XPointer

The XPointer Framework that XInclude uses to select a subset of a target document

#XPointer

XPointer identifies a fragment of an XML document. XInclude uses it to select part of a target document instead of the whole document element. There is no standalone XPointer API; the only way to reach it is the xpointer (or fragid) attribute on xi:include. See XInclude.

#How XInclude reaches XPointer

An xi:include with an xpointer or fragid attribute passes that attribute's value to the XPointer evaluator, together with the target document. The evaluator returns the matched nodes, or an empty result. An empty result triggers xi:fallback if present; see Fallback.

xml

<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
            href="catalog.xml" xpointer="chapter-3"/>

The same attribute with no href selects from the including document itself.

#The Framework: scheme-based parts

An XPointer value is either a shorthand (see Shorthand and element()) or a sequence of one or more scheme-based parts: scheme(data) scheme(data) .... The evaluator tries each part in order and returns the first one that matches at least one node.

Three scheme-based parts are supported:

Scheme	Purpose	Page
`element()`	Selects an element by ID or child sequence	Shorthand and element()
`xmlns()`	Binds a prefix for a following `xpath1()` part	Namespaces and xpath1()
`xpath1()`	Selects nodes with an XPath 1.0 expression	Namespaces and xpath1()

An unrecognized scheme name is not an error. The evaluator ignores that part and tries the next one. Only a value with no matching part at all, or a value that violates the Framework's own grammar, is malformed.

#No user extension point

XPointer has no user extension point. The set of supported schemes — shorthand, element(), xmlns(), and xpath1() — is fixed by the engine. There is no way to register a custom scheme, and no public API surfaces the evaluator directly; it is an internal detail of XInclude processing. Do not design a document format around an XPointer scheme the engine does not implement — the pointer part is silently skipped rather than rejected.

For C# developers: this is unlike an IXmlResourceResolver, which is a public extension point on XIncludeOptions. XPointer scheme resolution has no equivalent hook.