Using BizTalk's command xpath(msg,string) correctly

If you want to use BizTalk's xpath()-method in an Expression shape of Visual Studio's BizTalk Orchestration designer, you have to think of the return of the xpath carefully.

If you use the following:

[string] s = xpath(msg1, "/*[local-name()='Root' and namespace-uri()='http://TestBiztalkXPathCommand.Schema1'] /*[local-name()='name' and namespace-uri()='']")

to get the value at node "name", you'll receive the following error while running the orchestration:

...
Inner exception: There is an error in the XML document.
Exception type: InvalidOperationException
Source: System.Xml
[...]
Additional error information:
was not expected.
[...]
Target Site: System.Object Read_string()


The reason is that this XPath statements navigates to an XML node and, thus, an object of the class System.Xml.XmlNode is returned (and not a String). Since the XPath statement is not analyzed while building the DLL, the error occurs while running primary.

Thus, you should add /text() to your XPath statement, so that a string is returned. In this case, the following would work successfully:

[string] s = xpath(msg1, "/*[local-name()='Root' and namespace-uri()='http://TestBiztalkXPathCommand.Schema1'] /*[local-name()='name' and namespace-uri()='']/text()")


Charles Young's blogpost on xpath() was helpful to fix this problem.

Addition (June 3, 2008) : My colleague Christian found out that you should invoke some mysterious "string()"-method inside the xpath statement:

xpathExpression = "/*[local-name()='Root' and namespace-uri()='http://TestBiztalkXPathCommand.Schema1'] /*[local-name()='name' and namespace-uri()='']";
[string] s = xpath(msg1, "string(" + xpathExpression + "/text())")

Thus, you get less XPath errors (I don't know yet how that exactly works; but I know that string(...) is not an XPath expression).