System.Text.Json serialisation should include all public fields in a struct by default

[jessicah]

Description

Serializing a struct with the System.Text.Json serializer should serialize all public fields by default, and only exclude fields by default for classes.

Other information

I haven't tested out record types in C# 9, but would expect the behaviour there to be the same as serializing a struct.

[Symbai]

And can you tell us WHY it should do that?

[GrabYourPitchforks]

Possible dupe of #876.

[layomia]

Closing as a dupe of #876. When designing field support in JsonSerializer, we decided to not include fields when serializing and deserializing by default to avoid a breaking change from .NET 3.1 where fields were ignored. See #2192 (comment) for more info. You can opt into field support by setting JsonSerializerOptions.IncludeFields to true or by annotating each field to include with JsonIncludeAttribute.

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What's the rationale for including fields by default for structs, but not classes? I think this behavior would be very confusing.