How to validate PowerShell Function Parameters allowing empty strings?

Question

Please try this:

function f1 {     param(     [Parameter(Mandatory=$true,ValueFromPipelineByPropertyName=$true)]     [string]     $Text     )     $text }  function f2 {     param(     [Parameter(Mandatory=$true,ValueFromPipelineByPropertyName=$true)]     #[string]     $Text     )     $text }  function f3 {     param(     [Parameter(Mandatory=$False,ValueFromPipelineByPropertyName=$true)]     [string]     $Text     )     $text }  f1 '' f2 '' f3 '' 

Here f1 throws an error. Now try

f2 $null  f3 $null     

This time only f2 throws an error. What I want is a function f, so that

f '' # is accepted f $null # returns an error 

Answer 1

The Mandatory attribute blocks null and empty values and prompts you for a value. To allow empty values (including null) add the AllowEmptyString parameter attribute:

function f1 {     param(     [Parameter(Mandatory=$true,ValueFromPipelineByPropertyName=$true)]     [AllowEmptyString()]     [string]$Text     )     $text }