How do I assign a null value to a variable in PowerShell? - powershell

Question

I want to assign a null value to a variable called $dec, but it gives me errors. Here is my code:
import-module activedirectory
$domain = "domain.example.com"
$dec = null
Get-ADComputer -Filter {Description -eq $dec}

Answer 1

These are automatic variables, like $null, $true, $false etc.
about_Automatic_Variables, see https://technet.microsoft.com/en-us/library/hh847768.aspx?f=255&MSPPError=-2147217396
$NULL
$null is an automatic variable that contains a NULL or empty
value. You can use this variable to represent an absent or undefined
value in commands and scripts.
Windows PowerShell treats $null as an object with a value, that is, as
an explicit placeholder, so you can use $null to represent an empty
value in a series of values.
For example, when $null is included in a collection, it is counted as
one of the objects.
C:\PS> $a = ".dir", $null, ".pdf"
C:\PS> $a.count
3
If you pipe the $null variable to the ForEach-Object cmdlet, it
generates a value for $null, just as it does for the other objects.
PS C:\ps-test> ".dir", $null, ".pdf" | Foreach {"Hello"}
Hello
Hello
Hello
As a result, you cannot use $null to mean "no parameter value." A
parameter value of $null overrides the default parameter value.
However, because Windows PowerShell treats the $null variable as a
placeholder, you can use it scripts like the following one, which
would not work if $null were ignored.
$calendar = #($null, $null, “Meeting”, $null, $null, “Team Lunch”, $null)
$days = Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"
$currentDay = 0
foreach($day in $calendar)
{
if($day –ne $null)
{
"Appointment on $($days[$currentDay]): $day"
}
$currentDay++
}
output:
Appointment on Tuesday: Meeting
Appointment on Friday: Team lunch

Answer 2

Use $dec = $null
From the documentation:
$null is an automatic variable that contains a NULL or empty value. You can use this variable to represent an absent or undefined value in commands and scripts.
PowerShell treats $null as an object with a value, that is, as an explicit placeholder, so you can use $null to represent an empty value in a series of values.

Answer 3

If the goal simply is to list all computer objects with an empty description attribute try this
import-module activedirectory
$domain = "domain.example.com"
Get-ADComputer -Filter '*' -Properties Description | where { $_.Description -eq $null }

Answer 4

As others have said, use $null.
However, the handling of $null is not so simple.
In lists (or, more precisely, System.Array objects) $null is treated as a placeholding object when indexing the list, so ($null, $null).count outputs 2.
But otherwise $null is treated as a flag signifying that there is no content (no object; or, more precisely, a "null-valued expression", as reported by .GetType()), so ($null).count outputs 0.
Thus
$null.count; # Output = 0
($null).count; # Output = 0
(, $null).count; # Output = 1
($null, $null).count; # Output = 2
($null, $null, $null).count; # Output = 3
Note: the same output is returned from .count and .length in the above context.
Similarly if explicitly assigning any of the above to a variable, as in
$aaa = $null; $aaa.count
$bbb = ($null, $null); $bbb.count
which output, respectively, 0 and 2.
Similarly if looping with ForEach, as in
$aaa = $null; ForEach ($a in $aaa) {write-host "Foo" -NoNewLine}
$bbb = ($null, $null); ForEach ($b in $bbb) {write-host "Bar" -NoNewLine}
which output, respectively, nothing and BarBar.
However, note well that when operating on an individual item that has been returned from a list $null is again treated as a "null-valued expression", as can be confirmed by running
$xxx = ($null, "foo", $null); ForEach ($x in $xxx) {write-host "C=" $x.count "| " -NoNewLine}
which outputs C= 0 | C= 1 | C= 0 | .