using System;
using System.DirectoryServices;

namespace ConsoleApplication1
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            string name = "jim green"; // RDN format
            string logonName = @"xxx\jgreen"; //pre windows 2000 format
            Console.WriteLine(IsUserValid(name, "the-password"));
            Console.WriteLine(IsUserValid(logonName, "the-password"));
            Console.ReadLine();
        }

        public static bool IsUserValid(string name, string pwd)
        {
            DirectoryEntry rootEntry = new DirectoryEntry("LDAP://xxx.com", name.ToLower(), pwd,AuthenticationTypes.ServerBind);

            try
            {
                // Bind to the native object to force authentication
                Object nativeObject = rootEntry.NativeObject;
                return true;
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex);
                return false;
            }
        }
    }
}