How to Serialize Polymorphic Child Objects in Json.Net without the '$type' Field?

Json.Net polymorphic sub-object serialization without "$type" field

In the scenario of using Json.Net for object serialization, a common requirement is to represent polymorphic sub-objects in the parent class. While Json.Net's TypeNameHandling.Auto setting simplifies this task, it introduces a "$type" field to indicate the type of the child object. This may not be desirable for various reasons.

Challenge

The challenge is how to identify the type of the sub-object without relying on the "$type" field. One approach is to store the type information as an indicator in the parent class, as shown in the provided sample code. However, this comes with limitations related to accessibility and data management.

Solution

To work around these limitations, a more efficient solution is to add the subtype information directly as attributes to the base class. This allows serializing subtype enumerations when serializing objects assignable to a base class. During deserialization, a custom JsonConverter can be used to load the JSON data into a temporary object, determine the appropriate subclass based on the "Type" property, and perform deserialization accordingly.

Implementation details

Implementation involves creating a custom SubType enumeration and a dictionary mapping types to subtypes. The SubTypeClassBase base class contains a Type property that returns the subtype using a dictionary.

SubTypeClassConverter class acts as a JsonConverter responsible for deserialization. It checks the "Type" property in the JSON object, gets the corresponding subtype, and populates the corresponding subclass of the existing or new instance.

By taking this approach, you can use Json.Net to serialize and deserialize polymorphic sub-objects without introducing the "$type" field, ensuring flexibility and controllability of data representation.

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