This post is part of the F# Advent Calendar 2021. Many thanks to Sergey Tihon for organising these. Go checkout the other many and excellent posts.

This year, I’ve run out of Xmas themed topics. Instead, I’m just sharing a few tips from a recent project I’ve been working on…

I’m going to show…

  • Dev Containers for F# Development
  • A simple Giraffe Web Server
  • Automated HTTP Tests
  • Working with ASP.Net dependencies

You can see the full source code for this project on GitHub here

Dev Containers

Dev Containers are a feature of VS Code I was introduced to earlier this year and have since taken to using in all my projects.

They allow you to have a self contained development environment in DockerFile, including all the dependencies your application requires and extensions for Visual Studio Code.

If you have ever looked at the amount of things you have installed for various projects and wondered where it all came from and if you still need it - Dev Containers solves that problem. They also give you a very simple way to share things with your collaborators, no longer do I need a 10-step installation guide in a Readme file. Once you are setup for Dev Containers, getting going with a project that uses them is easy.

This blog is a GitHub Pages Site, and to develop and test it locally I had to install Ruby and a bunch of Gems, and Installing those on Windows is tricky at best. VS Code comes with some pre-defined Dev Container templates, so I just used the Jekyll one, and now I don’t have to install anything on my PC.

Dev Container for .NET

To get started, you will need WSL2 and the Remote Development Tools pack VS Code extension installed.

Then it just a matter of launching VS Code from in my WSL2 instance:

cd ~/xmas-2021
code .

Now in the VS Code Command Palette I select Remote Containers: Add Development Container Configuration Files… A quick search for “F#” helps get the extensions I need installed. In this case I just picked the defaults.

Once the DockerFile was created I changed the FROM to use the standard .NET format that Microsoft uses (the F# template may have changed by the time you read this) to pull in the latest .NET 6 Bullseye base image.




# [Choice] .NET version: 6.0, 5.0, 3.1, 6.0-bullseye, 5.0-bullseye, 3.1-bullseye, 6.0-focal, 5.0-focal, 3.1-focal
ARG VARIANT=6.0-bullseye

VS Code will then prompt to Repen in the Dev Container, selecting this will relaunch VS Code and build the docker file. Once complete, we’re good to go.

Creating the Projects

Now that I’m in VS Code, using the Dev Container, I can run dotnet commands against the terminal inside VS Code. This is what I’ll be using to create the skeleton of the website:

# install the template
dotnet new -i "giraffe-template::*"

# create the projects
dotnet new giraffe -o site
dotnet new xunit --language f# -o tests

# create the sln
dotnet new sln
dotnet sln add site/
dotnet sln add tests/

# add the reference from tests -> site
cd tests/
dotnet add reference ../site/
cd ..

I also update the projects target framework to net6.0 as the templates defaulted to net5.0.

For the site/ I updated to the latest giraffe 6 pre-release (alpha-2 as of now) and removed the reference to Ply which is no longer needed.

That done I could run the site and the tests from inside the dev container:

dotnet run --project site/

dotnet test

Next, I’m going to rip out most of the code from the Giraffe template, just to give a simpler site to play with.

Excluding the open’s it is only a few lines:

let demo = 
    text "hello world"

let webApp =
    choose [
        GET >=>
            choose [
                route "/" >=> demo
            ] ]

let configureApp (app : IApplicationBuilder) =

let configureServices (services : IServiceCollection) =
    services.AddGiraffe() |> ignore

let main args =
            fun webHostBuilder ->
                    |> ignore)

I could have trimmed it further, but I’m going to use some of the constructs later.

When run you can perform a curl localhost:5000 against the site and get a “hello world” response.


I wanted to try out self-hosted tests against this API, so that I’m performing real HTTP calls and mocking as little as possible.

As Giraffe is based on ASP.NET you can follow the same process as you would for testing as ASP.NET application.

You will need to add the TestHost package to the tests project:

dotnet add package Microsoft.AspNetCore.TestHost

You can then create a basic XUnit test like so:

let createTestHost () =
    .Configure(configureApp)    // from the "Site" project
    .ConfigureServices(configureServices)   // from the "Site" project
let ``First test`` () =
    task {
        use server = new TestServer(createTestHost())
        use msg = new HttpRequestMessage(HttpMethod.Get, "/")

        use client = server.CreateClient()
        use! response = client.SendAsync msg
        let! content = response.Content.ReadAsStringAsync()

        let expected = "hello test"
        Assert.Equal(expected, content)

If you dotnet test, it should fail because the tests expects “hello test” instead of “hello world”. However, you have now invoked your Server from your tests.


With this approach you can configure the site’s dependencies how you like, but as an example I’m going to show two different types of dependencies:

  1. App Settings
  2. Service Lookup

App Settings

Suppose your site relies on settings from the “appsettings.json” file, but you want to test with a different value.

Let’s add an app settings to the Site first, then we’ll update the tests…

    "MySite": {
        "MyValue": "100"

I’ve removed everything else for the sake of brevity.

We need to make a few minor changes to the demo function and also create a new type to represent the settings

type Settings = { MyValue: int }

let demo = 
    fun (next : HttpFunc) (ctx : HttpContext) ->

        let settings = ctx.GetService<IOptions<Settings>>()

        let greeting = sprintf "hello world %d" settings.Value.MyValue
        text greeting next ctx

And we need to update the configureServices function to load the settings:

let serviceProvider = services.BuildServiceProvider()
let settings = serviceProvider.GetService<IConfiguration>()
services.Configure<Settings>(settings.GetSection("MySite")) |> ignore

If you run the tests now, you get “hello world 0” returned.

However, if you dotnet run the site, and use curl you will see hello world 100 returned.

This proves the configuration is loaded and read, however, it isn’t used by the tests - because the appsettings.json file isn’t part of the tests. You could copy the file into the tests and that would solve the problem, but if you wanted different values for the tests you could create your own “appsettings.”json” file for the tests

    "MySite": {
        "MyValue": "3"

To do that we need function that will load the test configuration, and the add it into the pipeline for creating the TestHost:

let configureAppConfig (app: IConfigurationBuilder) =
  app.AddJsonFile("appsettings.tests.json") |> ignore

let createTestHost () =
    .ConfigureAppConfiguration(configureAppConfig)   // Use the test's config
    .Configure(configureApp)    // from the "Site" project
    .ConfigureServices(configureServices)   // from the "Site" project

Note: you will also need to tell the test project to include the appsettings.tests.json file.

    <Content Include="appsettings.tests.json" CopyToOutputDirectory="always" />

If you would like to use the same value from the config file in your tests you can access it via the test server:

let config = server.Services.GetService(typeof<IConfiguration>) :?> IConfiguration

let expectedNumber = config["MySite:MyValue"] |> int

let expected = sprintf "hello world %d" expectedNumber


In F# it’s nice to keep everything pure and functional, but sooner or later you will realise you need to interact with the outside world, and when testing from the outside like this, you may need to control those things.

Here I’m going to show you the same approach you would use for a C# ASP.NET site - using the built in dependency injection framework.

type IMyService =
    abstract member GetNumber : unit -> int

type RealMyService() =
    interface IMyService with
        member _.GetNumber() = 42

let demo = 
    fun (next : HttpFunc) (ctx : HttpContext) ->

        let settings = ctx.GetService<IOptions<Settings>>()
        let myService = ctx.GetService<IMyService>()

        let configNo = settings.Value.MyValue
        let serviceNo = myService.GetNumber()

        let greeting = sprintf "hello world %d %d" configNo serviceNo
        text greeting next ctx

I’ve create a IMyService interface and a class to implement it RealMyService.

Then in configureServices I’ve added it as a singleton:

services.AddSingleton<IMyService>(new RealMyService()) |> ignore

Now the tests fail again because 42 is appended to the results.

To make the tests pass, I want to pass in a mocked IMyService that has a number that I want.

let luckyNumber = 8

type FakeMyService() =
    interface IMyService with
        member _.GetNumber() = luckyNumber

let configureTestServices (services: IServiceCollection) = 
  services.AddSingleton<IMyService>(new FakeMyService()) |> ignore

let createTestHost () =
    .ConfigureAppConfiguration(configureAppConfig)   // Use the test's config
    .Configure(configureApp)    // from the "Site" project
    .ConfigureServices(configureServices)   // from the "Site" project
    .ConfigureServices(configureTestServices) // mock services after real ones

Then in the tests I can expect the luckyNumber:

let expected = sprintf "hello world %d %d" expectedNumber luckyNumber

And everything passes.


I hope this contains a few useful tips (if nothing else, I’ll probably be coming back to it in time to remember how to do some of these things) for getting going with Giraffe development in 2022.

You can see the full source code for this blog post here.