Converting empty string to $null back to the same variable

Question

We have a few imports via PowerShell in to Active Directory that have a couple fields that come across as an empty string from the datasource, but need to be set as $null within Active Directory.

Since there are quite a few of these fields, I attempted to create a function that will convert an empty string to $null.

The trouble is that if I set the variable back to itself, it remains an empty string. If I set it as a new variable, it works fine.

function Get-ValueOrNull
{
    param(
        [Parameter(Mandatory=$true)]
        [AllowEmptyString()]
        [string]$Value
    )

    if ([string]::IsNullOrEmpty($Value))
    {
        return $null
    }

    return [string]$Value
}

function Test-Function
{
    param(
        [Parameter(Mandatory=$true)]
        [AllowEmptyString()]
        [string]$TestValue
    )
    $TestValue = Get-ValueOrNull -Value $TestValue
    $TestValue2 = Get-ValueOrNull -Value $TestValue

    Write-Host "TestValue: $($TestValue -eq $null)"
    Write-Host "TestValue2: $($TestValue2 -eq $null)"
}

Test-Function -TestValue ""

Here the output is

PS C:\> .\Test-Function.ps1
TestValue: False
TestValue2: True

This is clearly something I'm not understanding about Types in PowerShell function parameters. I can change the [string]$TestValue to $TestValue, and it will work.

function Test-Function
{
    param(
        [Parameter(Mandatory=$true)]
        [AllowEmptyString()]
        $TestValue
    )
    ...
}
...

Output:

PS C:\> .\Test-Function.ps1
TestValue: True
TestValue2: True

The reason I'd like to preserve the [string] parameter type is to enforce that it should be a string or an empty string. Can someone explain what is going on here?

briantist · Accepted Answer · 2017-03-06 18:35:23Z

Once you've casted the variable as opposed to the value being assigned you are strictly typing that variable.

This is easier to see with an [int] because basically anything can be casted to a [string] successfully:

$v = [int]'5'
$v.GetType()  # int

$v = 'hello'
$v.GetType()  # string

[int]$v = '5'
$v.GetType()  # int

$v = 'hello'
# Exception:
# Cannot convert value "hello" to type "System.Int32". Error: "Input string was not in a correct format."

When you type a parameter, the variable that contains the parameter is the same way; you can reassign it, but the right side must be assignable / castabale / convertible to the left side's type.

A $null cast as [string] is an empty string:

([string]$null) -eq ([string]::Empty)  # True

You can still strongly type your parameter, if you use a different intermediate variable in the function that isn't, as you demonstrated with $TestValue2.

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