XML in a Nutshell, 3rd Edition

Allowing Mixed Content

The mixed attribute of the complexType element controls whether character data may appear within the body of the element with which it is associated. To illustrate this concept, Example 17-9 gives us a new schema that will be used to validate form-letter documents.

Example 17-9. formletter.xsd

<xs:schema xmlns:xsi="http://www.w3.org/2001/XMLSchema">
  <xs:element name="letter">
    <xs:complexType mixed="true"/>
  </xs:element>
</xs:schema>

This schema seems to declare a single element called letter that may contain character data and nothing else. But attempting to validate the following document produces an error, as shown in Example 17-10.

Example 17-10. formletterdoc.xml

<letter xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:noNamespaceSchemaLocation="formletter.xsd">Hello!</letter>

The following error is generated:

The content of element type "letter" must match "EMPTY".

This is because there’s no complex content for the letter element. Setting mixed to true is not the same as declaring an element that may contain a string. The character data may only appear in relation to other complex content, which leads to the subject of relative element positioning.