// Define a service contract and apply the ServiceKnownTypeAttribute
// to specify types to include when generating client code. 
// The types must have the DataContractAttribute and DataMemberAttribute
// applied to be serialized and deserialized. The attribute specifies the 
// name of a method (GetKnownTypes) in a class (Helper) defined below.
[ServiceKnownType("GetKnownTypes", typeof(Helper))]
[ServiceContract()]
public interface ICatalog
{
    // Any object type can be inserted into a Hashtable. The 
    // ServiceKnownTypeAttribute allows you to include those types
    // with the client code.
    [OperationContract]
    Hashtable GetItems();
}

// This class has the method named GetKnownTypes that returns a generic IEnumerable.
static class Helper
{
    public static IEnumerable<Type> GetKnownTypes(ICustomAttributeProvider provider)
    {
        System.Collections.Generic.List<System.Type> knownTypes =
            new System.Collections.Generic.List<System.Type>();
        // Add any types to include here.
        knownTypes.Add(typeof(Widget));
        knownTypes.Add(typeof(Machine));
        return knownTypes;
    }
}

[DataContract()]
public class Widget
{
    [DataMember]
    public string Id;
    [DataMember]
    public string Catalog;
}

[DataContract()]
public class Machine : Widget
{
    [DataMember]
    public string Maker;
}

// Apply the ServiceKnownTypeAttribute to the 
// interface specifying the type to include. Apply 
// the attribute more than once if needed.
[ServiceKnownType(typeof(Widget))]
[ServiceKnownType(typeof(Machine))]
[ServiceContract()]
public interface ICatalog2
{
    // Any object type can be inserted into a Hashtable. The 
    // ServiceKnownTypeAttribute allows you to include those types
    // with the client code.
    [OperationContract]
    Hashtable GetItems();
}