Hi Danny,

Your question is appropriate for xmlschema-dev@[...].org. You probably
shouldn't cross-post to xml-dev@[...].org. I'm only cross-posting
so that they know it's been answered; please post follow-ups to
xmlschema-dev@[...].org.

>  The problem seems to be around the use of simpleContent or
>  complexContent and these types:
> 
>  Here are my base definitions of the types that I use:
> 
>      <xsd:complexType name="C-255">
>         <xsd:simpleContent>
>            <xsd:extension base="C-255_NoID">
>               <xsd:attribute ref="id"/>
>            </xsd:extension>
>         </xsd:simpleContent>
>      </xsd:complexType>
>      <xsd:simpleType name="C-255_NoID">
>         <xsd:restriction base="xsd:string">
>            <xsd:maxLength value="255"/>
>            <xsd:minLength value="1"/>
>         </xsd:restriction>
>      </xsd:simpleType>
> 
>  I then have the following:
> 
>      <xsd:element name="HelpText">
>         <xsd:complexType>
>            <xsd:complexContent>
>               <xsd:extension base="C-255">
>                  <xsd:attribute ref="xml:lang"/>
>               </xsd:extension>
>            </xsd:complexContent>
>         </xsd:complexType>
>      </xsd:element>
> 
>  MS-XML indicates that instead of complexContent, the definition of
>  HelpText should use simpleContent. Is this correct? Is this a real
>  error or a preferred way to define this element?

Yes, it's a real error. If you did:

<xsd:element name="HelpText"> 
  <xsd:complexType> 
    <xsd:simpleContent> 
      <xsd:extension base="C-255"> 
        <xsd:attribute ref="xml:lang"/> 
      </xsd:extension> 
    </xsd:simpleContent> 
  </xsd:complexType> 
</xsd:element> 

then it would be just fine (and would be correct -- the HelpText
element has two attributes (which means it must be a complex type) but
has simple content (a string).

>  If MS-XML is correct, could we have made the Schema process any more
>  difficult? Why can't I use complexContent, with a type defined as a
>  complexType and simpleContent with a type defined as simpleType?

I agree that this can be confusing. XML Schema divides types into two
kinds: complex types, which allow element content and/or attributes;
and simple types, which don't. XML Schema then splits complex types
(which allow attributes) into two classes: those with purely textual
content (simple content) and those with element content (complex
content). Diagrammatically:

                 Type
                  /                 /                  /               Simple    Complex    
       allows attributes or elements?
            Type      Type
                       /                      /                       /                     
Simple  Complex       allows elements in content?
                Content  Content
                 
The restriction that you're running into is that you can't derive a
complex type with complex content from one with simple content. If you
need to extend a complex type with textual content to one that also
allows elements, then you have to start off with a complex type that
has complex content (allows elements in the first place) and is mixed
(to allow the text that you want). Similarly, you can't derive a
complex type from a simple type except to extend it by adding
attributes. I guess that this prevents you from doing:

<xs:complexType name="price"> 
  <xs:complexContent> 
    <xs:extension base="xs:decimal"> 
      <xs:element name="currency" type="xs:token" /> 
    </xs:extension> 
  </xs:complexContent> 
</xs:complexType> 

to validate:

  <price> 12.99<currency>GBP</currency></price>

but that's pretty bad XML design anyway (and I'll note that RELAX NG
doesn't support it either).

Cheers,

Jeni

---
Jeni Tennison
http://www.jenitennison.com/


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