I've given a fair read through msdn:datacontracts and I cannot find a out why the following does not work. So what is wrong here? Why isn't ExtendedCanadianAddress recognized by the datacontract serializer?

Type 'XYZ.ExtendedCanadianAddress' with data contract name 'CanadianAddress:http://tempuri.org/Common/Types' is not expected. Add any types not known statically to the list of known types - for example, by using the KnownTypeAttribute attribute or by adding them to the list of known types passed to DataContractSerializer.

Given:

namespace ABC  
{  
 [KnownType(typeof(Address))] 
 public abstract class Z  
 {
   //stuff
   //method that adds all types() in namespace to self
 }

 [KnownType(typeof(CanadianAddress))]  
 [DataContact(Name = "Address", Namespace = "http://tempuri.org/Types")]  
 public class Address : Z
 {}

 [DataContract(Name = "CanadianAddress", Namespace = "http://tempuri.org/Types")]
 public class CanadianAddress : Address
 {}
}

namespace XYZ
{
 [KnownType(typeof(ExtendedCanadianAddress))
 [DataContact(Name = "Address", Namespace = "http://tempuri.org/Types")] 
 public class ExtendedAddress : Address
 {
   //this serializes just fine
 }
 [DataContact(Name = "CanadianAddress", Namespace = "http://tempuri.org/Types")] 
 public class ExtendedCanadianAddress : CanadianAddress
 {
   //will NOT serialize
 }
}