PowerShell IsNullOrEmpty vs IsNullOrWhiteSpace

In PowerShell, IsNullOrEmpty vs IsNullOrWhiteSpace comes down to how they treat spaces. IsNullOrEmpty only catches $null and empty strings, while IsNullOrWhiteSpace also catches strings made only of spaces, tabs or line breaks:

[string]::IsNullOrEmpty('   ')        # False
[string]::IsNullOrWhiteSpace('   ')   # True

For user input and data from files, IsNullOrWhiteSpace is almost always the one you want. Below are both checks side by side, a common if-statement trap and parameter validation.

Side-by-side comparison

I ran both methods against the values that usually cause trouble:

$samples = [ordered]@{
    '$null'          = $null
    "'' (empty)"     = ''
    "'   ' (spaces)" = '   '
    'Tab'            = "`t"
    'Line break'     = "`r`n"
    'No-break space' = [string][char]0x00A0
    "'Austin'"       = 'Austin'
}
foreach ($name in $samples.Keys) {
    [pscustomobject]@{
        Value              = $name
        IsNullOrEmpty      = [string]::IsNullOrEmpty($samples[$name])
        IsNullOrWhiteSpace = [string]::IsNullOrWhiteSpace($samples[$name])
    }
}

Output:

Value          IsNullOrEmpty IsNullOrWhiteSpace
-----          ------------- ------------------
$null                   True               True
'' (empty)              True               True
'   ' (spaces)         False               True
Tab                    False               True
Line break             False               True
No-break space         False               True
'Austin'               False              False

The no-break space is the sneaky one. It often comes in when you copy text from a web page or Word, and IsNullOrWhiteSpace treats it as blank too.

	IsNullOrEmpty	IsNullOrWhiteSpace
$null	True	True
''	True	True
Spaces, tabs, line breaks	False	True
Any visible text	False	False
Available since	.NET Framework 2.0	.NET Framework 4.0

Both are .NET methods, so they work the same in PowerShell 7 and 5.1.

For a full guide to the first method, see PowerShell IsNullOrEmpty.

Why if ($value) isn’t enough

A plain if statement treats any non-empty string as True, including one that’s only spaces:

$city = '   '
if ($city) { 'if ($city) says: has a value' }
if ([string]::IsNullOrWhiteSpace($city)) { 'IsNullOrWhiteSpace says: blank' }

Output:

if ($city) says: has a value
IsNullOrWhiteSpace says: blank

So if ($city) works like IsNullOrEmpty, not IsNullOrWhiteSpace. Use the method whenever spaces could sneak in. The IsNullOrEmpty guide covers more of these checks.

Clean up blank values in imported data

CSV files and Excel exports often have cells with a stray space. IsNullOrWhiteSpace finds them along with the truly empty ones:

$rows = @'
Order,City
ORD-3001,Austin
ORD-3002,
ORD-3003,"   "
ORD-3004,Denver
'@ | ConvertFrom-Csv

$rows | Where-Object { [string]::IsNullOrWhiteSpace($_.City) } |
    ForEach-Object { "$($_.Order) has no city" }

Output:

ORD-3002 has no city
ORD-3003 has no city

IsNullOrEmpty would only have caught ORD-3002. For filtering objects in a pipeline, see Where-Object for null or empty values.

Reject blank parameters with validation attributes

Instead of checking inside a function, let PowerShell reject bad input. PowerShell 7.4 added ValidateNotNullOrWhiteSpace:

function Send-Report {
    param([ValidateNotNullOrWhiteSpace()][string]$City)
    "Report for $City"
}

try { Send-Report -City '   ' }
catch { "Failed. $($_.Exception.Message)" }

Output:

Failed. Cannot validate argument on parameter 'City