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         


        

Answer 1

$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)

Answer 2

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