Add json support for F# options, lists, sets, maps and records

[eiriktsarpalis]

Contributes to #29812 by adding support for the following types:

• F# options and struct options.
• F# lists, sets and maps.
• Fixes support for all F# record representations.

Note that the changes do not include support for discriminated unions, attempting to serialize a DU will now throw NotSupportedException. See #29812 (comment) for an explanation why we're doing this. We can revisit this approach in future releases of .NET

cc @dsyme @bartelink.

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Issue Details

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Contributes to #29812. Note that the changes do not include support for discriminated unions since it requires infrastructural changes touching on polymorphism support (cf. #30083).

cc @dsyme @bartelink.

Author:	eiriktsarpalis
Assignees:	eiriktsarpalis
Labels:	area-System.Text.Json
Milestone:	6.0.0

[jkotas]

How is this going to work with trimming? Does this need an explicit public API to opt-in?

[eiriktsarpalis]

How is this going to work with trimming?

The feature requires unreferenced code to function, so it can break when used with trimming. There is precedent for this, for example the converters for System.Collections.Immutable that intentionally don't take a static dependency to the types.

Does this need an explicit public API to opt-in?

Maybe, but I don't see why it shouldn't be the default. The current handling of F# types is broken one way or another.

[jkotas]

Maybe, but I don't see why it shouldn't be the default.

It is not pay-for-play. Every Json serializer construction is going to pay to the FSharp checks now.

[eiriktsarpalis]

Maybe, but I don't see why it shouldn't be the default.

It is not pay-for-play. Every Json serializer construction is going to pay to the FSharp checks now.

Of course, but that argument could be made for other rarely used supported types like IAsyncEnumerable and System.Collections.Immutable. We can measure the impact of adding F# checks to warmup time and make a call based on that, however my expectation is that users conscious of startup times will switch to source generators.

[eiriktsarpalis]

Misc user feedback collected from social media:

• Add support for ValueOption.
• Consider recognizing discriminated unions and throw NotSupportedException.

[En3Tho]

Aren't records supported by default in net5? I've even forked fsharp.stj repo to use record (de) serialization out of the box.

[eiriktsarpalis]

Aren't records supported by default in net5? I've even forked fsharp.stj repo to use record (de) serialization out of the box.

They generally happen to work, however struct record deserialization is currently broken. This change fixes that particular corner case and makes support official by adding tests for all record kinds.

[En3Tho]

Nice. Thank you for clarification.

[dsyme]

@eiriktsarpalis How about single case unions, list types, option types, are they covered?

[eiriktsarpalis]

Hi @dsyme, options and lists are supported special cases but other DUs have not been added yet.

[eiriktsarpalis]

FWIW I wrote a benchmark comparing cold start serialization for main and the PR branch: https://gist.github.com/eiriktsarpalis/ecb9259ee92940469d19989f81383dfd. Running statistical tests with 3% threshold seems to indicate no notable performance regressions.

[steveharter]

LGTM; however not familiar with F# so can't comment on the value add here without the support for "discriminated unions".

[eiriktsarpalis]

Heads up: I just pushed a commit that makes discriminated unions explicitly throw NotSupportedException. Rationale for not supporting the type can be found here. We can revisit our approach in future releases. cc @Zaid-Ajaj @dsyme

[eerhardt]

The trimming annotations LGTM.

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