View source for Not a Refnum Constant

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{{LabVIEW Palette Object Information|palette=Functions Palette/Programming/File I/O/File Constants{{!}}File Constants palette|type=constant|icon=File Constants Palette - Not a Refnum Constant.png}}

'''Not a Refnum Constant''' returns a refnum whose value is <code>Not A Refnum</code>. 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 <code>Error</code> subdiagram and wire the constant through the output tunnel and to the next file I/O operation. In the <code>No Error</code> subdiagram, wire the refnum output of the file I/O function through the Case structure and to the next file I/O operation.
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== Usage ==
{| class="wikitable"
! Data Type
! Name
! Description
|-
! [[File:Generic Refnum.png|link=Reference data type]]
| '''Reference'''
| '''Reference''' returns a refnum whose value is <code>Not A Refnum</code>.
|}

=== Comparing References ===
Do not test whether a refnum is invalid by comparing the refnum to the output returned by the '''Not a Refnum constant'''. The '''Not a Refnum constant''' returns a unique sequence of bits that is guaranteed to be an invalid refnum. However, because this sequence of bits is only one possible invalid refnum representation, it likely will not equal another refnum to which you compare it. Instead, use the [[Not A Number/Path/Refnum? function]] to test whether a refnum is invalid. The [[Not A Number/Path/Refnum? function]] returns a [[Boolean|Boolean data type]] value that is correct for all possible invalid refnum representations.

== History ==
{{ambox|text=History information is needed.  What changes have occurred over previous versions?}}
{| class="wikitable"
! Version
! Change(s)
|-
|[[File:LV2018.png|frameless|border|64x64px|LabVIEW 2018|link=LabVIEW 2018]]
|More info to come.
|}
== See Also ==
* [[Reference data type]]

== External Links ==
{{ambox|text=Add links to external resources that would also help.}}

[[Category:File Constants Palette]]