xml schema and using a choice as the document root

Question

I have a bit of a newbie xml schema question. I believe the answer is that what I need to do is not possible with schema, but I'd like to be sure. The problem is that I have a webservice that returns a response with one type of root element on success (say <Response>), and on a complete failure, returns a document with a different root element (say, <Exception>). So, basically, two completely different documents:

<Response>......</Response> OR
<Exception>....</Exception>

Is it possible to describe these two different documents with one schema document? It's like I want a choice as the first element under the schema element -- but that isn't valid syntax. I've tried a couple of variants that parse as valid xsd, but don't validate the documents. Any suggestions? Or is this simply not possible? Thanks very much in advance -- m

Answer 1

Actually, XML schema does allow you to define alternative root elements in a single schema, although not by using the choice element. Instead, all you need to do is list each of the possible roots as direct children of your schema element.

For example, given the following XML schema:

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
    <xs:element name="foo">
        ...
    </xs:element>
    <xs:element name="bar">
        ...
    </xs:element>
</xs:schema>

Either of the following documents would validate against it:

<foo>
    ...
</foo>

Or:

<bar>
    ...
</bar>