https://powershell.org/forums/topic/how-to-validate-multiple-items-in-a-parameter-using-validatescript/

IAmSirAskAlot.ps1

function Invoke-SomeTask {
    [CmdletBinding()]
    param(
        [Parameter(Mandatory, ValueFromPipeline)]
        [ValidateNotNullOrEmpty()]
        [string[]]
        $ComputerName
    )
    begin {}
    process {
        foreach ($Computer in $ComputerName) {
            try {
                if (Test-Connection $Computer -Quiet) {
                    # Do some task
                }
                else {
                    throw "$Computer is not reachable; the task was not performed."
                }
            }
            catch {
                Write-Error $_
            }
        }
    }
    end {}
}

Invoke-SomeTask -ComputerName 'TestSystem1', 'TestSystem2'