When I try to update some records for Opportunity, I would meet a record that have a null ID sometimes. So I have to keep it to be 0 when sending the MObject instance for this record. By Fiddler, I could find that the returned status information would contains a ID node without value in its xml string. But my code would causes an Exception showing me that XML serializing error because of incorrect-type string.
I find out that MObjStatus must have a specified id in the WSDL file. So I wish that the developers could specify 0 as the id for this issue.