//quote the input dict values
    //note: the return result for first para no '&'
    public string quoteParas(Dictionary<string, string> paras, bool spaceToPercent20 = true)
    {
        string quotedParas = "";
        bool isFirst = true;
        string val = "";
        foreach (string para in paras.Keys)
        {
            if (paras.TryGetValue(para, out val))
            {
                string encodedVal = "";
                if (spaceToPercent20)
                {
                    //encodedVal = HttpUtility.UrlPathEncode(val);
                    //encodedVal = Uri.EscapeDataString(val);
                    //encodedVal = Uri.EscapeUriString(val);
                    encodedVal = HttpUtility.UrlEncode(val).Replace("+", "%20");
                }
                else
                {
                    encodedVal = HttpUtility.UrlEncode(val); //space to +
                }

                if (isFirst)
                {
                    isFirst = false;
                    quotedParas += para + "=" + encodedVal;
                }
                else
                {
                    quotedParas += "&" + para + "=" + encodedVal;
                }
            }
            else
            {
                break;
            }
        }

        return quotedParas;
    }

    

Dictionary<string, string> postDataDict = genPostsrfPostDict(html, login, passwd, isKeepLogin);
postData += quoteParas(postDataDict);