# Converting the Result of a .NET Extension Function

This section explains how the value returned by a .NET extension function is converted to an XPath value.

The result type of the method is converted to an XPath value as follows.

- If the method returns void, the XPath value is an empty sequence.

- If the method returns null, the XPath value is an empty sequence.

- If the method is a constructor, the XPath value is of type "wrapped .NET object". The only way of using this is by passing it to another external function, or by converting it to one of the standard XPath data types as described above.

- If the returned value is an `XdmValue` or one of its subclasses such as `XdmNode` or `XdmAtomicValue`, then it is used as is, after unwrapping. (These types are defined in the Saxon.Api namespace). Note that if the method constructs a new XdmNode and returns it, then it must be built using the Name Pool that is in use by the transformation or query, which means in practice that it must be built using a `DocumentBuilder` derived from the same `Saxon.Api.Processor`.

- If the returned value is a .NET bool, the XPath result is a boolean.

- If the returned value is a .NET double, the XPath result is a double.

- If the returned value is a .NET float, the XPath result is a float.

- If the returned value is a .NET Int64, Int32, or Int16, or one of their object wrapper equivalents, the XPath result is an integer.

- If the returned value is a .NET string, the XPath result is a string.

- If the returned value is an instance of the Saxon class `net.sf.saxon.om.NodeInfo` (a node in a Saxon tree), the XPath value will be a sequence containing a single node.

- If the returned value is an instance of the Saxon class `net.sf.saxon.value.Value`, the returned value is used unchanged.

- If the returned value is is an instance of the Saxon class `net.sf.saxon.om.SequenceIterator` (an iterator over a sequence), the XPath value will be the sequence represented by this iterator. It is essential that this iterator properly implements the method `getAnother()` which returns a new iterator over the same sequence of nodes or values, positioned at the start of the sequence.

- If the returned value is an instance of the .NET interface `System.Collections.IEnumerable`, or if it is an array, the XPath value will be the sequence represented by the contents of this collection or array. The members of the list or array will each be converted to an XPath value, as if each member was supplied from a separate function call. An error is reported if the result contains a list or array nested within another list or array. The contents of the list or array are copied immediately on return from the function, so the original collection or array object itself may be safely re-used.

- If the result is any other Java object (including null), it is returned as a "wrapped Java object".

In XSLT, the system function `function-available(String name)` returns true if there appears to be a method available with the right name. The function also has an optional second argument to test whether there is a method with the appropriate number of arguments. However, it is not possible to test whether the arguments are of appropriate types. If the function name is "new" it returns true so long as the class is not an abstract class or interface, and so long as it has at least one constructor.

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