GetUserNameEx.ps1

param(
    [Parameter()]
    [ValidateSet("NameUnknown", "NameFullyQualifiedDN", "NameSamCompatible", "NameDisplay", "NameUniqueId", "NameCanonical", "NameUserPrincipal", "NameCanonicalEx", "NameServicePrincipal", "NameDnsDomain")]
    [string]
    $ExtendedNameFormat = "NameSamCompatible")

$extendedNameFormats = @{
    "NameUnknown"          = 0
    "NameFullyQualifiedDN" = 1
    "NameSamCompatible"    = 2
    "NameDisplay"          = 3
    "NameUniqueId"         = 6
    "NameCanonical"        = 7
    "NameUserPrincipal"    = 8
    "NameCanonicalEx"      = 9
    "NameServicePrincipal" = 10
    "NameDnsDomain"        = 12
}

$getUserNameExSig = @'
[DllImport("secur32.dll", CharSet=CharSet.Auto, SetLastError=true)]
public static extern byte GetUserNameEx (int nameFormat,
    System.Text.StringBuilder userName, ref int userNameSize);
'@

$getLastErrorSig = @'
[DllImport("Kernel32.dll")]
public static extern uint GetLastError();
'@

Add-Type -MemberDefinition $getUserNameExSig -Name Api1 -Namespace Sspi
Add-Type -MemberDefinition $getLastErrorSig -Name Api2 -Namespace Win32

$sb = New-Object System.Text.StringBuilder(1024)
$size = 1024

$result = [Sspi.Api1]::GetUserNameEx($extendedNameFormats[$ExtendedNameFormat], $sb, [ref]$size)

if ($result -eq 0) {
    $e = [Win32.Api2]::GetLastError()
    Write-Host "GetUserNameEx failed with error code $e"
} else {
    $sb.ToString()
}