下载文件使用流的形式控制

            if (!this.IsPostBack)
            {
                try
                {
                    if (SiteCommon.SiteContext.CurrentUser.IsLogon)
                    {
                        ds = bid.GetList(" ID = " + Request["id"].ToString() + " ");
                        if (ds != null)
                        {

                            string downFilePath = Server.MapPath("/" + ds.Tables[0].Rows[0]["url"].ToString()); // test with 3.8GB, ok // Server.MapPath("~/files/somefile.iso");
                            System.IO.FileInfo downFileInfo = new System.IO.FileInfo(downFilePath);

                            if (!downFileInfo.Exists) throw new Exception("文件不存在。");
                            const int CHUNK_SIZE = 10000;
                            byte[] buffer = new byte[CHUNK_SIZE];

                            Response.Clear();
                            using (System.IO.FileStream iStream = System.IO.File.OpenRead(downFilePath))
                            {
                                long dataLengthToRead = iStream.Length;
                                Response.ContentType = "application/octet-stream";
                                Response.AddHeader("Content-Disposition",
                                    "attachment; filename=" + Server.UrlPathEncode(downFileInfo.Name));
                                while (dataLengthToRead > 0 && Response.IsClientConnected)
                                {
                                    int lengthRead = iStream.Read(buffer, 0, CHUNK_SIZE);
                                    Response.OutputStream.Write(buffer, 0, lengthRead);
                                    Response.Flush();
                                    dataLengthToRead = dataLengthToRead - lengthRead;
                                }
                            }
                            Response.Close();
                        }
                    }
                    else 
                    {
                        Page.RegisterStartupScript("start", "<script>alert('请登录后再进行下载！')</script>");
                    }
                }
                catch (System.Exception ex)
                {
                    Page.RegisterStartupScript("start","<script>alert('下载的文件不存在！')</script>");
                    Console.Write(ex.ToString());
                }
            }