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XML Simple Attributes

Recall that attributes are used to provide additional information about an XML element. Only complex elements are allowed to contain attributes, however, the attributes themselves are considered simple types.

The syntax for declaring attributes in a Schema is shown below:

Attributes

<xsd:attribute name="attribute name" type="attribute data type"/>

Listing 3-9. Attribute syntax.

The attribute declaration is identical to the simple element. The attribute name is specified, followed by the attribute type.

In the following XML file, the Tree element is defined and includes an attribute tid:

Trees.xml

<Tree tid="1">Red Oak</Tree>

Listing 3-10. Attribute declaration.

The Schema file shows how to define the tid attribute:

Trees.xsd

<xsd:attribute name="tid" type="xs:integer"/>

Listing 3-11. Defining attributes.

When an XML element or attribute has a data type defined, it puts restrictions on the element's or attribute's content. An attribute defined in the Schema as type "integer" cannot be assigned the value "This is a String" in the XML file.

Like simple elements, attributes may have a default value using the default attribute or a fixed value using the fixed attribute. A default value is automatically assigned to the attribute when no other value is specified. A fixed value is also automatically assigned to the attribute, and you cannot specify another value.

The tid attribute is given a default value:

<xsd:attribute name="tid" type="xsd:integer" default="1111"/>

Listing 3-12. Assigning default values to attributes.

By default, attributes are optional. To specify that the attribute is required, include the use attribute:

<xsd:attribute name="tid" type="xsd:integer" default="1111" use="required"/>

Listing 3-13. Specifying required attributes.

The tid attribute is declared as a required attribute. If it is not coded in the XML document, the file will not be considered valid.


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