Newtonsoft JSON magic

In one of our applications, we had the following code:

	public abstract class Event
	{
	public Event(Guid operationId)
	{
	OperationId=operationId;
	}
	public Guid OperationId { get; }
	}

view raw Event.cs hosted with ❤ by GitHub

	public class ProductCreatedEvent: Event
	{
	public ProductCreatedEvent(Guid operationdId):base(operationdId)
	{
	}
	}

view raw ProductCreatedEvent.cs hosted with ❤ by GitHub

We were using events to share information between different modules in our Modulith. These same events were also stored in an Event Log as a source for possible projections into a read model.

However in case of the code above, the ‘operationId’ for was always {00000000-0000-0000-0000-000000000000} when deserializing the ProductCreated event.

Do you notice what is wrong?

Important to notice here is that our ‘OperationId’ is a readonly property that is set through the constructor. You may already wonder why the JSON serializer is smart enough to fill in this property if only a getter exists. This is because it has a neat trick where it looks at the constructor of the object and if it can find a corresponding parameter, it will call the constructor and pass on the parameter value.

Now that you know this, take a second look at the code above. The reason it fails is because there is a typo in the constructor of the ProductCreatedEvent class. It spells ‘operationdId’ instead of ‘operationdId’. This no longer matches with the serialized data and causes the ProductCreatedEvent to be constructed with a default guid value.

The fix is easy, just remove the typo:

	public class ProductCreatedEvent: Event
	{
	public ProductCreatedEvent(Guid operationId):base(operationId)
	{
	}
	}

view raw ProductCreatedEventFixed.cs hosted with ❤ by GitHub