Unqualified XSD global attribute references

Question

The following XML schema fails to validate with following XML instance document. Is there any way to rewrite the schema so the instance document validates, within the given constraints?

Constraints

• The attribute cannot be local to the element
• The instance document must be unchanged

(Invalid) Schema

<?xml version="1.0" encoding="utf-8"?>
<xs:schema targetNamespace="http://tempuri.org/XMLSchema.xsd"
    elementFormDefault="qualified"
    xmlns="http://tempuri.org/XMLSchema.xsd"
    xmlns:xs="http://www.w3.org/2001/XMLSchema">

    <xs:attribute name="sample-attribute" type="xs:string" />

    <xs:element name="sample-element">
        <xs:complexType>
            <xs:attribute ref="sample-attribute" use="required" />
        </xs:complexType>
    </xs:element>
</xs:schema>

Instance

<?xml version="1.0" encoding="utf-8"?>
<sample-element xmlns="http://tempuri.org/XMLSchema.xsd" sample-attribute="test" />

Answer 1

Yes.

Wrap the (global) xs:attribute element in an xs:attributeGroup element.

In the xs:element element, refer to the xs:attributeGroup element.

The name attribute of the xs:attributeGroup element can have the same value as the name attribute of the xs:attribute element.

Schema

<?xml version="1.0" encoding="utf-8"?>
<xs:schema targetNamespace="http://tempuri.org/XMLSchema.xsd"
    elementFormDefault="qualified"
    xmlns="http://tempuri.org/XMLSchema.xsd"
    xmlns:xs="http://www.w3.org/2001/XMLSchema">

    <xs:attributeGroup name="sample-attribute">
      <xs:attribute name="sample-attribute" type="xs:string" use="required"/>
    </xs:attributeGroup>

    <xs:element name="sample-element">
      <xs:complexType>
        <xs:attributeGroup ref="sample-attribute" />
      </xs:complexType>
    </xs:element>
</xs:schema>

Information not directly related to the question

This is not an extension to the answer above, nor an alternative answer, just related information that you might find helpful (it is not within the constraints of your question).

You could leave your original schema untouched, and explicitly qualify (add a namespace prefix to) the attribute name in the document instance, like this:

<?xml version="1.0" encoding="utf-8"?>
<t:sample-element
    xmlns:t="http://tempuri.org/XMLSchema.xsd"
    t:sample-attribute="test"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://tempuri.org/XMLSchema.xsd sample.xsd"/>

(Note the t: namespace prefix on both the root element name and the attribute name.)

Answer 2

Namespaces in XML states that "The namespace name for an unprefixed attribute name always has no value"; on the other hand you constrained the attribute not to be local, so the only way to do it (credit goes to @GrahamHannington) is to wrap it in an attribute group, thus allowing the attribute definition to be reused without being qualified.