How to validate PowerShell Function Parameters allowing empty strings?

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}

Here is the solution that fits the requirements with a caveat.

function f1{    param(    [Parameter(Mandatory=$true,ValueFromPipelineByPropertyName=$true)]    $Text    )    Write-Host 'Working'    $text}f1 ''f1 $null

Output:

Workingf1 : Cannot bind argument to parameter 'Text' because it is null.

Caveat

In order to fit the requirements we have to omit the explicit type declaration of [string]. The problem is that PowerShell tends to convert nulls to empty strings everywhere where the [string] type is specified. Thus, if we use the type declaration then null value actually never comes to the function.

P.S. Here is a related issue submitted:It isn't possible to pass null as null into a .NET method that has a parameter of type String

Just for the sake of completeness, if you want your input be validated against string type, you can do it afterward parameters declaration:

function f1{    param(    [Parameter(Mandatory=$true,ValueFromPipelineByPropertyName=$true)]    $Text    )    if (!($text -eq '') -and !($text -as [string])) {write-host "wrong type"; return }    $text}

This function behaves as follows:

• Throws "Cannot bind argument.." exception when input is $null
• Passes when input is an empty string
• Exits with message wrong type when input is not a string