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Subject: Schema extension vs OO
I have an AI to write a blurb about the differences between schema extension and OO inheritance, so here we go: XML instance documents and object instances often have widely different usages, requirements and constraints. In the general case, one can't introspect an XML instance document to discover its schema, the entire set of schema documents might not be readily available like a class library is, XML schemas may have much higher requirements of extensibility without coordination (they are the foundation for loose coupling). For these reasons and because of certain aspects of the schema language (for example the fact that derivation by restriction requires the repetition of the parent type and other reasons that make extension by composition more flexible and robust), extension by composition is often a preferable approach for XML than extension by subclassing. It is therefore not recommended in the general case to map OO classes to XML documents by replicating the class hierarchy using schema subclassing. Until Andrea corrects me... :-) William
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