DateOnly support

[kant2002]

I use Verify.Xunit + Argon.FSharp

I add AddFSharpConverters to serialize tuple.

VerifierSettings.AddExtraSettings(fun settings -> 
    //settings.Converters.Add(IsoDateConverter())
    settings.AddFSharpConverters()
    )

And (string * DateOnly * string) tuple. its serializes strings fine, but not DateOnly. I did try to add

type IsoDateConverter() = 
    inherit JsonConverter()
    override _.CanConvert(``type``: Type) =
        ``type`` = typeof<DateOnly>
    override _.ReadJson(reader: JsonReader, ``type``: Type, existingValue: obj, serializer: JsonSerializer) =
        let dateString = reader.Value :?> string
        DateOnly.ParseExact(dateString, "yyyy-MM-dd")
    override _.WriteJson(writer: JsonWriter, value: obj, settings: JsonSerializer) =
        writer.WriteValue(((value :?> DateOnly).ToString("yyyy-MM-dd")))

But seems to be it's not working and nothhing changed. It seems it would be good to add DateOnly suport for Argon probably.

I need probably some hints where should I add things.

[SimonCropp]

Argon already supports DateOnly/TimeOnly on net6.0+, and it works inside tuples. Verified:

Tuple<string, DateOnly, string>        -> {"Item1":"a","Item2":"2000-12-29","Item3":"b"}
(string, DateOnly, string) value tuple -> {"Item1":"a","Item2":"2000-12-29","Item3":"b"}

The relevant pieces are DefaultContractResolver.CanConvertToString (routes DateOnly/TimeOnly to a string contract), JsonSerializerInternalWriter.TryConvertToString (writes yyyy-MM-dd / HH:mm:ss.FFFFFFF), and ConvertUtils.TryConvert (ParseExact on the way back). Covered by DateTests.cs and TimeTests.cs.

So there is nothing to add here. What you are hitting is two separate Verify behaviors:

1. Verify scrubs dates by default. DateConverter calls writer.Counter.TryConvert(value, ...), which replaces the value with Date_1 (or Date_MinValue/Date_MaxValue). That is why the strings in your tuple look fine and the DateOnly does not. Turn it off with:

VerifierSettings.DontScrubDateTimes()

With scrubbing off, Verify's own converter writes yyyy-MM-dd anyway, which is exactly what your IsoDateConverter was producing. So you can drop the custom converter entirely.

2. Your converter can never fire as registered. Argon resolves converters first-match-wins: JsonSerializer.GetMatchingConverter walks settings.Converters by index and returns the first whose CanConvert returns true. Verify adds its DateConverter in SerializationSettings.BuildSettings, and AddExtraSettings callbacks run after that, so anything you add via settings.Converters.Add sits behind Verify's. To override a built-in you have to prepend:

settings.Converters.Insert(0, IsoDateConverter())

Also worth noting: AddFSharpConverters is unrelated to tuples. It only registers the F# list, map and discriminated-union converters. System.Tuple serializes as Item1/Item2/Item3 without it.