Mastering Windows PowerShell Scripting - Third Edition

The AllowNull attribute

AllowNull is used to permit explicit use of $null as a value for a Mandatory parameter:

function Test-AllowNull {    [CmdletBinding()]    param (        [Parameter(Mandatory)]        [AllowNull()]        [Object]$Parameter1    )}

AllowNull is effective for array parameters, and for parameters that use Object as a type. AllowNull is not effective for string parameters as the null value is cast to an empty string, and an empty string is still not permitted.