Monday, December 29, 2008

XML element order in WCF

Call me old-fashioned, but I’ve always held to the belief that in XML nesting is very important and order should never be important. I’ve always found it unbelievably stupid when some application depended on the order of elements in an XML document. To me XML is all about representing structured data, not about a rigorous document layout.

Imagine my surprise when I started testing my new WCF webservice with a request I wrote by hand. All of a sudden a member was left null no matter what I did. Because it was the last element in my hand-written request that remained null, my first though (after extensively verifying I didn’t make any typo’s) was that, maybe, for some obscure reason, the last member didn’t get deserialized correctly. So I put another member last and indeed it was also null. Then I put another member last and al of a sudden all where filled, except for the first one.

After a few hours of WTF?!! I found out that the ‘default’ serializer for WCF, the Microsoft favourite, the “Data Object Serializer” has a dependency on XML element order. After changing the serializer to the “XML serializer” (just remove the “DataContract” attribute from the classes and the “DataMember” attributes from the members of those classes and replace then with the “Serializable” atttribute and/or any of the “Xml*” attributes from the “System.Xml.Serialization” namespace and tack the “XmlSerializerFormat” attribute on the class that contains your WCF methods) it all just worked! Order wasn’t important anymore, as it should be.

What I don’t get is why Microsoft would promote the “Data Object Serializer” as the preferred methods for WCF. The “XML Serializer” is much more flexible if you want to control what the resulting XML document should look like and XML element order doesn’t matter! The only advantage to using the “Data Object Serializer” is that it doesn’t serialize members of classes that do not have the required attribute (opt-in instead of the opt-out method of the “XML Serializer”) and that you can serialize private members, which can come in handy from time to time. But it has a dependency on XML element order!!

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