[Bug]: Missing type discriminator when using JsonDerivedType

[Rick-van-Dam]

Describe the bug 🐞

I ran into this error:

The JSON payload for polymorphic interface or abstract type 'CleanAspCore.Api.Endpoints.Weapons.CreateWeaponRequest' must specify a type discriminator. Path: $ | LineNumber: 0 | BytePositionInLine: 10

It seems refit is not taking into account JsonDerivedType. I assume because its not using the base type that contains those attributes.

Step to reproduce

1. Use json serialization with derived classes: https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/polymorphism
2. Send a request
3. Notice the type discriminator property is missing which causes problems when deserializing

Reproduction repository

https://github.com/reactiveui/refit

Expected behavior

The type disriminator is included in the json payload

Screenshots 🖼️

No response

IDE

Rider

Operating system

Windows

Version

11

Device

PC

Refit Version

8.0.0

Additional information ℹ️

No response

[Rick-van-Dam]

I stepped through the refit code and I indeed see object being used as the generic type parameter because the type of the parameter is not passed into Refit.IHttpContentSerializer.ToHttpContent[T]. This is turn causes polymorphic serialization to fail.

What needs to happen is that the type of the parameter is passed into the serializer. So if I have this:

    [Post("/weapons")]
    Task<HttpResponseMessage> CreateWeapon(ICreateWeaponRequest createJobRequest);

Then the serializer would need to use ICreateWeaponRequest so that it know it needs to use polymorphism. This is due to the information thats on this interface and is also mentioned in the microsoft docs:

[JsonDerivedType(typeof(CreateBowRequest), typeDiscriminator: "bow")]
[JsonDerivedType(typeof(CreateSwordRequest), typeDiscriminator: "sword")]
public interface ICreateWeaponRequest;

This could be as simple as adding this to Refit\RequestBuilderImplementation.cs:916:

var parameterType = restMethod.MethodInfo.GetParameters()[restMethod.BodyParameterInfo.Item3].ParameterType
var content = serializer.ToHttpContent(param, parameterType);

And modify Refit.SystemTextJsonContentSerializer.ToHttpContent<T> like this:

        public HttpContent ToHttpContent<T>(T item, Type serializationType)
        {
            var content = JsonContent.Create(item, serializationType, options: jsonSerializerOptions);

            return content;
        }

Some more changes would be needed to make everything compile but iam not familiar enough with the refit source code to judge if this is robust enough. Also since its a public type this will be a breaking change.