Always serialize discriminator as the first property

[ahusseini]

Is your feature request related to a problem? Please describe the problem.

An unhandled exception has occurred while executing the request.
      Microsoft.AspNetCore.Http.BadHttpRequestException: Failed to read parameter "List<ResolveThingRequest> requests" from the request body as JSON.
       ---> System.Text.Json.JsonException: The metadata property is either not supported by the type or is not the first property in the deserialized JSON object. Path: $[0].$type | LineNumber: 0 | BytePositionInLine: 44.

related to:

• JsonSerializer: Allow out-of-order reading of metadata properties. dotnet/runtime#72604
• Kiota should send properties in OAS declaration order, not alphabetic order #4680
• Populate the discriminator property for derrived types. #4618

ResolveThingRequest:
  type: object
  discriminator:
    propertyName: $type
    mapping:
      ResolveThingARequest: '#/components/schemas/ResolveThingARequest'
      ResolveThingBRequest: '#/components/schemas/ResolveThingBRequest'
  additionalProperties: false
  required:
  - $type
  properties:
    $type:
      type: string
    contextId:
      type: integer
      format: int32
    name:
      type: string
      nullable: true

Client library/SDK language

Csharp

Describe the solution you'd like

Kiota should not include the discriminator property when reordering alphabetically.

Additional context

Although this will be fixed in .net 9, this seems counter intuitive and breaking.

[baywet]

Hi @ahusseini
Thanks for using kiota and for reaching out.
This is something we're unlikely to do, or even accept.
As stated in #4680 the JSON spec explicitly calls out that objects with unordered properties. And you can see that in this issue, the requested order was different from this one here.
Digging into the discriminator issue from STJ, it seems this is something they've fixed for .NET 9. And if the higher compatibility/preview state is a blocker for you, there are multiple workarounds from the community on the thread.

[ahusseini]

Hey @baywet, that's understandable, although a bit strange to take this position when:

• Kiota is generating incompatible client code for standard .net servers, and anyone who is not able to upgrade to .net 9 is stuck with a workaround
• System.Text.Json and JSON.Net both provide property order attributes which are written out to the OpenAPI spec
• although you say the JSON spec says there should be no defined order, you are choosing a defined order by reordering things alphabetically

IMO this is unneeded friction, I just want to generate compliant code for minimal APIs. I've worked around this by creating an empty base class for all objects, so that $type is the first and only property. Anyways, I don't want to rehash the reordering thread so I'll close this.