I built an XPath expression to extract the contents of an XML element.  I tested this XPath in a utility I created that executes the expression, and writes the results to the file system.  The XPath expression worked perfectly, and returned the contents of the XML element.  Here's a quick and dirty example ...

My XML file:

<ns0:Example xmlns:ns0="http://example/">
 <ns0:Contents>1169861</ns0:Contents>
</ns0:Example>

And my XPath expression:

/*[local-name()='Example' and namespace-uri()='http://example/']/*[local-name()='Contents' and namespace-uri()='http://example/']/text()

(Don't forget the "/text()" at the end, or you'll get the entire element!)

And, using my utility, I get the following (expected!) output:

1169861

However, when run in BizTalk 2006, I got that ever-so-annoyingly-generic error message:

Object reference not set to an instance of an object.

I confirmed that this error occured when I set my orchestration's local numeric variable equal to the the following expression in my BizTalk orchestration:

xPathExpression = "/*[local-name()='Example' and namespace-uri()='http://example/']/*[local-name()='Contents' and namespace-uri()='http://example/']/text()";

contents = xpath(msgExample, xPathExpression);

What gives?  The XPath expression works perfectly in my utility, but it doesn't in a BizTalk orchestration?  So, I decided see if I could find any help on Google.  No joke, I searched for "biztalk xpath gotcha" and found Aaron Saikovski's blog detailing this problem.  His note indicated that, in BizTalk 2004, you had to wrap your XPath expressing with string()!  It's that simple.  So, changing my XPath to the following ...

xPathExpression = "string(/*[local-name()='Example' and namespace-uri()='http://example/']/*[local-name()='Contents' and namespace-uri()='http://example/']/text())";

... saved the day!

What a fun quirk!  Thanks Aaron!