What is the difference between xsd and xsi?

Question

What exactly is the difference between XML Schema Document and XML Schema Instance ?

• xmlns:xsd="http://www.w3.org/2001/XMLSchema"
• xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

Please elaborate.

Quasimodo

What exactly is the difference - well, the exact difference is that they are different schemas that don't have common elements. A helpful link would be w3.org/TR/xmlschema-1/#Instance_Document_Constructions. – Platform

Answer 1

xsd and xsi Similarities

• Both are XML namespace prefixes, abbreviations for an XML namespace.
• Both are, as are all namespace prefixes, arbitrarily named; other namespace prefix abbreviations could equally well be used. However, both prefixes are conventional and therefore recommended. (An also-conventional alternative to xsd is xs.)

xsd and xsi Differences

• The xsd (or xs) prefix referring to the Schema Namespace (http://www.w3.org/2001/XMLSchema) is used in XML Schemas (XSDs) for the elements, attributes, and types of the W3C XML Schema Recommendation itself. (This is possible because XML Schema is itself XML.)

• The xsi prefix referring to the The Schema Instance Namespace http://www.w3.org/2001/XMLSchema-instance is used in XML document instances for several special attributes defined by the XML Schema Recommendation:

  • xsi:type allows an XML instance to associate element type information directly rather than through an XSD. See How to restrict the value of an XML element using xsi:type in XSD?

  • xsi:nil allows an empty element to be considered to be valid when the XSD might not otherwise have allowed it.

  • xsi:schemaLocation and xsi:noNamespaceSchemaLocation provide hints to the XML processor as to how to associate an XSD with an XML document. Use xsi:schemaLocation when there is a namespace; use xsi:noNamespaceSchemaLocation when there is no namespace.

See Also

• Namespace related attributes in XML and XML Schema (XSD)
• How to restrict the value of an XML element using xsi:type in XSD?

Dunfermline

.NET's XmlSerializer seems to introduce these namespace prefixes even though (1) they aren't used in the document it produces, and (2) there is no schema to validate the document it produces. My takeaway is this is all by convention, which I suppose makes some sense, if not a whole lot of sense. – Ieyasu

To include unused namespace prefix declarations of any sort is unnecessary but not wrong. Any serializer that includes unused namespace prefix declarations is simply not being as parsimonious as it could be. – Dunfermline

Actually, the XmlSerializer uses the XMLSchemaInstance namespace to handle abstract classes, via the xsi:type attribute. It is also possible to generate schemas using XSD.exe. I've never seen the schemas embedded directly in the serialized XML file, but wouldn't surprise me if this was possible too. – Expediency

There's a Schema Namespace, there's an Instance Namespace, so what's the use of the Default Namespace? – Yeseniayeshiva

@MinhNghĩa: A default namespace is specified on an element via xmlns=" default namespace URI " and places that element and its descendents in the given namespace, by default. A default namespace can be declared in any XML document, including even an XSD, where it can be used to avoid having to specify xsd or xs namespace prefixes on elements throughout the XSD. See also Namespace related attributes in XML and XML Schema (XSD) – Dunfermline

Answer 2

http://www.w3.org/2001/XMLSchema

The Simple Version : This is the namespace used within an XML Schema (XSD). An XML schema is used to describe what's valid within an XML instance document.

The Less Simple Version : This is the namespace of an XML Schema that describes the structure of an XML Schema. In other words a schema that describes itself.

An XML Schema (XSD) must be written using the types defined within this schema.

For Example.

<xs:schema elementFormDefault="qualified" xmlns:xs="http://www.w3.org/2001/XMLSchema">
    <xs:element name="MyElement" type="xs:string" />
</xs:schema>

http://www.w3.org/2001/XMLSchema-instance

This is a namespace used within XML Instance documents to provide additional data to the XML Parser that is processing it. It describes the attributes xsi:schemalocation, xsi:noSchemalocation, xsi:type and xsi:nil which the XML parser can use to assist it with validation.

For Example.

<MyElement xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
           xsi:noNamespaceSchemaLocation="MySchema.xsd">
    string
</MyElement>

Chishima