What would cause a WCF service to return an object of type “object”

Question

Per my other post about WCF service return values, I'm consuming a web service from another company, and when I add the service reference inside Visual Studio, the return value of the method is an object of type object.

The author of the web service showed me the code, and it actually returns a typed object.

Am I missing something, or is the proxy class supposed to return a typed value?

Is there a setting for generating the proxy class, or the actual service?

UPDATE:

I looked at the actual classes behind the WCF service and realized that the return value of the service method is actually returning an interface, that the concrete type implements. The concrete type is marked with the [DataContract] attribute ( and appropriate [DataMember] attributes), but the interface has no such attributes. Could this be causing the service to set the return type as object?

Answer 1

Hypothetically, if you were the service developer, you could use a KnownTypeAttribute:

[DataContract]
[KnownType(typeof(MyConcreteClass))]
public interface IMyInterface {
}

[DataContract]
public class MyConcreteClass : IMyInterface {
}

I haven't personally tried this with an interface, but I have tried it with an abstract base class and it works fine. When the client receives the return value, it can successfully downcast to the derived class.

It might be that the service code actually does this, and the problem lies with svcutil.exe not generating the proxy classes accurately enough.

Although you don't control the service code, you do control the client proxy code. You could try manually editing the proxy classes that svcutil.exe gave you to add the KnownTypeAttribute yourself. By doing this, you are taking control of the behaviour of the DataContractSerializer at your end, and as long as you take care not to change the wire format of the data by mistake, it should all still work.

Answer 2

The proxy class is a generated file and as such it can contain mistakes. If you have a copy of the data contract you are free to change the proxy class to use the correct type rather than System.Object and things ought to work properly.

The Visual Studio "Add Service Reference" tool and svcutil.exe are very good at generating proxy classes but they are not perfect. The files that they generate are yours to modify and I would encourage you to simply modify the operation to return the proper data contract.

Answer 3

We had a similar problem when consuming a java web service from WCF.

In our case the type that it said that it was returning was a limited version of what was actually being returned.

What worked for us was to cast the object to the type that was expected. After that the data was available.

Therefore, to fix your problem you could try to cast the object to the type expected.