the underlying connection was closed the connection was closed unexpectedly. c#

System.Object stringpost = "User=" + User + "&passwd=" + password + "&mobilenumber=" + Mobile_Number + "&message=" + Message + "&MType=" + Mtype + "&DR=" + DR + "&SID=" + SID;

            //string functionReturnValue = null;
            //functionReturnValue = "";

            HttpWebRequest objWebRequest = null;
            HttpWebResponse objWebResponse = null;
            StreamWriter objStreamWriter = null;
            StreamReader objStreamReader = null;

            try
            {
                string stringResult = null;

                objWebRequest = (HttpWebRequest)WebRequest.Create("http://www.smscountry.com/SMSCwebservice.asp");
                objWebRequest.Method = "POST";

                if ((objProxy1 != null))
                {
                    objWebRequest.Proxy = objProxy1;
                }

                // Use below code if you want to SETUP PROXY.
                //Parameters to pass: 1. ProxyAddress 2. Port
                //You can find both the parameters in Connection settings of your internet explorer.

                //WebProxy myProxy = new WebProxy("YOUR PROXY", PROXPORT);
                //myProxy.BypassProxyOnLocal = true;
                //wrGETURL.Proxy = myProxy;
                
                objWebRequest.ContentType = "application/x-www-form-urlencoded";

                objStreamWriter = new StreamWriter(objWebRequest.GetRequestStream());
                objStreamWriter.Write(stringpost);
                objStreamWriter.Flush();
                objStreamWriter.Close();

                objWebResponse = (HttpWebResponse)objWebRequest.GetResponse();
                objStreamReader = new StreamReader(objWebResponse.GetResponseStream());
                stringResult = objStreamReader.ReadToEnd();

                objStreamReader.Close();
                return (stringResult);
            }
            catch (Exception ex)
            {
                return (ex.Message);
            }