[API Proposal]: STJ SystemTextJsonOutputFormatter output type selector

[angelaki]

Background and motivation

Right now the default SystemTextJsonOutputFormatter internally decides what type to serialize (in WriteResponseBodyAsync). In my case I'd like to keep the controlers response type (an Interface) that now gets broken down to it's actual class.

Right now the type to serialize gets chosen here:

aspnetcore/src/Mvc/Mvc.Core/src/Formatters/SystemTextJsonOutputFormatter.cs

Line 74 in 333629e

JsonTypeInfo? jsonTypeInfo = null;

and checked here:

aspnetcore/src/Mvc/Mvc.Core/src/Formatters/SystemTextJsonOutputFormatter.cs

Line 91 in 333629e

if (jsonTypeInfo is not null)

It'd be great to have an overrideable method where the jsonTypeInfo is selected.

API Proposal

    protected JsonTypeInfo? GetTypeInfo(OutputFormatterWriteContext context)
    {
        // context.ObjectType reflects the declared model type when specified.
        // For polymorphic scenarios where the user declares a return type, but returns a derived type,
        // we want to serialize all the properties on the derived type. This keeps parity with
        // the behavior you get when the user does not declare the return type.
        // To enable this our best option is to check if the JsonTypeInfo for the declared type is valid,
        // if it is use it. If it isn't, serialize the value as 'object' and let JsonSerializer serialize it as necessary.
        JsonTypeInfo? jsonTypeInfo = null;
        if (context.ObjectType is not null)
        {
            var declaredTypeJsonInfo = SerializerOptions.GetTypeInfo(context.ObjectType);

            var runtimeType = context.Object?.GetType();
            if (declaredTypeJsonInfo.ShouldUseWith(runtimeType))
            {
                jsonTypeInfo = declaredTypeJsonInfo;
            }
        }
        return jsonTypeInfo;
    }

API Usage

    protected override JsonTypeInfo? GetTypeInfo(OutputFormatterWriteContext context)
    {
        if (context.ObjectType?.IsInterface == true) { return SerializerOptions.GetTypeInfo(context.ObjectType); }
        return base.GetTypeInfo(context);
    }

Alternative Designs

No response

Risks

No response

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-text-json, @gregsdennis
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[eiriktsarpalis]

cc @captainsafia