Get-UsersFullName.ps1

function Get-UsersFullName {
  $NAME_DISPLAY = [int] 3
  $MethodDefinition = @'
[DllImport("secur32.dll", CharSet=CharSet.Auto, SetLastError=true)]
[return: MarshalAs(UnmanagedType.I1)]
public static extern int GetUserNameEx (int nameFormat, StringBuilder userName, ref UInt32 userNameSize);
'@
  $type = Add-Type -MemberDefinition $MethodDefinition `
    -NameSpace Win32 -Name "Secur32" `
    -Using "System.Text" -PassThru
  $sb = New-Object -TypeName "System.Text.StringBuilder" -ArgumentList 1024
  $rv = [int]([Win32.Secur32]::GetUserNameEx($NAME_DISPLAY, $sb, [ref][UInt32]($sb.Capacity)))

  return $sb.ToString()
}

This code doesn't work yet. I get the following...

Exception calling "GetUserNameEx" with "3" argument(s): "Cannot marshal 'return value': Invalid managed/unmanaged type combination
(Int32/UInt32 must be paired with I4, U4, or Error)."
At C:\Users\bruces\Documents\WindowsPowerShell\Microsoft.PowerShell_profile.ps1:82 char:3
+   $rv = [int]([Win32.Secur32]::GetUserNameEx($NAME_DISPLAY, $sb, [ref][UInt32]($ ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
    + FullyQualifiedErrorId : MarshalDirectiveException