This is what Saxon says on the matter:
 
Validation error on line 9 column 32 of file:/c:/temp/test.xml:
  Invalid element <Item>: the element is declared to be abstract
Validation of source document failed
 
However, the schema spec doesn't give you a simple boolean function
isValid(instance, schema). For example in 3.3.4 Element Locally Valid
(Element) we read:
 
For an element information item to be locally
<http://www.w3.org/TR/xmlschema-1/#key-vn> =B7valid=B7 with respect to =
an
element declaration all of the following must be true: 
1 The declaration must not be  =
<http://www.w3.org/TR/xmlschema-1/#key-null>
=B7absent=B7.
2 Its  <http://www.w3.org/TR/xmlschema-1/#e-abstract> {abstract} must be
false.
3 ...
 
So it's clear that your Item element is not locally valid with respect =
to
this element declaration.
 
But from 3.3.4 Element Locally Valid (Type) we can see that the element =
is
locally valid with respect to the type xs:string.
 
In 5.2 we see that there are three ("primary") ways of assessing the
schema-validity of an element. Method 1 allows you to ask "is Item a =
valid
xs:string", and the answer is yes. Method 2 allows you to ask "is Item a
valid Item", and the answer is no. Method 3 allows you to ask "is Item
valid?" - which is closest to your question, and what the spec says is:
 
"The processor starts from
<http://www.w3.org/TR/xmlschema-1/#cvc-assess-elt> Schema-Validity
Assessment (Element) (=A73.3.4) with no stipulated declaration or =
definition,
and either  <http://www.w3.org/TR/xmlschema-1/#key-sva> =B7strict=B7 or
<http://www.w3.org/TR/xmlschema-1/#key-lva> =B7lax=B7 assessment ensues,
depending on whether or not the element information and the schema =
determine
either an element declaration (by name) or a type definition (via =
xsi:type)
or not."
 
Schema-Validity Assessment (Element) clause 1 says that "for =
schema-validity
to be assessed", *either* 1.1 or 1.2 must be true. 1.1 is true if the
element is valid against the element declaration; 1.2 is true if it is =
valid
against the xsi:type. This suggests that the element has to be valid =
against
either the element declaration or the xsi:type, but not necessarily =
against
both.
 
What's more, there's a note: "Note: In general if clause
<http://www.w3.org/TR/xmlschema-1/#c-ed> 1.1 above holds clause
<http://www.w3.org/TR/xmlschema-1/#c-td> 1.2 does not, and vice versa. =
When
an xsi:type  <http://www.w3.org/TR/xml-infoset/#infoitem.element>
[attribute] is involved, however, clause
<http://www.w3.org/TR/xmlschema-1/#c-td> 1.2 takes precedence, as is =
made
clear in  <http://www.w3.org/TR/xmlschema-1/#cvc-elt> Element Locally =
Valid
(Element) (=A73.3.4)." This note isn't as helpful as one would like, =
however,
because  <http://www.w3.org/TR/xmlschema-1/#cvc-elt> Element Locally =
Valid
(Element) says that the element is invalid if the element declaration
specifies "abstract", regardless of xsi:type - in other words, the =
element
declaration, in this respect at least, actually takes precedence over
xsi:type!
 
It's all pretty murky. Saxon has decided the issue one way, which I =
think is
the common-sense way, but I think one could take a different =
interpretation.
 
Interesting question.
 
Michael Kay
http://www.saxonica.com/
 
 
  _____  

From: xmlschema-dev-request@w... [mailto:xmlschema-dev-request@w...] =
On
Behalf Of Stan Kitsis
Sent: 11 January 2006 00:21
To: xmlschema-dev@w...
Subject: abstract elements and xsi:type substitution



Is the following valid? Why or why not?

 

Thanks,

Stan Kitsis

 

******* XSD **************

 

<?xml version="1.0" encoding="utf-8" ?> 

<xs:schema targetNamespace="foo"

                  elementFormDefault="qualified"

                  xmlns="foo"

                  xmlns:xs="http://www.w3.org/2001/XMLSchema">

  

  <xs:element name="Item" type="xs:string" abstract="true"/>

</xs:schema>

 

******* XML **************

<?xml version="1.0" encoding="utf-8"?>

<Item xmlns="foo" 

      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 

      xmlns:xs="http://www.w3.org/2001/XMLSchema"

          xsi:type="xs:string">text</Item>