//计算文件的MD5码
        public static string GetFileMD5(string path)
        {
            string ret = "";
            FileStream fs = new FileStream(path, FileMode.Open, FileAccess.Read);
            MD5CryptoServiceProvider md5 = new MD5CryptoServiceProvider();
            byte[] md5byte = md5.ComputeHash(fs);
            int i, j;
            foreach (byte b in md5byte)
            {
                i = Convert.ToInt32(b);
                j = i >> 4;
                ret += Convert.ToString(j, 16);
                j = ((i << 4) & 0x00ff) >> 4;
                ret += Convert.ToString(j, 16);
            }
            return ret;
        }

        //字符串加密
        public static string GetStringMD5(string str, int size)
        {
            string ret = "";
            if (size == 32)
            {
                byte[] b = System.Text.Encoding.Default.GetBytes(str);
                b = new MD5CryptoServiceProvider().ComputeHash(b);
                for (int i = 0; i < b.Length; i++)
                {
                    ret += b[i].ToString("x").PadLeft(2, '0');
                }
            }
            else if (size == 16)
            {
                MD5CryptoServiceProvider md5 = new MD5CryptoServiceProvider();
                ret = BitConverter.ToString(md5.ComputeHash(System.Text.UTF8Encoding.Default.GetBytes(str)), 4, 8);
                ret = ret.Replace("-", "");
            }
            return ret;
        }