protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            int id=int.Parse(Request.QueryString["id"]); //页数传过来的id
            getdownloadFilePath(id);
        }
    }

    public void getdownloadFilePath(int id)
    {
        string sqlTable = " download where DownloadID=" + id;//到下载中表去找对应的文件名
        DataSet ds = clsMySql.GetDataTable(sqlTable);
        string fileName = ds.Tables[0].Rows[0]["DownloadFile"].ToString();//取数据库文件名
        string path = Server.MapPath("./download/")+fileName;//得到映射路径
        Response.Write(path);//测试一下

        //以下是简单的下载方法，有很多地方可以完善！！
        System.IO.FileInfo theFile = new System.IO.FileInfo(path);
        Response.Clear();
        Response.AddHeader("Content-Disposition", "attachment; filename=" + theFile.Name);
        Response.AddHeader("Content-Length", theFile.Length.ToString());
        Response.ContentType = "application/octet-stream";
        Response.WriteFile(theFile.FullName);
        Response.End();

    }