public static string MyUrlDeCode(string str, Encoding encoding)

{

    if (encoding == null)

    {

        Encoding utf8 = Encoding.UTF8;

        //首先用utf-8进行解码                    

        string code = HttpUtility.UrlDecode(str.ToUpper(), utf8);

        //将已经解码的字符再次进行编码.

        string encode = HttpUtility.UrlEncode(code, utf8).ToUpper();

        if (str == encode)

            encoding = Encoding.UTF8;

        else

            encoding = Encoding.GetEncoding("gb2312");

    }

    return HttpUtility.UrlDecode(str, encoding);

}