Function parameter validation in Powershell

Question

Why does a [string] casted parameter with the 'value' $null in this example never throw an error (empty or $null), but a string with the value '$null' always throws? I would expect if passing a mandatory parameter, it is checked for $null/emptyness and thus an error is always thrown in these cases:

Function test_M_NoE ( [Parameter(Mandatory=$true)] [ValidateNotNullOrEmpty()] [string] $x ) {}

# test cases. Uncomment one:
[string]$x = [string]$null 
# $x = [string]$null
# [string]$x = $null
# $x = $null 

"1:"; test_M_NoE [string]$x # never error
"2:"; test_M_NoE $x         # always error

Converting $null to [string] in PowerShell results in an empty string (eg. "" or [String]::Empty), so the argument passed is never actually $null once you cast to [string]. But in your case, [string]$x is not an actual cast expression - it's parsed in argument mode, and interpreted as an expandable string, eg. no different that test_M_NoE "[string]$x" — Mathias R. Jessen
– Mathias R. Jessen, Commented Sep 20, 2021 at 12:05
@MathiasR.Jessen Ok, but an empty string should also throw (ValidateNotNullOrEmpty) and it always does in the last statement. Why not if I cast the parameter? — jamacoe
– jamacoe, Commented Sep 20, 2021 at 12:09
The expression "[string]$null" results in a non-empty string — Mathias R. Jessen
– Mathias R. Jessen, Commented Sep 20, 2021 at 12:09
$x = [string]$null; test_M_NoE $x yields an error. What's the difference? — jamacoe
– jamacoe, Commented Sep 20, 2021 at 12:11

Mathias R. Jessen · Accepted Answer · 2021-09-20 12:19:55Z

2

The reason this works:

test_M_NoE [string]$x

Is that [string]$x is not being interpreted the way you expect.

Let's change your test function definition to help us better see what's actually going on:

function test_M_NoE {
  param(
    [Parameter(Mandatory=$true)]
    [ValidateNotNullOrEmpty()]
    [string]$x
  )

  Write-Host "Argument value passed was: '$x'"
}

Now, let's try again:

PS ~> $x = $null
PS ~> test_M_NoE [string]$x
Argument value passed was: '[string]'

Aha! The argument expression [string]$x did not result in an empty string - it resulted in the literal string value [string].

This is due to the fact that PowerShell attempts to parse command arguments differently from anything else. From the about_Parsing help topic:

Argument mode is designed for parsing arguments and parameters for commands in a shell environment. All input is treated as an expandable string unless it uses one of the following syntaxes: [...]

So really, PowerShell interprets our argument expression like a double-quoted string:

test_M_NoE "[string]$x"

At which point the behavior makes sense - $x is $null, so it evaluates to an empty string, and the result of the expression "[string]$x" is therefore just [string].

Enclose the argument expression in the $(...) subexpression operator to have it evaluated as a value expression instead of as an expandable string:

test_M_NoE $([string]$x)

answered Sep 20, 2021 at 12:19

Mathias R. Jessen

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2 Comments

jamacoe Over a year ago

That makes sense now, thank you. To me very unexpected though. So as a consequence I suppose one should not cast function parameters at all and always make sure the parameter already has got the correct type, true?

Mathias R. Jessen Over a year ago

@jamacoe no, not at all. My recommendation would be: don't do anything - PowerShell will automatically attempt to convert/coerce the argument to the target type, so your attempts at manually converting the argument to [string] explicitly is completely unnecessary.

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