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    copied!<p>What you require is a <a href="http://www.w3schools.com/Schema/el_choice.asp" rel="nofollow">choice element</a> in your schema. It'll allow you to express that only one from a set of elements may appear.</p> <p>EDIT: you'll need to specify a name for the element, either directly via <code>name</code> or in reference to an element defined elsewhere via <code>ref</code>.</p> <p>Your only alternative <strong>as far as I know</strong> is to have something like this:</p> <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt; &lt;xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" attributeFormDefault="unqualified"&gt; &lt;xs:element name="PetCage"&gt; &lt;xs:complexType&gt; &lt;xs:sequence&gt; &lt;xs:element name="animal" type="Animal" minOccurs="1" maxOccurs="1"/&gt; &lt;/xs:sequence&gt; &lt;/xs:complexType&gt; &lt;/xs:element&gt; &lt;xs:complexType name="Animal" abstract="true"&gt; &lt;xs:attribute name="Name" type="xs:string" /&gt; &lt;/xs:complexType&gt; &lt;xs:complexType name="Cat"&gt; &lt;xs:complexContent&gt; &lt;xs:extension base="Animal"&gt;&lt;/xs:extension&gt; &lt;/xs:complexContent&gt; &lt;/xs:complexType&gt; &lt;/xs:schema&gt; </code></pre> <p>This would validate XML files of a form like this:</p> <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt; &lt;PetCage&gt; &lt;animal xsi:type="Cat" Name="String"/&gt; &lt;/PetCage&gt; </code></pre> <p>Unfortunately this leaves you tied to that <code>animal</code> element name. I doubt that XML Schema allows you to specify a specific type and lets you use a wildcard for the name. XML Schema validates based on element names and, if the element is defined with an abstract type, the <code>xsi:type</code> attribute. There is an <code>xs:any</code> declaration, but that can't be tied to any specific type. The following will validate as best as possible, but still allows any element defined under the schema root to be used:</p> <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt; &lt;xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" attributeFormDefault="unqualified"&gt; &lt;xs:element name="PetCage"&gt; &lt;xs:complexType&gt; &lt;xs:sequence&gt; &lt;xs:any processContents="strict" /&gt; &lt;/xs:sequence&gt; &lt;/xs:complexType&gt; &lt;/xs:element&gt; &lt;xs:complexType name="Animal" abstract="true"&gt; &lt;xs:attribute name="Name" type="xs:string" /&gt; &lt;/xs:complexType&gt; &lt;xs:element name="cat" /&gt; &lt;xs:complexType name="Cat"&gt; &lt;xs:complexContent&gt; &lt;xs:extension base="Animal"&gt;&lt;/xs:extension&gt; &lt;/xs:complexContent&gt; &lt;/xs:complexType&gt; &lt;/xs:schema&gt; </code></pre> <p>It would validate this correctly:</p> <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt; &lt;PetCage&gt; &lt;cat Name="snookums"/&gt; &lt;/PetCage&gt; </code></pre> <p>... but it wouldn't stop you from putting a PetCage in a PetCage.</p> <p>I'm hoping someone with good knowledge of the XML Schema spec can confirm that this is correct or, preferably, offer the desired solution.</p>
 

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