datacontract - WCF: Returning a derived object for a contract with base object (DataContractResolver)

Question

I have have a WCF derived/base contract issue. I have a server interface/contract that returns a BaseThing object:

[OperationContract]
BaseThing Get_base_thing();

The server that implements this has a DerivedThing (derived from BaseThing) and wants to return this as a BaseThing. How to tell WCF that I only want to transport the BaseThing part of DerivedThing?

If in Get_base_thing I just return a reference to a DerivedThing then I get a SerializationException server side.

I think I need to define a DataContractResolver and I looked at the MSDN article Using a Data Contract Resolver but this is not 100% clear (to me a least).

How should my DataContractResolver look to tell WCF to only transport the base part of the derived object I pass it?

Is there some way to do this more simply just with KnownType attribue?

Answer 1

KnownType will not resolve this issue.

It sounds as if you have a serious divergence between the object model you're using at the server and the service contracts you're using. There seems to be 3 possible solutions:

1) Data Contract Resolver as you've identified to make it automatic across all your operations. There are a number of examples out there including this one: http://blogs.msdn.com/b/youssefm/archive/2009/06/05/introducing-a-new-datacontractserializer-feature-the-datacontractresolver.aspx.

2) Align your object model to better match your service contracts. That is, use containment rather than inheritance to manage the BaseThing-DerivedThing relationship. That way you work with DerivedThing at the server and simply return DerivedThing.BaseThing over the wire. If BaseThing needs to get transmitted from client to server, this will also work better.

3) Use explicit conversion using something like AutoMapper so it is obvious in your operations that there is a divergence between the objects being used at the server and those exposed to the outside world.