Sending xls file in attachment

HttpContext.Current.Response.Clear();
HttpContext.Current.Response.AddHeader("content-disposition", "attachment;filename=offline_data.xls");
HttpContext.Current.Response.Charset = "";

HttpContext.Current.Response.ContentType = "application/ms-excel";
System.IO.StringWriter stringWrite = new System.IO.StringWriter();
System.Web.UI.HtmlTextWriter htmlWrite = new HtmlTextWriter(stringWrite);


HttpContext.Current.Response.Write(stringWrite.ToString())

private void ATestMethod()
    {

        int i_result = 0;

        //IMP: Make sure you are getting data into dtResult
        DataTable dtResult = Fetching_Query_Data_In_Datatable(strDatatype, from, to, strHscode, strComb, strProd);


        StringBuilder dataToExport = new StringBuilder();

        dataToExport.Append("<table style=border:solid 1px blue;>");
        dataToExport.Append("<tr>");

        foreach (DataColumn dCol in dtResult.Columns)
        {
            dataToExport.Append("<td style=border:solid 1px blue;>");
            dataToExport.Append(HttpContext.Current.Server.HtmlEncode(dCol.ColumnName));
            dataToExport.Append("</td>");
        }

        dataToExport.Append("</tr>");

        foreach (DataRow dRow in dtResult.Rows)
        {
            dataToExport.Append("<tr>");
            foreach (object obj in dRow.ItemArray)
            {
                dataToExport.Append("<td style=border:solid 1px blue;>");
                dataToExport.Append(HttpContext.Current.Server.HtmlEncode(obj.ToString()));
                dataToExport.Append("</td>");
            }
            dataToExport.Append("</tr>");
        }

        dataToExport.Append("</table>");

        if (!string.IsNullOrEmpty(dataToExport.ToString()))
        {
            string strSub = "Offline data";

            HttpContext.Current.Response.Clear();
            HttpContext.Current.Response.AddHeader("content-disposition", "attachment;filename=offline_data.xls");
            HttpContext.Current.Response.Charset = "";

            HttpContext.Current.Response.ContentType = "application/ms-excel";

            System.IO.StringWriter stringWrite = new System.IO.StringWriter(dataToExport); //Added dataToExport

            System.Web.UI.HtmlTextWriter htmlWrite = new HtmlTextWriter(stringWrite);

            HttpContext.Current.Response.Write(stringWrite.ToString());
            SmtpClient mailcleint = new SmtpClient();
            string SenderId = EmailTo.ToString().Trim();
            if (SenderId != "")
            {
                string SenderName = "Team Cybex";

                MailAddress MailRecepient = new MailAddress(EmailTo.ToString().Trim());
                MailAddress MailSender = new MailAddress(SenderId, SenderName);
                MailMessage MailMsg = new MailMessage(MailSender, MailRecepient);

                /******************** SEND Email TO Users **************************************/

                MailMsg.Body = "Hello";
                MailMsg.DeliveryNotificationOptions = DeliveryNotificationOptions.OnFailure;
                MailMsg.IsBodyHtml = true;
                var a = System.Net.Mail.Attachment.CreateAttachmentFromString(stringWrite.ToString(), "application/vnd.xls");
                MailMsg.Attachments.Add(a);
                MailMsg.Priority = MailPriority.Normal;
                MailMsg.Subject = strSub.ToString();
                SmtpClient mailclient = new SmtpClient();
                mailclient.Credentials = new System.Net.NetworkCredential("myID", "my password");
                mailclient.Host = "xxxx.xx.xx.xx";

                mailclient.Send(MailMsg);
                i_result = 1;
            }
            HttpContext.Current.Response.End(); //Need this line of code.

        }