Validating XML with XSDs ... but still allow extensibility

Question

Maybe it's me, but it appears that if you have an XSD

<?xml version="1.0" encoding="utf-8"?>
<xs:schema attributeFormDefault="unqualified" elementFormDefault="qualified" xmlns:xs="http://www.w3.org/2001/XMLSchema"&gt;
    <xs:element name="User">
        <xs:complexType>
            <xs:sequence>
                <xs:element name="GivenName" />
                <xs:element name="SurName" />
            </xs:sequence>
            <xs:attribute name="ID" type="xs:unsignedByte" use="required" />
        </xs:complexType>
    </xs:element>
</xs:schema>

that defines the schema for this document

<?xml version="1.0" encoding="utf-8" ?>
<User ID="1">
    <GivenName></GivenName>
    <SurName></SurName>
</User>

It would fail to validate if you added another element, say EmailAddress, and mix up the order

<?xml version="1.0" encoding="utf-8" ?>
<User ID="1">
    <SurName></SurName>
    <EmailAddress></EmailAddress>
    <GivenName></GivenName>
</User>

I don't want to add EmailAddress to the document and have it be marked optional.

I just want an XSD that validates the bare minimum requirements that the document must meet.

Is there a way to do this?

EDIT:

marc_s pointed out below that you can use xs:any inside of xs:sequence to allow more elements, unfortunately, you have to maintain the order of elements.

Alternatively, I can use xs:all which doesn't enforce the order of elements, but alas, doesn't allow me to place xs:any inside of it.

Answer 1

You should be able to extend your schema with the <xs:any> element for extensibility - see W3Schools for details.

<?xml version="1.0" encoding="utf-8"?>
<xs:schema attributeFormDefault="unqualified" elementFormDefault="qualified" xmlns:xs="http://www.w3.org/2001/XMLSchema"&gt;
    <xs:element name="User">
        <xs:complexType>
            <xs:sequence>
                <xs:element name="GivenName" />
                <xs:element name="SurName" />
                <xs:any minOccurs="0" maxOccurs="unbounded" processContents="lax" />
            </xs:sequence>
            <xs:attribute name="ID" type="xs:unsignedByte" use="required" />
        </xs:complexType>
    </xs:element>
</xs:schema>

When you add the processContents="lax" then the .NET XML validation should succeed on it.

See MSDN docs on xs:any for more details.

Update: if you require more flexibility and less stringent validation, you might want to look at other methods of defining schemas for your XML - something like RelaxNG. XML Schema is - on purpose - rather strict about its rules, so maybe that's just the wrong tool for this job at hand.

Answer 2

After reading of the answer of marc_s and your discussion in comments I decide to add a little.

It seems to me there are no perfect solution of your problem Chad. There are some approaches how to implement extensible content model in XSD, but all me known implementation have some restrictions. Because you didn't write about the environment where you plan to use extensible XSD I can you only recommend some links which probably will help you to choose the way which can be implemented in your environment:

1. http://www.xfront.com/ExtensibleContentModels.html (or http://www.xfront.com/ExtensibleContentModels.pdf) and http://www.xfront.com/VariableContentContainers.html
2. http://www.xml.com/lpt/a/993 (or http://www.xml.com/pub/a/2002/07/03/schema_design.html)
3. http://msdn.microsoft.com/en-us/library/ms950793.aspx

Answer 3

Well, you can always use DTD :-) except that DTD also prescribes ordering. Validation with "unordered" grammar is terribly expensive. You could play with xsd:choice and min and max occurs but it's probably going to balk as well. You could also write XSD extensions / derived schemas.

The way you posed the problem it looks like you don't really want XSD at all. You can just load it and then validate whatever minimum you want with XPaths, but just protesting against XSD, how many years after it became omni-present standard is really, really not going to get you anywhere.