How Can I Handle Known and Unknown JSON Fields During Deserialization in .NET?

Handling JSON known and unknown fields in .NET deserialization

When processing JSON data, you often encounter situations where there are both known fields and unknown fields. Known fields can be mapped to specific properties in the class, while unknown fields require special handling to avoid data loss.

Using a custom contract parser for JSON .NET

One way to manage unknown fields is to leverage a custom contract parser in JSON .NET. However, achieving this can be challenging.

Limitations of DataContract serializer

DataContract serializer does not allow overriding deserialization, so is not suitable for this case.

Dynamic Objects and Serialization

Serializing and deserializing to dynamic objects can provide a solution, but it is a tedious process and involves post-processing.

DynamicObject inheritance

Inheriting from the DynamicObject class also doesn't solve the problem because the serializer relies on reflection and does not call custom methods for dynamic objects.

A simpler solution: JsonExtensionDataAttribute

Instead of using complex techniques, consider JsonExtensionDataAttribute in JSON .NET (version 5.0 and above). This attribute allows unknown fields to be stored anonymously in attributes of type IDictionary<string, JToken>.

public class Product
{
    public string id { get; set; }
    public string name { get; set; }
    [JsonExtensionData]
    public Dictionary<string, JToken> UnknownFields { get; set; }
}

Using this approach, the JSON data will be successfully deserialized and known and unknown fields can be accessed through the class instance.

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