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if (Session["txt"] != "")
            {   //获取文件路径
                string path = Server.MapPath("File/") + Session["txt"].ToString();
                //初始化 FileInfo 类的实例，它作为文件路径的包装
                FileInfo fi = new FileInfo(path);
                
                //判断文件是否存在
                if (fi.Exists)
                {
                    //将文件保存到本机上
                    Response.Clear();
                    Response.AddHeader("Content-Disposition", "attachment; filename=" + Server.UrlEncode(fi.Name));
                    Response.AddHeader("Content-Length", fi.Length.ToString());
                    Response.ContentType = "application/octet-stream";
                    Response.Filter.Close();
                    Response.WriteFile(fi.FullName);
                    Response.End();
                }
            }