public static void TransferFile(string filepath)
    {
       // StreamWriter sw = new StreamWriter(@"E:/NDadmin/ndfile/2.txt", true, Encoding.UTF8);

        try
        {
            FileStream filestream = new FileStream(filepath, FileMode.Open, System.IO.FileAccess.Read, System.IO.FileShare.Read);
         //   sw.WriteLine("文件完成读取");
            long filesize = filestream.Length;
         //   sw.WriteLine("查询大小完成");
            //让客户端浏览器正确识别这个文件的类型和文件大小
            string filename = System.IO.Path.GetFileName(filepath).ToLower();
        //    sw.WriteLine("识别大小完成");

            HttpContext.Current.Response.ContentType = "application/octet-stream";
         //    sw.WriteLine("制定类型完成");
            HttpContext.Current.Response.AddHeader("Content-Disposition", "attachment; filename=\"" + HttpUtility.UrlEncode(filename, System.Text.Encoding.UTF8) + "\";");
         //    sw.WriteLine("文件位置制定完成");
            HttpContext.Current.Response.AddHeader("Content-Length", filesize.ToString());

             //sw.WriteLine("长度制定完成");
            //将文件中的数据发送到客户端
            byte[] filebuffer = new byte[filesize];
            //sw.WriteLine("获取文件数据");
            filestream.Read(filebuffer, 0, (int)filesize);
             //sw.WriteLine("发送ING");
            HttpContext.Current.Response.BinaryWrite(filebuffer);
             //sw.WriteLine("发送完成");
            filestream.Close();
              //sw.WriteLine("释放");
        }
        catch (Exception ex)
        {
            HttpContext.Current.Response.Write(ex.Message);
            throw ex;
            
              //sw.WriteLine("报错了！" + ex.Message);
              //sw.WriteLine("报错了！" + ex.TargetSite);

        }
        finally
        {
            // sw.Flush();
            //sw.Close();
        }
    }