WithUnwrapOption(false) still serializes None as null

[cmeeren]

When using WithUnwrapOption(false), None still serializes to null.

(I am serializing for logging/assertion/debugging purposes and want None/ValueNone to serialize to the strings "None"/"ValueNone", as I expect is the intended functionality with WithUnwrapOption(false).)

open System.Text.Json
open System.Text.Json.Serialization
open Xunit

let options = JsonSerializerOptions()
options.Converters.Add(
    JsonFSharpOptions
        .Default()
        .WithUnwrapOption(false)
    |> JsonFSharpConverter
)
Assert.Equal("\"None\"", JsonSerializer.Serialize(None))

This fails with:

Xunit.Sdk.EqualException
Assert.Equal() Failure
          ↓ (pos 0)
Expected: "None"
Actual:   null
          ↑ (pos 0)

[cmeeren]

Could this be related to Option<_> using [<CompilationRepresentation(CompilationRepresentationFlags.UseNullAsTrueValue)>]?

[Tarmil]

Yes, it is on purpose that None and ValueNone are serialized as null. I am open to adding an option to customize this though!

[cmeeren]

Would be great! I am not serializing for idiomatic JSON, but for displaying structures of F# data (in a better way than %A), so displaying None instead of null would help my users.

[cmeeren]

Note by the way that the docs for WithUnwrapOption are misleading:

FSharp.SystemTextJson/src/FSharp.SystemTextJson/Options.fs

Lines 368 to 372 in 4869244

	/// If set, `None` is represented as null,
	/// and `Some x` is represented the same as `x`.
	/// This flag is included in Default.
	member this.WithUnwrapOption([<Optional; DefaultParameterValue true>] set: bool) =
	this.WithUnionEncodingFlag(JsonUnionEncoding.UnwrapOption, set)

It says that if set, None is serialized as null. While this technically doesn't say that if it's not set, then None is not serialized as null, it does give a strong impression that that's the intended meaning.

Also relevant here:

FSharp.SystemTextJson/src/FSharp.SystemTextJson/Options.fs

Lines 56 to 59 in 4869244

	/// If set, `None` is represented as null,
	/// and `Some x` is represented the same as `x`.
	/// This flag is included in Default.
	| UnwrapOption = 0x00_00_04_00

[cmeeren]

Hi! Just wondering if it would be possible to implement this soon-ish (say, during the coming week or two)? I need to decide whether to work around this, but the point is moot if this could be supported upstream. 🙂

Yes, it is on purpose that None and ValueNone are serialized as null.

By the way, ValueNone is serialized as ValueNone, not null (regardless of WithUnwrapOption).