authentication in active directory

using System.Security;
        public User Login(string login_name, string password){
            ICredValidator cv = new LdapCredValidator(); //LanCredValidator();
            User u = new User(conn);
            if (cv.ValidateCred("MY-DOM", login_name, password))
            {
                // validation sucess
            }else Throw("Login failed for {0}",login_name);
            return u;
        }