param($KNOWNFOLDERID)

$Pinvoke = @’
[DllImport("shell32.dll")]
public static extern int SHGetKnownFolderPath(
     [MarshalAs(UnmanagedType.LPStruct)] Guid rfid,
     uint dwFlags,
     IntPtr hToken,
     out String pszPath  // API uses CoTaskMemAlloc
     );
‘@

$type = Add-Type -MemberDefinition $Pinvoke
    -Name Win32Utils -Namespace SHGetKnownFolderPath
    -PassThru

$type::SHGetKnownFolderPath($KNOWNFOLDERID)

int SHGetKnownFolderPath(     [MarshalAs(UnmanagedType.LPStruct)] Guid rfid,     uint dwFlags,     IntPtr hToken,     out String pszPath  )

SHGetKnownFolderPath FDD39AD0-238F-46AF-ADB4-6C85480369C7 0 0 [ref]$path

Argument: '4' should be a System.Management.Automation.PSReference. Use [ref].
At line:1 char:56
+ New-Item Function:Global:SHGetKnownFolderPath -Value { [Microsoft.PowerShell.Com ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [], MethodException
    + FullyQualifiedErrorId : NonRefArgumentToRefParameterMsg

[environment]::getfolderpath("mydocuments")

$mydocuments = [environment]::getfolderpath("mydocuments")