datasource verification

Is there a simple way (QI) to determine the current  datasource if the
resource has a urn style value?

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for example if a resource (about)  has no value then and a user clicks on a
tree item
with a click event code  (flags="dont-build-content")

      var object =
widget.builderView.getResourceAtIndex(widget.currentIndex)
      var
urn=object.QueryInterface(Components.interfaces.nsIRDFResource).Value;

then the current datasource is returned

BUT if resource has a valid urn such as

           <RDF:Description about="urn:abc" />

then only the urn is returned

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PS I did think about work arounds such as deleting the urn , then QI the
resource then
restore the urn - but this seems a bit crude (even it would work)