public async Task <string> GetPublicKeyAsync()
{
string url = "https://fraud.mch.weixin.qq.com/risk/getpublickey";
XmlDocument xmlDocument = new XmlDocument();
var root = xmlDocument.CreateElement("xml");
var idElement = xmlDocument.CreateElement("mch_id");
idElement.InnerText = _mchId;
var nonceElement = xmlDocument.CreateElement("nonce_str");
nonceElement.InnerText = Helper.GetNonceStr(32);
var signTypeElement = xmlDocument.CreateElement("sign_type");
signTypeElement.InnerText = "MD5";
root.AppendChild(idElement);
root.AppendChild(nonceElement);
root.AppendChild(signTypeElement);
xmlDocument.AppendChild(root);
string sign = WeChatSignHelper.CreateMd5SignByXml(xmlDocument, _secretKey);
var signElement = xmlDocument.CreateElement("sign");
signElement.InnerText = sign;
root.AppendChild(signElement);
string body = XmlSerializeHelper.XmlToString(xmlDocument);
var result =
await _client.ExecutePostRequest(url, new Dictionary <string, string>(), body, _path, _password,
"xml/text");
Check(XmlSerializeHelper.StringToObject <PayResponse>(result));
XmlDocument res = new XmlDocument();
res.LoadXml(result);
return(res.FirstChild.SelectSingleNode("pub_key").InnerText);
}