private static void GetAllUsersInAD()

        {

            DirectorySearcher searcher = new DirectorySearcher();

            searcher.SearchRoot = new DirectoryEntry("LDAP://a14907.com", "username", "pwd", AuthenticationTypes.Encryption);

            searcher.Filter = "(objectClass=USER)";

            searcher.SearchScope = SearchScope.Subtree;

            searcher.Sort = new SortOption("name", SortDirection.Ascending);

            searcher.PageSize = int.MaxValue - ;

            searcher.PropertiesToLoad.AddRange(new string[] { "samaccountname", "company", "department", "mail" });

            SearchResultCollection results = searcher.FindAll();

            foreach (SearchResult item in results)

            {

                //操作

            }

        }