How to capture output with XUnit 2.0 and FSharp style tests

Question

Normally I write my unit tests in F# as

open Swensen.Unquote
open Xunit

module MyTests =

    [<Fact>]
    let ``SomeFunction should return 10`` () =
        let a = SomeFunction()
        test <@ a = 10 @>


    [<Fact>]
    let ``SomeOtherFunction should return 11`` () =
        let a = SomeFunction()
        test <@ a = 11 @>

If I wish to log to the console from xunit ( according to http://xunit.github.io/docs/capturing-output.html ) one needs to write a constructor that takes an ITestOutputHelper and then use that instead of Console.WriteLine and family.

using Xunit;
using Xunit.Abstractions;

public class MyTestClass
{
    private readonly ITestOutputHelper output;

    public MyTestClass(ITestOutputHelper output)
    {
        this.output = output;
    }

    [Fact]
    public void MyTest()
    {
        var temp = "my class!";
        output.WriteLine("This is output from {0}", temp);
    }
}

however fsharp modules are static classes and the tests are static methods. There is no constructor to inject the output helper.

Is there a way to get access to the current output helper for the current test. I know I could rewrite my fsharp tests to be non static classes but that is undesired.

After looking at the XUnit source.

https://github.com/xunit/xunit/blob/e64f566b75f93cd3cec27f950759d82832bfe44b/src/xunit.execution/Sdk/Frameworks/Runners/TestClassRunner.cs#L90

I'm pretty sure this is an overlooked case. There is no injection of the helper into static classes.

Answer 1

If xUnit does not have any alternative mechanism for injection of the parameters, then I guess the only option is to define the tests as methods in an F# object type. I also prefer writing tests as functions using let, but the following simple object type does not look too bad:

open Swensen.Unquote
open Xunit
open Xunit.Abstractions

type MyTests(output:ITestOutputHelper) =

    [<Fact>]
    member __.``SomeFunction should return 10`` () =
        let a = SomeFunction()
        output.WriteLine("Some function returned {0}", a)
        test <@ a = 10 @>

It would be nice if xUnit supported some other option for this - I suspect that they might be open to suggestions, if it is something that would not be too awkward (perhaps using a method parameter?)

But unless xUnit adds support for some other method, I think you'll need to use F# object with methods.