I'm trying using LDAP to authenticate user, but I have a problem with LDAP. This is my code string hostOrDomainName = "MrHand-PC"; string targetOu = "cn=Huy Pham,ou=people,dc=example,dc=com"; // create a search filter to find all objects string ldapSearchFilter = "uid=pdhuy"; // establish a connection to the directory LdapConnection connection = new LdapConnection(hostOrDomainName); Console.WriteLine("\r\nPerforming a simple search ..."); SearchRequest searchRequest = new SearchRequest(targetOu, ldapSearchFilter, System.DirectoryServices.Protocols.SearchScope.OneLevel, null); // cast the returned directory response as a SearchResponse object SearchResponse searchResponse = (SearchResponse)connection.SendRequest(searchRequest); << **Throw exception: The distinguished name contains invalid syntax.** Can anyone help my solve this problem. Thank you so much.