What's the difference between ref and type in an XML schema?

Question

Consider the following schema:

<?xml version="1.0" encoding="ISO-8859-1" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

    <xs:complexType name="Root">
        <xs:sequence>
            <xs:element ref="Child" />
            <xs:element name="Child2" type="Child" />
        </xs:sequence>
        <xs:attribute ref="Att" />
        <xs:attribute name="Att2" type="Att" />
    </xs:complexType>

    <xs:complexType name="Child">
        <xs:attribute ref="Att" />
    </xs:complexType>

    <xs:attribute name="Att" type="xs:integer" />

</xs:schema> 

The ref to "Child" on line 6 fails, while the type on line 7 validates. For the attribute, the ref succeeds while the type fails. I'm trying to understand why.

My understanding of ref was that it simply referred to another element and specified that you expect to see an instance of the referred type (with the name given in the definition) at that location. Obviously I'm wrong, so what does ref actually mean?

Answer 1

Using ref=".." you are "pasting" existing element/attribute defined on the other place. Using type=".." you are assigning some structure (defined in complextype/simpletype) to new element/attribute. Look at following:

<?xml version="1.0" encoding="ISO-8859-1" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:tst="test" targetNamespace="test">

    <xs:complexType name="Root">
        <xs:sequence>
            <xs:element ref="tst:Child" />
            <xs:element name="Child2" type="tst:ChildType" />
        </xs:sequence>
        <xs:attribute ref="tst:AttRef" />
        <xs:attribute name="Att2" type="tst:AttType" />
    </xs:complexType>

    <xs:complexType name="ChildType">
        <xs:attribute ref="tst:AttRef" />
    </xs:complexType>

    <xs:element name="Child">
    </xs:element>

    <xs:simpleType name="AttType">
        <xs:restriction base="xs:string">
            <xs:maxLength value="10" />
        </xs:restriction>
    </xs:simpleType>

    <xs:attribute name="AttRef" type="xs:integer" />

</xs:schema> 

Answer 2

Within a content model, the xs:element element may be either:

1. An element declaration (giving the name and type of the element) or
2. A reference to a top-level element declaration (giving the name of the element as a way of identifying it, and deferring to the actual declaration for the type).

(The same name/ref alternation applies to attribute declarations and attribute references, and there is a similar dichotomy between inline type definitions and references to named types.)

In your example, there is no top-level declaration for an element named Child, so the ref attribute fails. I hope this helps.