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XML values ​​of the same attribute in different elements cannot be repeated

In my C # application, I am using an XSD file to validate XML files. Is there a way to include a warning in the XSD file that the attribute values ​​are repeated Reference_numberin different elements?

<?xml version="1.0" encoding="utf-8"?>
<xs:schema attributeFormDefault="unqualified" elementFormDefault="qualified" xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="Items">
    <xs:complexType>
      <xs:sequence>
        <xs:element maxOccurs="unbounded" name="Item">
          <xs:complexType>
            <xs:sequence>
              <xs:element name="Name" type="xs:string" />
              <xs:element name="Reference_number" type="xs:string" />
              <xs:element name="Price" type="xs:float" />
            </xs:sequence>
          </xs:complexType>
        </xs:element>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
</xs:schema>
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2 answers

You can use xs:uniqueto restrict Reference_numberas unique:

<?xml version="1.0" encoding="utf-8"?>
<xs:schema attributeFormDefault="unqualified"
           elementFormDefault="qualified"
           xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="Items">
    <xs:complexType>
      <xs:sequence>
        <xs:element maxOccurs="unbounded" name="Item">
          <xs:complexType>
            <xs:sequence>
              <xs:element name="Name" type="xs:string" />
              <xs:element name="Reference_number" type="xs:string" />
              <xs:element name="Price" type="xs:float" />
            </xs:sequence>
          </xs:complexType>
        </xs:element>
      </xs:sequence>
    </xs:complexType>
    <xs:unique name="UniqueItemRefNum">
      <xs:selector xpath="Item" /> 
      <xs:field xpath="Reference_number" /> 
    </xs:unique>
  </xs:element>
</xs:schema>

Then, for example, the following XML is valid:

<?xml version="1.0" encoding="utf-8"?>
<Items xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:noNamespaceSchemaLocation="try.xsd">
  <Item>
    <Name>name1</Name>
    <Reference_number>r1</Reference_number>
    <Price>1.00</Price>
  </Item>
  <Item>
    <Name>name2</Name>
    <Reference_number>r2</Reference_number>
    <Price>2.00</Price>
  </Item>
</Items>

But the following XML will be invalid (note the duplicate Reference_number):

<?xml version="1.0" encoding="utf-8"?>
<Items xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:noNamespaceSchemaLocation="try.xsd">
  <Item>
    <Name>name1</Name>
    <Reference_number>r1</Reference_number>
    <Price>1.00</Price>
  </Item>
  <Item>
    <Name>name2</Name>
    <Reference_number>r1</Reference_number>
    <Price>2.00</Price>
  </Item>
</Items>
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For this purpose you should use an element key or a unique element. something like that:

<?xml version="1.0" encoding="utf-8"?>
<xs:schema attributeFormDefault="unqualified" elementFormDefault="qualified" xmlns:xs="http://www.w3.org/2001/XMLSchema">
    <xs:element name="Items">
        <xs:complexType>
            <xs:sequence>
                <xs:element maxOccurs="unbounded" name="Item">
                    <xs:complexType>
                        <xs:sequence>
                            <xs:element name="Name" type="xs:string" />
                            <xs:element name="Reference_number" type="xs:string" />
                            <xs:element name="Price" type="xs:float" />
                        </xs:sequence>
                    </xs:complexType>
                </xs:element>
            </xs:sequence>
        </xs:complexType>
        <xs:key name="referenceKey">
            <xs:selector xpath="Item"/>
            <xs:field xpath="Reference_number"/>
        </xs:key>
    </xs:element>
</xs:schema>
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