被上面催的急 新手大家 怎么把gridview导到excel后存在服务器

private void ToExcel(Control ctl){      string FileName = "z.xls";  HttpContext.Current.Response.Charset = "UTF-8";  HttpContext.Current.Response.ContentEncoding = System.Text.Encoding.UTF8;  HttpContext.Current.Response.ContentType = "application/ms-excel";  HttpContext.Current.Response.AppendHeader("Content-Disposition", "attachment;filename=" + "" + FileName);  ctl.Page.EnableViewState = false;  System.IO.StringWriter tw = new System.IO.StringWriter();  HtmlTextWriter hw = new HtmlTextWriter(tw);  ctl.RenderControl(hw);  HttpContext.Current.Response.Write(tw.ToString());  //到这里将gridview保存到excel表z.xls了， 执行HttpContext.Current.Response.End()后提示将excel保存到客户端，  //会有小框 【打开】【保存】【取消】,可是怎么将表存到服务器呢？我在后面加了SaveAs代码有问题啊！  HttpFileCollection files = HttpContext.Current.Request.Files;  System.Web.HttpPostedFile myFile = files[FileName];  string ppath =  @"D:/excel/"+ FileName ;  myFile.SaveAs(ppath);//保存上载文件的内容。  // 这里报错未将对象引用设置到对象的实例。感觉是System.Web.HttpPostedFile myFile = files[FileName];这句的问题，怎么找到gridview保存后的excel文件z.xls保存在服务器上呢？  HttpContext.Current.Response.End();  }