Comparing Against $NULL

If you want to find out whether a variable contains $Null (nothing), always make sure you keep $null on the left side of the comparison. Most of the time, the order does not really matter:

 
PS C:\> $a = $null

PS C:\> $b = 12

PS C:\> $a -eq $null
True

PS C:\> $b -eq $null
False
 

However, if a variable contains an array, placing the array on the left side of the comparison operator makes it work like a filter. So now order becomes vital:

# this all produces inconsistent and fishy results
 
$a = $null
$a -eq $null  # works: returns $true
 
$a = 1,2,3
$a -eq $null  # fails: returns $null
 
$a = 1,2,$null,3,4
$a -eq $null  # fails: returns $null
 
$a = 1,2,$null,3,4,$null,5
$a -eq $null  # fails: returns array of 2x $null
($a -eq $null).Count

If you place the variable on the left side, PowerShell checks for $null values inside the array and returns these, or $null if there are no such values.

If you place the variable on the right side, PowerShell checks whether the variable is $null.

# by reversing the operands, all is FINE:
 
$a = $null
$null -eq $a  # works: $true
 
$a = 1,2,3
$null -eq $a  # works: $false
 
$a = 1,2,$null,3,4
$null -eq $a  # works: $false
 
$a = 1,2,$null,3,4,$null,5
$null -eq $a  # works: $false

This can be eliminated by placing $null on the left rather than on the right side of the comparison.

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