public static void saveStream(WebResponse respon, string fullPath)
        {
            Stream internetStream = respon.GetResponseStream();

            MD5 md5 = new System.Security.Cryptography.MD5CryptoServiceProvider();

            try
            {
                byte[] retVal = md5.ComputeHash(internetStream);

                string str_md5 = BitConverter.ToString(retVal);//http响应流的md5//这里很耗费时间

                string str2_md5 = getmd5forfile();//本地文件的md5

                bool isequals = str_md5.Equals(str2_md5);//二者md5比较
            }
            catch (Exception)
            {

                throw;
            }

            internetStream.Close();

        }