Question

Suppose I have an XML like this:

<foo ...>
   <bar a="s1" b="s2" />
   <bar a="s3" />
</foo>

What I’d like is to define in the XSD is that the default value of attribute b should be the value of attribute a. Is that possible?

Thanks in advance!

Answer 1

Short answer: no.

Medium answer, according to “XML Schema Part 1: Structures Second Edition”, section “3.2.1 The Attribute Declaration Schema Component”: “default specifies that the attribute is to appear unconditionally in the ·post-schema-validation infoset·, with the supplied value used whenever the attribute is not actually present”

Long and practical answer: the key is “post-schema-validation infoset”. So ask yourself what you need the schema document for?

• if you need it to validate incoming XML documents, then just use use="required" for attribute b
• if you need it to build objects (e.g., using some (un)marshalling technologies such as Java’s JAXB or .NET’s XmlSerializer) then you must explicitely add some post-processing which satisifies this “post-schema-validation-infoset” term – you should detect if the value of b wasn’t set and set it yourself – that’s your business requirement
• if you need to communicate your intent (*I/my app will treat empty b as having the same value as a) add documentation. XML Schema doesn’t have such concept. Using extrapolation – that would be the same requirement as “the value of b will be the current USD/EUR ratio” – it’s just computed value