WCF - serializing inherited types

I have these classes: ``` [DataContract] public class ErrorBase {} [DataContract] public class FileMissingError: ErrorBase {} [DataContract] public class ResponseFileInquiry { [DataMember] public List<ErrorBase> errors {get;set;}; } ``` An instance of the class ResponseFileInquiry is what my service method returns to the client. Now, if I fill ResponseFileInquiry.errors with instances of ErrorBase, everything works fine, but if I add an instance of inherited type FileMissingError, I get a service side exception during serialization: ``` Type 'MyNamespace.FileMissingError' with data contract name 'FileMissingError' 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.' ``` So serializer is getting confused because it's expecting the List to contain the declared type objects (ErrorBase) but it's getting inherited type (FileMissingError) objects. I have the whole bunch of error types and the List will contain combinations of them, so what can I do to make it work?