Json deserialization of polymorphic types does not expose the discriminator value

Start by reproducing the shown System.Text.Json polymorphic deserialization case with the Preference model, FallBackToNearestAncestor, IgnoreUnrecognizedTypeDiscriminators, and JsonExtensionData. Compare the resulting extension data with the input discriminator, then determine how the fallback behavior should expose that value and verify it with cases such as the distinct legume and beans payloads.

When using polymorphic types with System.Text.Json and configuring FallBackToNearestAncestor and IgnoreUnrecognizedTypeParameters to true you cannot tell the Json type discriminator of unrecognized objects.

To give a real-world example, I talk to a preferences API which returns multiple types of preferences, so I have a base class/record Preference

[JsonPolymorphic(UnknownDerivedTypeHandling = 
  JsonUnknownDerivedTypeHandling.FallBackToNearestAncestor,
  IgnoreUnrecognizedTypeDiscriminators = true)]

[JsonDerivedType(typeof(......)]
public record Preference
{
    [JsonConstructor]
    public Preference() {}

    [JsonExtensionData]
    public Dictionary<string, JsonElement>? ExtensionData {get; set;}
}

The idea being that if the api needs a new type of preference I don't have a derived class for then deserialization will still succeed, falling back to create an instance of Preference, with the unknown properties exposed in the ExtensionData.

Then in parsing a collection of preferences I can use pattern matching to act on the derived types.

However, when deserialization falls back to using the preference class I get everything in the ExtensionData except for the type discriminator value. This makes it impossible to make a reasoned decision about what to do with the data.

If the preferences API added two new types which have the same shape, say

{
    "$type": "legume",
    "description": "a type of legume"
},
{
    "$type": "beans",
    "description" : "the best type of legume"
}

then after deserialization I would get is the description in the extension data with no way of telling the types apart. So, you need to expose the type property somewhere.

My suggestion would be if deserialization has fallen back because the type is unknown and there is an extension data property then also insert the type value into that, or, if the fallback class/record has a property of string where the JsonPropertyName value matches the TypeDiscriminatorPropertyName property on the JsonPolymorphic attribute, feed it into that as well.

If you go the property router then during serialization any JsonInclude property which has a JsonPropertyName which matches the TypeDiscriminatorPropertyName property on the JsonPolymorphicattribute could be ignored by default.