Why does passing $null to a parameter with AllowNull() result in an error?

Question

Consider the following code:

function Test
{
    [CmdletBinding()]
    param
    (
        [parameter(Mandatory=$true)]
        [AllowNull()]
        [String]
        $ComputerName
    ) 
    process{}
}

Test -ComputerName $null

Based on the official documentation for AllowNull I was expecting that $ComputerName could either be [string] or $null. However, running the above code results in the following error:

[14,24: Test] Cannot bind argument to parameter 'ComputerName' because it is an empty string.

Why doesn't passing $null for $ComputerName work in this case?

I can't give you a definitive explanation, but to me it looks like $null is cast to an empty string, due to the parameter type, and thus isn't covered by the validation attribute anymore. Using AllowEmptyString() instead works. I'd consider this a documentation bug. — Ansgar Wiechers
– Ansgar Wiechers, Commented Aug 5, 2015 at 22:09
But you can't do Test -ComputerName with no value specified? Why not? — john v kumpf
– john v kumpf, Commented Oct 17, 2024 at 15:11
@johnvkumpf Because the attribute parameter has argument Mandatory=true. — alx9r
– alx9r, Commented Oct 18, 2024 at 15:40

user4003407 · Accepted Answer · 2015-08-05 22:30:19Z

8

$null, when converted to [string], return empty string not $null:

[string]$null -eq $null # False
[string]$null -eq [string]::Empty # True

If you want to pass $null for [string] parameter you should use [NullString]::Value:

[string][NullString]::Value -eq $null # True
Test -ComputerName ([NullString]::Value)

answered Aug 5, 2015 at 22:30

user4003407

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1 Comment

Vladislav Zalesak Over a year ago

Just one warning, tihs doesnt work on 2.0 as far as i could say

Aaron Jensen · Accepted Answer · 2015-08-05 22:15:37Z

3

You also need to add the [AllowEmptyString()] attribute if you plan on allowing nulls and empty strings.

function Test
{
    [CmdletBinding()]
    param
    (
        [parameter(Mandatory=$true)]
        [AllowNull()]
        [AllowEmptyString()]
        [String]
        $ComputerName
    ) 
    process{}
}

Test -ComputerName $null

answered Aug 5, 2015 at 22:15

Aaron Jensen

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Comments

Grady Patterson · Accepted Answer · 2022-02-08 23:45:02Z

0

As noted, when cast to a [string], $null becomes an empty string (""). To avoid this, one option is to type the param as an object - then in the body test first for $null, then check that $MyParam.GetType().Name is "String".

answered Feb 8, 2022 at 23:45

Grady Patterson

1

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