1 string  resp=string.Empty   2 string cert = Server.MapPath("apiclient_cert.p12");  3    string password = mch_id;（商户号） 4     ServicePointManager.ServerCertificateValidationCallback = new RemoteCertificateValidationCallback(CheckValidationResult); 5  6   //这两句特别注意 7  8   // X509Certificate cer = new X509Certificate(cert, password);//线下发布需要添加的 9      X509Certificate2 cer = new X509Certificate2(cert, password, X509KeyStorageFlags.PersistKeySet | X509KeyStorageFlags.MachineKeySet);//线上发        布需要添加10 11    HttpWebRequest webrequest = (HttpWebRequest)HttpWebRequest.Create(url);12         webrequest.ClientCertificates.Add(cer);13         webrequest.Method = "post";14         webrequest.ContentType = "application/x-www-form-urlencoded";15         webrequest.ContentLength = xml.Length;16         HttpWebResponse response = null;17    try18         {19             StreamWriter swRequestWriter = new StreamWriter(webrequest.GetRequestStream());20             swRequestWriter.Write(xml);21 22             if (swRequestWriter != null)23                 swRequestWriter.Close();24 25             response = (HttpWebResponse)webrequest.GetResponse();26             using (StreamReader reader = new StreamReader(response.GetResponseStream(), Encoding.Default))27             {28                 resp = reader.ReadToEnd();29             }30         }31  catch (Exception exp)32         {33             throw exp;34         }35         finally36         {37             if (response != null)38                 response.Close();39         }

分享几个退款时遇到的问题
1、退款提示:订单号非法
  原因：退款的订单号错误导致的
2、退款时提示：基础连接已关闭，或被意外关闭（验证证书的问题）
  原因： 首先要通过证书请求退款的接口，其次要把下面的这行语句

 // X509Certificate cer = new X509Certificate(cert, password);//线下发布需要添加的
 修改成这样的才可以 X509Certificate2 cer = new X509Certificate2(cert, password, X509KeyStorageFlags.PersistKeySet | X509KeyStorageFlags.MachineKeySet);//线上发        布需要添加
3、 Miss Parameters  
  原因：传参数时，参数的名字和文档大小写不一致
4、每次退款，退款的订单号不能相同，否则无法退款