WCF - probem with serializing inherited types

Question

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???

Answer 1

Try this:

[DataContract]
[KnownType(typeof(FileMissingError))]
public class ErrorBase {}

As the error message states, any information that cannot be know statically (like the polymorphic relationship you have expressed here) must be supplied via attributes. In this case you need to specify that your FileMissingError data contract is a known type of its base class, ErrorBase.

Answer 2

You should add KnownType attribute to your base class

[DataContract]
[KnownType(typeof(FileMissingError))]
public class ErrorBase {}

Read more about KnownType attribute in this blog