XML Schema Part 0: Primer Second Edition

March 3, 2006

XML Schema Part 0: Primer Second Edition

In general, an element is required to appear when the value of minOccurs is 1 or more. The maximum number of times an element may appear is determined by the value of a maxOccurs attribute in its declaration. This value may be a positive integer such as 41, or the term unbounded to indicate there is no maximum number of occurrences. The default value for both the minOccurs and the maxOccurs attributes is 1. Thus, when an element such as comment is declared without a maxOccurs attribute, the element may not occur more than once. Be sure that if you specify a value for only the minOccurs attribute, it is less than or equal to the default value of maxOccurs, i.e. it is 0 or 1. Similarly, if you specify a value for only the maxOccurs attribute, it must be greater than or equal to the default value of minOccurs, i.e. 1 or more. If both attributes are omitted, the element must appear exactly once.

Basically, if it is missing it is assumed to be a value of 1 for both minOccurs and maxOccurs. So, a definition of

<xsd:element ref=”Keys” />

Would be interpreted as

<xsd:element ref=”Keys” minOccurs=”1″ maxOccurs=”1″ />

Just in case you were wondering.

(note: this entry was updated several times to get the code formatting to appear correctly for me in FireFox. I didn’t try IE.)

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