Typed results (such as Created<T>) do not emit type discriminator from attribute-based polymorphic serialization introduced in .NET 7

[dkokotov]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

If I have a type hierarchy with two classes A and B, where B is derived from A and A contains JsonPolymorphic and JsonDerivedType annotations, and I then have a request delegate that returned Created<A>, the serialized response does not contain the type discriminator property.

Expected Behavior

I would expect the response to include the type discriminator property as described in the docs for polymorphic serialization.

However, it does not.

Steps To Reproduce

[JsonPolymorphic(TypeDiscriminatorPropertyName = "type")]
[JsonDerivedType(typeof(B), typeDiscriminator: "b")]
[JsonDerivedType(typeof(A), typeDiscriminator: "a")]
public record A {
    public string SomeProp { get; init; }
}

public record B {
    public string SomeOtherProp { get; init; }
}

// in Program.cs
builder.Map("/try", () => return TypedResults.Created<A>("/newone", new B { SomeProp = "foo", SomeOtherProp = "bar" }));
// this will serialize to { "SomeProp": "foo", "SomeOtherProp": "bar" } 
// but I would expect it to be { "type": "b", "SomeProp": "foo", "SomeOtherProp": "bar" } 

Exceptions (if any)

No response

.NET Version

7.0.100

Anything else?

I believe this is caused by the considerations discussed in #43894 and #41724 to support backwards-compatible polymorphic serialization. Presumably due to those changes, ultimately the serialization code from Created<T> calls JsonSerializer.Serialize explicitly with the runtime type of the value passed to Created<T>. but the documentation for the type discriminator feature explicitly requires the base type to be used.

[dkokotov]

I think this issue got closed accidentally because I mentioned it in a PR where I implemented a workaround (in a private repo). It looks like when that PR got merged Github for some reason decided to close this issue as well.

[captainsafia]

Triage: @brunolins16 has been doing some investigation in this area after reviewing issue #44852. Closing this one as a dupe.