Coming from a Java background, this is the way I'm thinking:

The server provides an object to the client. This object should be able to execute on the server.

Server:

private string _S = "A";

public interface IFoo { void Bar(); }

private class Foo : IFoo {
    void Bar() { _S = "B";}
}

public IFoo GetFoo() { return new Foo(); }

Client:

IFoo foo = serverChannel.GetFoo();
foo.Bar();

Is this possible? Or is my understanding wrong and this is not how it works in WCF? What would be a better design?