Created attachment 3671 [details]
This program will serialize A before B under Microsoft .NET and B before A under Mono.
Setting the Order property on MessageBodyMemberAttribute should change the order of members of a MessageContract during serialization, as is documented here:
However, under Mono it does not change member order. It can, however, be set such that it throws an IndexOutOfRangeException as noted in bug #11309.