Json Deserialization Override Property

Question

Calling out the tech: C#, Newtonsoft.Json.

We have a class which contains a property named SchemaVersion that we use to inform of the Json document version once that object is ultimately serialized in to our CosmosDb.

Currently this property is set in the object class constructor, and for simplicity assume it's "1.0"

So far we've only added to our class & incremented "1.0" to "2.0", and so when we deserialize document not of latest SchemaVersion always want to 'upgrade' it and this is done by simply checking the value of that property..

private string _schemaVersion;
public string SchemaVersion
{
    get => _schemaVersion != "2.0" ? "2.0" : _schemaVersion;
    set => _schemaVersion = value;
}

New objects still have their SchemaVersion set with the object constructor. This feels a bit clunky to me.. but it does the job.

I know though, that there will come a point where we need to refactor the class for some as yet unknown reason. We'll try to avoid this as much as possible.. but i can see it happening.

The challenge then becomes how do we maintain conversions between

• "1.0" --> "x.0"
• "2.0" --> "x.0"
• "3.0" --> "x.0"
• "4.0" --> "x.0"

Are there any recommended ways in which this could best be handled? The best idea that i have right now is a custom JsonConverter, which i have written before to deserialize unknown Json documents to a specific object based on a property within that document... but i wonder if that is over-complicated as we will always be deserializing into a latest known shape...

I fear that this could get complex quickly if we end up with many versions, and their conversions to the latest version all need to be maintained separately..

Answer 1

Given sufficient complexity and time you will eventually need to implement converter classes that convert old versions of the object to new versions.

These converters will have more logic that you can sensibly squeeze into derserialiation classes, so I would avoid any idea of your class being more than a single version.

ie. the schema version, or simply version, should be a constant. I would probaby use the assembly version of the containing model dll.

When you load old versions of the serialised object, check the version before deserialisation and load the appropraite converter for that version.

For simple point releases with no breaking changes that converter class might be a simple default deserialiser to the current version of the class.

But when you get a breaking change you can add the custom logic to switch to your conversion logic for the particular version you are attempting to load (or throw an exception for an unknown version)