WCF exposing generic type ‘T’

I suppose this is possible, though I'm not sure you'd want this. I'd take the following approach (untested, not sure if it works). First create the following project structure in your solution:

• ServiceInterfaces
• ServiceImplementations (references ServiceInterfaces and ModelClasses)
• ModelClasses
• Host (references ServiceInterfaces and ServiceImplementations)
• Client (references ServiceInterfaces and ModelClasses)

In ServiceInterfaces you have an interface like this (I skipped the namespaces, etc to make the example shorter):

[ServiceContract]
public interface IMyService<T>
{
    T GetObject(int id);
}

In ServiceImplementations you have a class that implements IMyService<T>:

public class MyService<T> : IMyService<T>
{
    T GetObject(int id)
    {
        // Create something of type T and return it. Rather difficult
        // since you only know the type at runtime.
    }
}

In Host you have the correct configuration for your service in an App.config (or Web.config) file and the following code to host your service (given that it is a stand-alone app):

ServiceHost host = new ServiceHost(typeof(MessageManager.MessageManagerService))
host.Open();

And finally in Client you use a ChannelFactory<TChannel> class to define a proxy:

Binding binding = new BasicHttpBinding(); // For the example, could be another binding.
EndpointAddress address = new EndpointAddress("http://localhost:8000/......");
IMyService<string> myService =
    ChannelFactory<IMyService<string>>.CreateChannel(binding, address);
string myObject = myService.GetObject(42);

Again, I'm not sure if this works. The trick is to share your service interfaces (in ServiceInterfaces) and domain model objects (in ModelClasses) between the host and the client. In my example I use a string to return from the service method but it could be any data contract type from the ModelClasses project.

You simply can't. It's not possible to do that, as soap does not support this. See this article, which mentions how to get around generics, by creating an intermediate local object that is called and casts the object before calling the WCF operation.