How to use a ValidateSet and still allow $null on an Array parameter?

Question

I've got a method that needs to accept an array of strings as a parameter, but that array can only contain valid strings. My problem is that if I ensure to [AllowNull()], and also [AllowEmptyCollection()], the method still fails

function SomethingSomethingAuthType {
    param(
        [parameter(Mandatory=$true,position=0)] 
        [ValidateSet('anonymousAuthentication','basicAuthentication','clientCertificateMappingAuthentication','digestAuthentication','iisClientCertificateMappingAuthentication','windowsAuthentication')] 
        [AllowNull()] 
        [AllowEmptyCollection()] 
        [array] $authTypes
    )

    $authTypes | % {
        Write-Host $_ -f Green
    }

}

SomethingSomethingAuthType $null

SomethingSomethingAuthType : Cannot validate argument on parameter 'authTypes'. The argument is null, empty, or an element of the argument collection contains a null value. Supply a collection that does not contain any null values and then try the command again. At line:16 char:32 + SomethingSomethingAuthType $null + ~~~~~ + CategoryInfo : InvalidData: (:) [SomethingSomethingAuthType], ParameterBindingValidationException + FullyQualifiedErrorId : ParameterArgumentValidationError,SomethingSomethingAuthType

What do I need to do to allow $null to be passed in, but also validate the set to ensure the appropriate types?

Chase Florell · Accepted Answer · 2015-04-15 17:14:00Z

The answer here would be to use an [Enum[]] instead of [array], and remove the ValidateSet all together.

function SomethingSomethingAuthType {
    param(
        [parameter(Mandatory=$true,position=0)] 
        [AllowNull()] 
        [AllowEmptyCollection()] 
        [AuthType[]] $authTypes
    )

    Write-Host 'Made it past validation.'

    if(!$authTypes) { return }

    $authTypes | % {
        Write-Host "type: $_" -f Green
    }

}

# Check if the enum exists, if it doesn't, create it.
if(!("AuthType" -as [Type])){
 Add-Type -TypeDefinition @'
    public enum AuthType{
        anonymousAuthentication,
        basicAuthentication,
        clientCertificateMappingAuthentication,
        digestAuthentication,
        iisClientCertificateMappingAuthentication,
        windowsAuthentication    
    }
'@
}

# Testing
# =================================

SomethingSomethingAuthType $null                                          # will pass
SomethingSomethingAuthType anonymousAuthentication, basicAuthentication   # will pass

SomethingSomethingAuthType invalid                                        # will fail
SomethingSomethingAuthType anonymousAuthentication, invalid, broken       # will fail

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