Converting Atomic Values and Sequences

The following conversions are supported between the static type of the supplied value of the argument, and the declared .NET type of the argument. The mappings are given in order of preference; a type that appears earlier in the list has smaller "conversion distance" than one appearing later. These priorities are used to decide which method to call when the class has several methods of the same name. Simple classes (such as boolean) are acceptable wherever the corresponding wrapper class (Boolean) is allowed. Class names shown in italics are Saxon-specific classes.

If the value is a .NET external object (typically the result of a call to another extension function), then the underlying .NET object is passed to the method. An error will occur if it is the wrong .NET type.

If the required type is one of the types used in the Saxon.Api namespace to represent XPath values (for example XdmValue, XdmNode, XdmAtomicValue), then the value is converted to an instance of this class and passed over unchanged. If the expected type is XdmAtomicValue and the supplied value is a node, the node will be atomized (this must result in a single atomic value). Use of types such as XdmValue is available only when the query or transformation is invoked using the .NET API, it does not work when running from the command line.

If the static type of the supplied value allows a sequence of more than one item, then Saxon looks for a method that expects a net.sf.saxon.om.SequenceIterator, a net.sf.saxon.om.Sequence, an ICollection or an array, in that order. (The first two classes are Saxon-specific.) In all these cases except the last, the item type of the supplied value plays no role.

Nodes in the supplied sequence are atomized only if the .NET method requires an atomic type such as an integer or string. If the method requires an ICollection, then the contents of the sequence will be supplied unchanged. The objects in the list will typically be Saxon net.sf.saxon.om.Item objects. (Saxon does not yet recognize the generic types in .NET 2.0, which allow the item type of a collection to be declared.) If atomization is required, you can force it by calling the data() function.

If the required type is an array, Saxon will attempt to create an array of the required type. This will not always succeed, for example if the array has type X[] where X is an interface rather than a concrete class. If it is an array of items or nodes, the nodes in the supplied sequence will be inserted into the array directly; if it is an array of a type such as integer or double, the sequence will first be atomized.

If the supplied value is a singleton (a sequence of one item) then the type of that item is decisive. If it is a sequence of length zero or more than one, then the general rules for a sequence are applied, and the types of the items within the sequence are irrelevant.

If the supplied value contains more than one item and only a single item is expected, an error is reported. There is no implicit extraction of the first value (as happened in earlier releases).

Saxon tries to select the appropriate method based on the static type of the arguments to the function call. If there are several candidate methods, and there is insufficient information available to decide which is most appropriate, an error is reported. The remedy is to cast the arguments to a more specific type.