Dotnet 8 JsonSerializerOptions Breaking change

[MoishyS]

I have a Attribute to change JSON serialization from camelCase to PascalCase, which worked fine in dotnet 7, but in dotnet 8 i am getting an error System.InvalidOperationException: JsonSerializerOptions instance must specify a TypeInfoResolver setting before being marked as read-only

public class PascalCaseJson : ActionFilterAttribute
{
    public override void OnActionExecuted(ActionExecutedContext context)
    {
        if (context.Result is ObjectResult objectResult)
            objectResult.Formatters.Add(new SystemTextJsonOutputFormatter(new JsonSerializerOptions()));
    }
}

Adding TypeInfoResolver = new DefaultJsonTypeInfoResolver() fixes the issue, but i couldn't find any documentation on this breaking change.

[pinkfloydx33]

Note that you shouldn't create a new instance of the options each time either.

[eiriktsarpalis]

@MoishyS presumably this is using trimming or Native AOT? You appear to be impacted by this intentional breaking change: https://learn.microsoft.com/en-us/dotnet/core/compatibility/serialization/8.0/publishtrimmed.

If you are using trimming or Native AOT, then you should consider using the source generator instead of the reflection-based DefaultJsonTypeInfoResolver type which can lead to unexpected runtime errors related to trimming and Native AOT.

[MoishyS]

@eiriktsarpalis i am having this issue for a regular aspnet app, without trimming or Native AOT

[eiriktsarpalis]

@MoishyS can you share a minimal reproduction that demonstrates the issue?

[MoishyS]

@eiriktsarpalis see https://github.com/MoishyS/SerializerAttribute/blob/main/SerializerAttribute/PascalCaseJson.cs

[eiriktsarpalis]

That repo is not public, it would help if you could extract a self contained console app or a ZIP file that everyone reading the issue can access.

[MoishyS]

@eiriktsarpalis i made it public

[eiriktsarpalis]

Thanks. This looks like a moderately sized project that I won't be able to take a look at right now. It would help immensely if you could reduce the bug to the smallest possible console app (one that fits in a github comment). Thanks.

[MoishyS]

@eiriktsarpalis i used dotnet new webapi -o SerializerAttribute -controllers to create a new app. the only change i made was to add the [PascalCaseJson] to the controller and create the PascalCaseJson Attribute

public class PascalCaseJson : ActionFilterAttribute
{
    public override void OnActionExecuted(ActionExecutedContext context)
    {
        if (context.Result is ObjectResult objectResult)
            objectResult.Formatters.Add(new SystemTextJsonOutputFormatter(new JsonSerializerOptions()));
    }
}

[eiriktsarpalis]

Moving to aspnetcore for further triage. The error should be expected with Native AOT or PublishTrimmed=true however the OP is claiming that this is not the case here.