XPointer

Namespaces and xpath1()

Bind namespace prefixes with xmlns() and select nodes with the xpath1() scheme, under a wall-clock timeout

#Namespaces and xpath1()

#The xmlns() scheme

xmlns() binds a namespace prefix for use in a later xpath1() part in the same pointer. Its data is prefix=uri:

xmlns(c=http://example.com/catalog) xpath1(//c:book)
                                  

xmlns() never selects a node by itself. It only prepares a binding. Place it before the xpath1() part that needs the binding. Parts run in the order they appear, so the binding must exist before the expression that uses it evaluates.

The xml prefix is always bound to the XML namespace, so xml:id and other xml:-prefixed attributes are reachable in an xpath1() expression without an explicit xmlns() part.

A malformed xmlns() data string — one with no =, or an empty prefix — is a fatal, malformed-pointer error.

#The xpath1() scheme

xpath1() selects nodes with an XPath 1.0 expression, evaluated against the target document with System.Xml's built-in XPath 1.0 engine:

xpath1(//book[@category='fiction'])
                                  

An expression that is not valid XPath 1.0 is a fatal, malformed-pointer error. The error message includes the offending expression text.

#The evaluation timeout

xpath1() evaluation runs under a wall-clock budget, because System.Xml's XPath engine cannot be cancelled mid-evaluation once it starts. The budget is MaxXPathEvalMilliseconds on XIncludeOptions, 5000 milliseconds by default.

An evaluation that exceeds the budget raises a fatal error at the deadline. The abandoned evaluation keeps running on a dedicated background thread, off the thread pool, so it does not starve pooled work or the caller. Discard a document that hits this deadline instead of reusing it, because that background thread still reads it.

Set MaxXPathEvalMilliseconds to 0 or a negative value to remove the budget. Do this only against a trusted document; an unbounded expression over an adversarial document can then run without a wall-clock limit.

var options = new XIncludeOptions
{
    Enabled = true,
    MaxXPathEvalMilliseconds = 2000,
};
                                  

#Combining with other parts

An xpath1() part can follow element() in the same pointer, so the pointer falls through to an XPath expression when the ID-based lookup finds nothing. See Shorthand and element() for the element() scheme and its child-sequence syntax.