导出大量数据，解决npoi导出内存溢出

/// <summary>
        /// 使用OLEDB导出Excel
        /// </summary>
        /// <param name="dt">数据集</param>
        /// <param name="filepath">文件目录和文件名</param>
        /// <param name="tablename">SHEET页名称</param>
        /// <param name="pagecount">每页记录数</param>
        public static void Export(DataTable dt, string filepath, string tablename, int pagecount,string UserID)
        {
            string docupath = System.Web.HttpContext.Current.Server.MapPath("~\\NPOI\\" + UserID + "\\");
            //建立数据夹
            if (!Directory.Exists(docupath))
            {
                Directory.CreateDirectory(docupath);
            }
            //else
            //{
            //    File.Delete(docupath);
            //    Directory.CreateDirectory(docupath);
            //}
            //excel 2003格式
            string name = docupath + filepath;
            string connString = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + name + ";Extended Properties=Excel 8.0;";
            //Excel 2007格式
            //string connString = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" + name + ";Extended Properties=Excel 12.0 Xml;";
            try
            {
                using (OleDbConnection con = new OleDbConnection(connString))
                {
                    con.Open();

                    //开始分页
                    if (dt.Rows.Count > pagecount)
                    {
                        int page = dt.Rows.Count / pagecount + 1; //总页数
                        for (int i = 0; i < page; i++)
                        {
                            //建新sheet和表头
                            StringBuilder strSQL = new StringBuilder();
                            string tabname = tablename + i.ToString();
                            strSQL.Append("CREATE TABLE ").Append("[" + tabname + "]"); //每60000项建一页
                            strSQL.Append("(");
                            for (int j = 0; j < dt.Columns.Count; j++)
                            {
                                strSQL.Append("[" + dt.Columns[j].ColumnName + "] text,");
                            }
                            strSQL = strSQL.Remove(strSQL.Length - 1, 1);
                            strSQL.Append(")");

                            OleDbCommand cmd = new OleDbCommand(strSQL.ToString(), con);
                            cmd.ExecuteNonQuery();

                            //准备逐条插入数据
                            for (int j = i * pagecount; j < (i + 1) * pagecount; j++)
                            {
                                if (i == 0 || j < dt.Rows.Count)
                                {
                                    StringBuilder tmp = new StringBuilder();
                                    StringBuilder strfield = new StringBuilder();
                                    StringBuilder strvalue = new StringBuilder();
                                    for (int z = 0; z < dt.Columns.Count; z++)
                                    {
                                        strfield.Append("[" + dt.Columns[z].ColumnName + "]");
                                        strvalue.Append("'" + dt.Rows[j][z].ToString() + "'");
                                        if (z != dt.Columns.Count - 1)
                                        {
                                            strfield.Append(",");
                                            strvalue.Append(",");
                                        }
                                        else
                                        {
                                        }
                                    }
                                    cmd.CommandText = tmp.Append(" insert into [" + tabname + "]( ")
                                        .Append(strfield.ToString())
                                        .Append(") values (").Append(strvalue).Append(")").ToString();
                                    cmd.ExecuteNonQuery();
                                }
                            }
                        }
                    }
                    con.Close();
                    //no = count;
                }
                Console.WriteLine("OK");
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
            }
            GC.Collect();
            Table_Export_BIG(UserID);
        }
        /// <summary>
        /// 根据userid找到服务器地址路径下文件夹压缩并下载下来
        /// </summary>
        /// <param name="操作人id_文件夹名字"></param>
        public static void Table_Export_BIG(string UserID)
        {
            try
            {
                //DataTable dtout = DataAccess.GetTable("select top 1 * from IMN009");
                #region 打包发送主目录;
                string strInDirectoryPath = System.Web.HttpContext.Current.Server.MapPath("~\\NPOI\\" + UserID);//路径

                //if (File.Exists(strInDirectoryPath))//必须判断要复制的文件是否存在
                //{
                //    File.Copy(strInDirectoryPath, pSaveFilePath, true);//三个参数分别是源文件路径，存储路径，若存储路径有相同文件是否替换
                //}
                string strOutFilePath = System.Web.HttpContext.Current.Server.MapPath("~\\NPOI\\" + UserID + ".zip");

                DownLoadFilesHelp.CompressDirectory(strInDirectoryPath, strOutFilePath, UserID);
                //CopyDirectory(strInDirectoryPath, pSaveFilePath);
                string fileName = UserID + ".zip";//客户端保存的文件名
                string filePath = System.Web.HttpContext.Current.Server.MapPath("~\\NPOI\\" + UserID + ".zip");//路径
                string file = System.Web.HttpContext.Current.Server.MapPath("~\\NPOI\\" + UserID);

                FileInfo fileInfo = new FileInfo(strOutFilePath);
                System.Web.HttpContext.Current.Response.Clear();
                System.Web.HttpContext.Current.Response.ClearContent();
                System.Web.HttpContext.Current.Response.ClearHeaders();
                System.Web.HttpContext.Current.Response.AddHeader("Content-Disposition", "attachment;filename=" + fileName);
                System.Web.HttpContext.Current.Response.AddHeader("Content-Length", fileInfo.Length.ToString());
                System.Web.HttpContext.Current.Response.AddHeader("Content-Transfer-Encoding", "binary");
                System.Web.HttpContext.Current.Response.ContentType = "application/octet-stream";
                System.Web.HttpContext.Current.Response.ContentEncoding = System.Text.Encoding.GetEncoding("gb2312");
                System.Web.HttpContext.Current.Response.WriteFile(fileInfo.FullName);
                System.Web.HttpContext.Current.Response.Flush();
                File.Delete(filePath);
                DirectoryInfo dir = new DirectoryInfo(file);
                if (dir.Exists)
                {
                    DirectoryInfo[] childs = dir.GetDirectories();
                    foreach (DirectoryInfo child in childs)
                    {
                        child.Delete(true);
                    }
                    dir.Delete(true);
                }

                System.Web.HttpContext.Current.Response.End();
                #endregion
                //DataAccess.Despose();
                
            }
            catch (Exception ex)
            {

                //zAlert(ex.Message.ToString().Replace("\r\n", ""));
            }
        }