/// <summary>
    /// 转为GBK
    /// </summary>
    /// <param name="str">要转换的字符串</param>
    /// <returns>GBK字符串</returns>
    private static string ToGBK(this string str)
    {
        byte[] buffer = Encoding.GetEncoding(936).GetBytes(str);
        string gbk = Encoding.GetEncoding(936).GetString(buffer);
        return gbk;
    }