XMLDSig Specify an element reference as an XPath expression

Question

Someone who has experience with xml digital signature, please help.

From http://www.w3.org/TR/xmldsig-core/ I realized that in xmldsig I can use this code

<Signature ..> <SignedInfo> ... <Reference URI="#xpointer(anyXPathExpression)"> ... 

So, if I have such a document

 <Document xmlns="my-document"> <DocumentData> <Name>Nobuo document</Name> </DocumentData> </Document> 

And I want a sign /Document/DocumentData element, I can use such a signature

 <Document xmlns="swis-document"> <DocumentData> <Name>Nobuo document</Name> </DocumentData> <Signature xmlns="http://www.w3.org/2000/09/xmldsig#"> <SignedInfo> ... <Reference URI="#xpointer(/*[local-name()='Document'][1]/*[local-name()='DocumentData'][1])"> ... 

Am I right in my thoughts?

I tried to specify such a URI when using the SignedXml class in the .NET Framework. And the ComputeSignature method ComputeSignature with a CryptoGraphicsException:Malformed reference element error.

SignedXml does not support XPath, I suppose?