Question

I want to have a function on my WCF service with its return type as an interface, but when I call it from a client I receive a System.Object, not the class that implements the interface which the service sent.

Here is some sample code:

[ServiceContract]
public interface IService
{
    [OperationContract]
    string SayHello();

    [OperationContract]
    IMyObject GetMyObject();
}

public interface IMyObject
{
    int Add(int i, int j);
}

[DataContract]
public class MyObject : IMyObject
{
    public int Add(int i, int j)
    {
        return i + j;
    }
}

In the implementation of this service I have:

public class LinqService : IService
{
    public string SayHello()
    {
        return "Hello";
    }

    public IMyObject GetMyObject()
    {
        return new MyObject();
    }
}

SayHello() works well, but GetMyObject() returns a System.Object. How can change this code so that GetMyObject() returns an object which implements IMyObject?

Edit 1

Changed the code as follow:

using System.Runtime.Serialization;
using System.ServiceModel;

[ServiceContract]
public interface IService
{
    [OperationContract]
    string SayHello();

    [OperationContract]
    IMyObject GetMyObject();
}

[ServiceKnownType(typeof(MyObject))]
public interface IMyObject
{
    [OperationContract] 
    int Add(int i, int j);
}

[DataContract]
public class MyObject:IMyObject
{
    public int Add(int i, int j)
    {
        return i + j;
    }
}

But no success!

Answer 1

All WCF contract argument and return types have to be serializable; interfaces aren’t. This question explores the same issue with an answer revolving around the KnownType attribute; if you’re going to be passing back various implementations of IMyObject I’d recommend this, otherwise you’ll have to change the return type to MyObject.