I googled for it for few houres, but didn't resolve it, any hints will be 
thinkfull.

I want to use the following XSD file to guarantee <element1> is unique, but 
it doesn't work.

The following is my XSD file and XML file:

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

<xs:element name="root" type="rootType">
 <xs:unique name="PK_element1">
  <xs:selector xpath="./element1"></xs:selector>
  <xs:field xpath="."></xs:field>
 </xs:unique>
</xs:element>

<xs:complexType name="rootType">
 <xs:choice>
  <xs:group ref="elementGroup" maxOccurs="unbounded" minOccurs="0" />
 </xs:choice>
</xs:complexType>

<xs:group name="elementGroup">
 <xs:choice>
  <xs:element name="element1" type="xs:string" />
  <xs:element name="element2" type="xs:string" />
 </xs:choice>
</xs:group>

</xs:schema>


----------- test.xml ----------------
<?xml version="1.0" encoding="UTF-8"?>

<root xmlns="http://www.aray.com"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://www.aray.com test.xsd"
>
 <element1>1</element1>
 <element2>1</element2>
 <element1>1</element1>
 <!-- 
    I expect this xml file is not valide, because the two <element1> has the 
same content.
    But it is valide
 -->
</root>



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