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Returns a refnum whose value is Not A Refnum. You can use this refnum as an output from structures and subVIs when an error occurs.

For example, you can use the not a refnum constant to verify that the refnum is valid before passing the refnum to the next file I/O operation. Wire the error out cluster of a file I/O function to the selector terminal of a Case structure. Place the not a refnum constant in the Error subdiagram and wire the constant through the output tunnel and to the next file I/O operation. In the No Error subdiagram, wire the refnum output of the file I/O function through the Case structure and to the next file I/O operation.