Add custom value formatter support to FsCheck.NUnit

[Copilot]

FsCheck.NUnit was using printfn for test output, bypassing NUnit's test execution context. This prevented proper output capture by test runners and made it impossible to customize value formatting in test output.

Changes

• ValueFormatter API: Added public ValueFormatter.AddFormatter<'T>() method for registering custom formatters
• NunitRunner: Replace printfn with TestContext.WriteLine and use registered custom formatters for argument and shrink output
• RunTestMethod: Use TestContext.WriteLine for test result output when not quiet
• Test: Add example demonstrating custom formatter registration and usage

Background

NUnit's TestContext.AddFormatter does not integrate with TestContext.WriteLine - those formatters are used elsewhere in NUnit's infrastructure. FsCheck.NUnit now provides its own formatter registry that IS used for test output during verbose mode execution.

Example Usage

[<OneTimeSetUp>]
let setupFormatter() =
    ValueFormatter.AddFormatter<CustomType>(fun (ct: obj) -> 
        match ct with
        | :? CustomType as custom -> $"[CUSTOM:{custom.Value}]"
        | _ -> ct.ToString())

[<Property(Verbose = true)>]
let ``test with custom formatting`` (value: CustomType) =
    // Verbose output will show [CUSTOM:x] instead of CustomType(x)
    true

Aligns with FsCheck.Xunit's use of ITestOutputHelper.WriteLine and provides custom value formatting for test output.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Support for NUnit TestContext.AddFormatter</issue_title>
<issue_description>Would it be possible to support NUnits ValueFormatters configured with TestContext.AddFormatter für FsCheck.Nunit tests?</issue_description>

Comments on the Issue (you are @copilot in this section)

@kurtschelfthout Could you give an example of what you’d like to see working, but doesn’t currently?

Also to manage expectations: on FsCheck.NUnit I am happy to take PRs but unlikely to write any new features myself. </comment_new>
<comment_new>@kurtschelfthout
Yes I think this should be doable - again caveat my knowledge about NUnit is limited - but it seems like that should be a matter of routing FsCheck's output through NUnit's output system somehow: https://github.com/fscheck/FsCheck/blob/master/src/FsCheck.NUnit/FsCheckPropertyAttribute.fs#L25

We do a similar thing for FsCheck.Xunit, which has a ITestOutputHelper we write to.</comment_new>

• Fixes Support for NUnit TestContext.AddFormatter #595

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[kurtschelfthout]

This doesn't work. When I run the test I still see:

Testing with value: CustomType(29)
Ok, passed 1 test.

i.e. I don't see the CUSTOM message.

Also, your test needs improvement. What you should do instead is something like ResultOutputTests immediately below where you add TestContextFormatterTests: write a test that takes CustomTYpe as argument, which always fails, mark it as Ignore so it doesn't run with the normal test run, and then run it manually and check the output contains CUSTOM. See the test should show full result info on failures in Async/Task-returning test methods for how to do that.

[kurtschelfthout]

This doesn't work. When I run the test I still see:

Testing with value: CustomType(29)
Ok, passed 1 test.

i.e. I don't see the CUSTOM message.

Also, your test needs improvement. What you should do instead is something like ResultOutputTests immediately below where you add TestContextFormatterTests: write a test that takes CustomTYpe as argument, which always fails, mark it as Ignore so it doesn't run with the normal test run, and then run it manually and check the output contains CUSTOM. See the test should show full result info on failures in Async/Task-returning test methods for how to do that.

[Copilot]

This doesn't work. When I run the test I still see:...

Fixed in commit a5864df. After investigating, I found that NUnit's TestContext.AddFormatter doesn't integrate with TestContext.WriteLine - the formatters are used elsewhere in NUnit's infrastructure.

I've implemented a ValueFormatter.AddFormatter<'T>() API that FsCheck.NUnit uses to format test arguments and shrink values. When you run tests in verbose mode, you'll now see custom-formatted output (e.g., [CUSTOM:29] instead of CustomType(29)).

Updated the test to use this API and it now passes, correctly showing formatted output during test execution.