Improve feature parity between JsonSerializerOptions and JsonSourceGenerationOptionsAttribute

[ithline]

EDIT See #57321 (comment) for an API proposal.

Original Proposal

Currently source generator for JSON serialization in `System.Text.Json` creates a static property named `Default` that receives default `JsonSerializerOptions` defined by the source generator. These can be modified by using `JsonSourceGenerationOptionsAttribute`, but several options are missing and cannot be set, e.g. `PropertyNameCaseInsensitive`, `NumberHandling`, `DictionaryKeyPolicy`...

It would be really helpful if these missing options could be added to the JsonSourceGenerationOptionsAttribute or at least disable generating of the Default property, so we could provide our own configuration for the default instance.

Tagging subscribers to this area: @eiriktsarpalis, @layomia
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Issue Details

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Currently source generator for JSON serialization in System.Text.Json creates a static property named Default that receives default JsonSerializerOptions defined by the source generator. These can be modified by using JsonSourceGenerationOptionsAttribute, but several options are missing and cannot be set, e.g. PropertyNameCaseInsensitive, NumberHandling, DictionaryKeyPolicy...

It would be really helpful if these missing options could be added to the JsonSourceGenerationOptionsAttribute or at least disable generating of the Default property, so we could provide our own configuration for the default instance.

Author:	Ithline
Assignees:	-
Labels:	area-System.Text.Json, untriaged
Milestone:	-

[layomia]

Have you tried using the generated ctor on your context type that takes a JsonSerializerOptions instance?

e.g.

[JsonSerializable(typeof(Foo))]
internal partial sealed class MyContext : JsonSerializerContext { }

JsonSerializerOptions options = new() { ... };
MyContext context = new(options);

JsonSerializer.Serialize(foo, context.Foo);

[ithline]

Yes, that can be used as a workaround, but the generator still forces public API of my custom JsonSerializerContext type. I work with a third party API that needs some specific options set to work and having a Default property with basically invalid settings is not a good design. Everyone on the team would need to be aware not to use this property but instead use some other instance.

Ideally even the constructors would not be automatically generated and left to the developer.
I use need metadata generation to get rid of trimming warnings and don't need the sped up pre-generated serializer.

[eiriktsarpalis]

@layomia given that there's a workaround, does this meet the bar for 6.0.0?

[eiriktsarpalis]

Moving to 7.0.0, we need to think about API design for this particular use case and we have a 6.0 workaround via the generated constructors.