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System.Text.Json Polymorphism Type Discriminator Does Not Support Subtypes Class Polymorphism #101286
Description
Description
System.Text.Json Polymorphism Type Discriminator Does Not Support Subtypes Class Polymorphism
System.Text.Json Version
9.0.0-preview.3.24172.9
Test
var jsonText = """
{"time":1713524900,"self_id":123456789,"post_type":"meta_event","meta_event_type":"lifecycle","sub_type":"connect"}
""";
var eventBase = JsonSerializer.Deserialize<EventBase>(jsonText, options: new()
{
AllowOutOfOrderMetadataProperties = true
});
Console.WriteLine(eventBase);[JsonPolymorphic(TypeDiscriminatorPropertyName = "post_type")]
[JsonDerivedType(typeof(MetaEvent), "meta_event")]
// ......
public record EventBase
{
[JsonPropertyName("time")]
public long Time { get; init; }
[JsonPropertyName("self_id")]
public long SelfId { get; init; }
[JsonPropertyName("post_type")]
public virtual string PostType { get; init; } = "unknown";
}
[JsonPolymorphic(TypeDiscriminatorPropertyName = "meta_event_type")]
[JsonDerivedType(typeof(MetaLifecycleEvent), "lifecycle")]
// ......
public record MetaEvent : EventBase
{
[JsonPropertyName("post_type")]
public override string PostType { get; init; } = "meta_event";
[JsonPropertyName("meta_event_type")]
public virtual string MetaEventType { get; init; } = "unknown";
}
public record MetaLifecycleEvent : MetaEvent
{
[JsonPropertyName("meta_event_type")]
public override string MetaEventType { get; init; } = "lifecycle";
[JsonPropertyName("sub_type")]
public required string SubType { get; init; }
}
Activity
eiriktsarpalis commentedon Apr 19, 2024
I tried running the reproduction and it seems to be working as expected, i.e. the discriminator
"post_type" : "meta_event"is being mapped to an instance of typeMetaEvent, when deserializing asEventBase. I'm not entirely sure what your expected behavior is in this case, but I have two remarks based on what I'm seeing:JsonDerivedType/JsonPolymorphicannotations onMetaEventare not inherited byEventBase. These set up independent sets of configurations depending on whether you are callingDeserialize<EventBase>orDeserialize<MetaEvent>.Hope this helps.
bunnyi116 commentedon Apr 19, 2024
How should I deserialize into the MetaLifecycleEvent class?
EventBase is the base class for all events, MetaEvent is a broad category of events, with MetaLifecycleEvent belonging to the MetaEvent class, which in turn belongs to EventBase.
bunnyi116 commentedon Apr 19, 2024
I think that when serializing/deserializing, if a subclass specified, it should check whether the subclass has the
JsonPolymorphicorJsonDerivedTypeattributes. If these attributes exist, the process should continue.eiriktsarpalis commentedon Apr 19, 2024
You would need to add a
JsonDerivedTypeannotation forMetaLifecycleEventinEventBase. If you're feeling fancy (and you're not working with Native AOT or trimmed applications) you can write a contract customizer that looks up the type hierarchy using reflection and automatically appends JsonDerivedAttribute annotations to the base.That might work when using reflection-based serialization, but it might not be possible using the source generator which needs to know the entire hierarchy at compile time. Therefore, it is not supported (though there are ways to achieve this via contract customization).
bunnyi116 commentedon Apr 19, 2024
I don't need AOT.
Could you help me write a simple example using custom contracts for my sample data? I'm not very familiar with it.
Also, I hope STJ can find a way to support AOT in the future.