Checking for NULL in PS

PowerShell Help
1

Hello Scripting Experts,

I have extracted a simple file via the Import-Excel command as follows:
> Import-Excel $infile -Noheader -Dataonly -Startrow 2 | Select P2,P3,P4,P5 |
Foreach-Object { If ($_.P2 = NULL ???) …

My objective is to check the property P2 for a NULL (i.e., empty) value (not quite the same as blanks).
Will the following lines work: (the first line assigns to $Null the empty string using single-quote)
$Null = ‘’
ForEach-Object { If (($_.P2 -EQ $Null) -EQ $True) { true branch } Else { … }?

Would be grateful for any tips, advice or suggestions.

2

The variable $null already exists on Powershell … always. You don’t need to create or assign. If you like to check for $null you can simply do 

if ($_.P2 -eq $null) {…}

3

I use this

if([string]::IsNullOrWhiteSpace($variabletocheck))
4

Many thanks, much appreciated.

5

One other possibility that is a little more a case-by-case basis is that you can use the fact that when cast to bool, $null becomes $false, and most other things that contain data become $true. So you can do:

if (-not $_.P2) { 
# code here 
}

If $_.P2 is either $null, zero, a zero-length array, or contains the value $false, the if statement would execute. Just about anything else is $true and would skip the code in the script block.

6

“If $_.P2 is either $null, zero, a zero-length array, or contains the value $false, the if statement would execute”
But not if it’s Whitespace.

$P2 = "  "

if ( -Not $P2) { 
Write-Host "White space" 
} else { Write-Host "something" }

if([string]::IsNullOrWhiteSpace($P2)) { 
Write-Host "White space"
} else { Write-Host  "something" }
7

Thank you all Messrs. Barnetson, Sallow and Soyk. Your replies shed a lot of light and provide many interesting highlights. I have performed your examples hands-on, and I am extremely grateful for your posts. In the course of my “immersion” with NULL, I came across a very informative and interesting site/post, with a short article on the same subject:

I thought it interesting to extract that portion of the article to share this author’s “all in one” NULL function:
#-----------------------------------------------------
function IsNull($objectToCheck) {
if ($objectToCheck -eq $null) { return $true }

if ($objectToCheck -is [String] -and $objectToCheck -eq [String]::Empty) { return $true }

if ($objectToCheck -is [DBNull] -or $objectToCheck -is [System.Management.Automation.Language.NullString]) { return $true }
return $false

}
#----------------------------------------------------
Sincerest thanks to you all for sharing your expertise.