powershellclass.psm1

#FAILS
class Test {
    [void] static Foo($zipfile) {
        Add-Type -AssemblyName System.IO.Compression.FileSystem  
        $ziparchive = [System.IO.Compression.ZipFile]::OpenRead($zipfile) #fails at runtime, and ISE puts squiggly lines under System.IO.Compression.ZipFile
        $ziparchive.Entries | % { Write-Host $_.Name }
    }

}

powershellFunction.ps1

#WORKS
Function ListZip {
    param(
        [string]$zipfile
    )
    Add-Type -AssemblyName System.IO.Compression.FileSystem
    $ziparchive = [System.IO.Compression.ZipFile]::OpenRead($zipfile)
    $ziparchive.Entries | % { Write-Host $_.Name }
}

Nevermind... that works