatively, tests can also be run from your IDE or Visual Studio Code with the Ionide and .NET Core Test Explorer extensions installed

Additional Resources

If you’d like a deeper look into F# or XUnit here are some of my other posts which cover those:

Nabeel Valley