C# extension functions: simple interface

A simple extension function can be registered in SaxonCS by calling the method

processor.RegisterExtensionFunction(QName, XdmItemType, Func<XdmValue[], XdmValue>)

The first argument is the name of the function, for example new QName("urn:math", "sqrt").

The second argument is the function type. This is most conveniently provided using XPath ItemType syntax, for example processor.ParseItemType("function(xs:double?) as xs:double?").

The third argument is the implementation of the function, which can conveniently be supplied as a simple lambda expression. For example, a square-root function might be:

proc.RegisterExtensionFunction( new QName("urn:math", "sqrt"), proc.ParseItemType("function(xs:double) as xs:double"), val => new XdmAtomicValue(Math.sqrt((XdmAtomicValue)val[0][0].AsDouble()))

This can then be invoked from XPath as Q{urn:math}sqrt(2), or as m:sqrt(2) if the namespace prefix m has been bound to the URI urn:math.

The expression val[0][0] selects the first item in the sequence that was supplied as the first argument; this is then cast to an XdmAtomicValue to make the AsDouble() method available; this extracts the value as a C# double. Saxon takes care of the type checking (and where necessary, type conversion) to ensure that there will be exactly one argument, and that it will be a single item of type xs:double: the function implementation does not need to repeat these checks.

The main restrictions of the simple interface are that the extension function has no access to static or dynamic context information. To avoid these restrictions, use the full interface described on the next page.