Description
Background and motivation
When using System.Text.Json to deserialize to a polymorphic type decorated with the [JsonPolymorphic] attribute, there is the option set IgnoreUnrecognizedTypeDiscriminators = true and allow deserialization to fallback to instantiating the base type. This useful feature could potentially be improved by allowing additional JSON metadata properties such as the $type discriminator value to be captured on the base type in the fallback case.
The [JsonExtensionData] property can be leveraged to accumulate the unbound JSON properties into a dictionary - and in the fallback scenario described above this would include all properties not explicitly defined on the base type. However the metadata properties, the $type discriminator in particular, is not captured and is effectively lost in such scenarios.
A common scenario where I've wished for such a feature is deserializing batches of events from a stream or json object arrays where the events can contain many different subtypes and the producer(s) add new types semi-frequently. I often want to be able to capture and log a sample of new/unrecognized types for further investigation. Adding a clean out-of-the-box way to do this with the deserialized C# object would go a very long ways in simplifying such processes.
API Proposal
Provide a mechanism, such as a [JsonMetadata] or a [JsonUnrecognizedTypeMetadata] attribute, that can be used indicate to the deserializer when & where such metadata value(s) should be captured.
API Usage
[JsonPolymorphic(IgnoreUnrecognizedTypeDiscriminators = true)]
[JsonDerivedType(typeof(Doggo), "DoggoV1"]
[JsonDerivedType(typeof(Kitteh), "KittehV1"]
public class Animal
{
[JsonExtensionData]
[JsonMetadata]
public Dictionary<string, JsonElement>? ExtensionData { get; set; }
}Alternative Designs
No response
Risks
No response