Using BizTalk xpath() Function to Retrieve Optional Nodes

I often suffer a bit of trepidation when trying to retrieve values for optional XML nodes using the built-in xpath() function in an orchestration, as I wonder if it will result in an exception of some kind if the node isn’t found in the message.  This concern is heightened by reading Microsoft’s own documentation of the xpath function on MSDN:

The engine is not schema-aware, so you can only read values from or write values to a node that exists in the containing message (the complete path must exist), or the engine will raise an exception. This is true even if you supply a default value.

The conclusion from this is that if the node doesn’t exist in the message instance, you’ll get a runtime error trying to read from it using xpath(), right?

Wrong! In fact, as Leonid Ganeline proved in an older blog post, you will either get an empty string or “null”, depending on whether you wrap the XPATH expression in a string() function or not:

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