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.
