Find out if variable is null or an array containing null variables in PowerShell

Question

I am trying to find out if a variable is null or is an array containing null variables.

So for example, a short expression that will match 'false' for each of the following, but 'true' for anything else:

$nullvariable = $null
$nullvariable1 = @($null)
$nullvariable2 = @($null, $null)
$nullvariable3 = @("1", $null)

# Update - Also need the following to match 'false' as it also breaks Compare-Object
$nullvariable4 = @()

Context: I have some Compare-Object calls where I am trying to avoid 'Cannot bind argument to parameter 'ReferenceObject' because it is null' errors. The first answer here: https://stackoverflow.com/a/45634900/12872270 works but it's not very legible/comprehensible code.

The remaining answers use an 'if' statement which is more legible, but in my testing Compare-Object doesn't just fail for $null, but also any array containing null entries - the given examples using 'if' don't account for that.

Detecting an array containing null variables seems like a separate problem in its own right anyway, hence the question.

"I am trying to find out if a variable is null or is an array containing null variables." $null -in $variable would work for all those examples in your question — Santiago Squarzon
– Santiago Squarzon, Commented Mar 3, 2023 at 15:22
@SantiagoSquarzon Fab! Never thought of using that. Make it as an answer and I'll happily accept :) — Minkus
– Minkus, Commented Mar 3, 2023 at 15:25
@SantiagoSquarzon And to be honest I'm not sure I've seen that syntax recommended anywhere, not even in the usually fantastic learn.microsoft.com/en-us/powershell/scripting/learn/deep-dives/… — Minkus
– Minkus, Commented Mar 3, 2023 at 15:26

Santiago Squarzon · Accepted Answer · 2023-03-03 15:32:22Z

2

Since containment operators can evaluate against scalars as well as collections, in this case you can either use -contains or -in with any of your variables and the result would be $true:

$null -in $nullvariable         # True
$nullvariable1 -contains $null  # True
$null -in $nullvariable2        # True
$nullvariable3 -contains $null  # True

answered Mar 3, 2023 at 15:32

Santiago Squarzon

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Minkus Over a year ago

OK, found another scenario where Compare-Object fails - $nullvariable4 = @() This fails with this check. Any other ideas?

Minkus · Accepted Answer · 2024-10-14 11:25:22Z

0

Improving on Santiago Squarzon's helpful answer, which is correct for all the instances I originally supplied, but Compare-Object can still fail when $nullvariable is empty:

$nullvariable = $null
$nullvariable1 = @($null)
$nullvariable2 = @($null, $null)
$nullvariable3 = @("1", $null)
$nullvariable4 = @()

Adding another check as follows seems to do the job:

(!$nullvariable -or $null -in $nullvariable)    # True
(!$nullvariable1 -or $null -in $nullvariable1)  # True
(!$nullvariable2 -or $null -in $nullvariable2)  # True
(!$nullvariable3 -or $null -in $nullvariable3)  # True
(!$nullvariable4 -or $null -in $nullvariable4)  # True

answered Oct 14, 2024 at 11:25

Minkus

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