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The required attribute is missing

A required attribute is missing from an element.

The message, as .NET reports it
The required attribute 'currency' is missing.

Why it happens

  • The attribute is declared use="required" and the document leaves it out.
  • Serialized output: XmlSerializer leaves out an [XmlAttribute] property that is null.

How to fix it

Add the attribute with a valid value. The message doesn't name the element: use the line and position of the validation event.

.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 required but missing. It checks the whole document in your browser and lists every schema error with its line and column, not only the first.

Element 'order': The attribute 'currency' is required but missing.

Example

Validated against order.xsd, below.

Reports the error
<order>
  <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>