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The attribute is not declared

An element carries an attribute its type in the schema does not declare.

The message, as .NET reports it
The 'vat' attribute is not declared.

Why it happens

  • A misspelt attribute name or a different letter case.
  • An attribute from a newer version of the format, validated against an older schema.

How to fix it

Remove or rename the attribute, or validate against the schema version the document was written for. The message doesn't name the element: the line and position in ValidationEventArgs.Exception do.

.NET throws an XmlSchemaValidationException at the first schema error unless XmlReaderSettings.ValidationEventHandler is set, which then receives every one with its line and position. A well-formedness error is an XmlException and ends reading whatever the settings.

The same problem in xmlvalid

xmlvalid validates with libxml2, which reports it as The attribute is not allowed. It checks the whole document in your browser and lists every schema error with its line and column, not only the first.

Element 'order', attribute 'vat': The attribute 'vat' is not allowed.

Example

Validated against order.xsd, below.

Reports the error
<order currency="EUR" vat="20">
  <id>A-1</id>
  <qty>1</qty>
</order>
Fixed
<order currency="EUR">
  <id>A-1</id>
  <qty>1</qty>
</order>
order.xsd 
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="order">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="id" type="xs:string"/>
        <xs:element name="qty" type="xs:int"/>
        <xs:element name="status" type="Status" minOccurs="0"/>
        <xs:element name="code" type="Code" minOccurs="0"/>
        <xs:element name="note" type="Short" minOccurs="0"/>
      </xs:sequence>
      <xs:attribute name="currency" type="xs:string" use="required"/>
    </xs:complexType>
  </xs:element>
  <xs:simpleType name="Status">
    <xs:restriction base="xs:string">
      <xs:enumeration value="open"/>
      <xs:enumeration value="paid"/>
    </xs:restriction>
  </xs:simpleType>
  <xs:simpleType name="Code">
    <xs:restriction base="xs:string">
      <xs:pattern value="[A-Z]{3}-[0-9]{4}"/>
    </xs:restriction>
  </xs:simpleType>
  <xs:simpleType name="Short">
    <xs:restriction base="xs:string">
      <xs:maxLength value="10"/>
    </xs:restriction>
  </xs:simpleType>
</xs:schema>