Classes that inherit from ValidateEnumeratedArgumentsAttribute may be used to test each of the elements in an array (when associated with an array-based parameter), or a single item (when associated with a scalar parameter).

Classes that implement ValidateEnumeratedArgumentsAttribute must inherit from  System.Management.Automation.ValidateEnumeratedArgumentsAttribute. The class must implement a Validate method that is marked as abstract in the ValidateEnumeratedArgumentsAttribute class.

The ValidateElement method accepts one argument with the System.Object type. This is shown in the .NET reference: https://docs.microsoft.com/en-us/dotnet/api/system.management.automation.validateenumeratedargumentsattribute.validateelement?view=powershellsdk-1.1.0.

The ValidateElement method does not return any output; it either runs successfully or throws an error. The error will be displayed to the end user.

The following validates that an IP address used as an argument falls in a private address range. If the address is not part of a private range, or not a valid IP address, the command will throw an error:

using namespace System.Management.Automation

class ValidatePrivateIPAddressAttribute : ValidateEnumeratedArgumentsAttribute {
    Hidden $ipAddress = [IPAddress]::Empty

    Hidden [Boolean] IsValidIPAddress([String]$value) {
        return [IPAddress]::TryParse($value, [Ref]$this.ipAddress)
    }

    Hidden [Boolean] IsPrivateIPAddress([IPAddress]$address) {
        $bytes = $address.GetAddressBytes()
        $isPrivateIPAddress = switch ($null) {
            { $bytes[0] -eq 192 -and 
              $bytes[1] -eq 168 } { $true; break }
            { $bytes[0] -eq 172 -and 
              $bytes[1] -ge 16 -and
              $bytes[2] -le 31 } { $true; break }
            { $bytes[0] -eq 10 } { $true; break }
            default { $false }
        }
        return $isPrivateIPAddress
    }

    [Void] ValidateElement([Object]$element) {
        if (-not $element -is [IPAddress]) {
            if ($this.IsValidIPAddress($element)) {
                $element = $this.ipAddress
            } else {
                throw '{0} is an invalid IP address format' -f $element
            }
        }
        if (-not $this.IsPrivateIPAddress($element)) {
            throw '{0} is not a private IP address' -f $element
        }
    }
}

The attribute defined in the preceding code may be used with any parameter to validate IP addressing, as shown in the following short function:

function Test-Validate {
    [CmdletBinding()]
    param (
        [ValidatePrivateIPAddress()]
        [IPAddress]$IPAddress
    )
 
    Write-Host $IPAddress
}

Validation like this can be implemented with ValidateScript, which also inherits from ValidateEnumeratedArgumentsAttribute. ValidateScript can call functions, centralizing the validation code.