How to constrain set of attributes in xsd - xsd

Question

All
Please suggest me how to restrict in xsd schema the following:
<root>
<node action="action1" parameter="1" />
</root>
I need to require attribute "parameter" only if attribute "action" is defined.
Thanks,

Answer 1

W3C Schema doesn't have the ability to express conditionally required attributes.
Schematron is a great tool for validating that documents adhere to custom validation scenarios in which content is conditionally required.
You could define those attributes as optional in your schema, and then use Schematron to validate it against those conditional rules.

Answer 2

I created this xsd to attempt to solve the problem.
<?xml version="1.0" encoding="utf-8"?>
<xs:schema id="XMLSchema1"
targetNamespace="http://tempuri.org/XMLSchema1.xsd"
elementFormDefault="qualified"
xmlns="http://tempuri.org/XMLSchema1.xsd"
xmlns:mstns="http://tempuri.org/XMLSchema1.xsd"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
>
<xs:group name="populated">
<xs:sequence >
<xs:element name="node">
<xs:complexType>
<xs:attributeGroup ref="actionattrib" />
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:group>
<xs:group name="unpopulated">
<xs:sequence >
<xs:element name="node">
<xs:complexType>
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:group>
<xs:attributeGroup name ="actionattrib">
<xs:attribute name="action1" type="xs:string" />
<xs:attribute name="parameter" type="xs:int" />
</xs:attributeGroup>
<xs:element name="root">
<xs:complexType>
<xs:choice minOccurs ="0">
<xs:group ref="populated" />
<xs:group ref ="unpopulated" />
</xs:choice>
</xs:complexType>
</xs:element>
</xs:schema>
And a test method:
public static void Go()
{
string nameSpace = "http://tempuri.org/XMLSchema1.xsd";
XmlSchemaSet schemas = new XmlSchemaSet();
schemas.Add(nameSpace, "XMLSchema1.xsd");
XDocument myDoc1 = new XDocument(
new XElement(XName.Get("root", nameSpace),
new XElement( XName.Get("node", nameSpace ))
)
);
myDoc1.Validate(schemas, (o, e) => { Console.WriteLine(e.Message); });
}
And finally got this exception when validating:
Multiple definition of element 'http://tempuri.org/XMLSchema1.xsd:node' causes the content model to become ambiguous. A content model must be formed such that during validation of an element information item sequence, the particle contained directly, indirectly or implicitly therein with which to attempt to validate each item in the sequence in turn can be uniquely determined without examining the content or attributes of that item, and without any information about the items in the remainder of the sequence.
Which matches Mads result. His answer should be accepted.