asp.net Gridview分页保存选项

#region //'revision: 1.00 created date: 2013/08/02 created id: una [#1300071]增加多選框
        /// <summary>
        /// session獲取多選框值
        /// </summary>
        private void rememberoldvalues()
        {
            arraylist categoryidlist = new arraylist();
            string index = "";
            foreach (gridviewrow row in gridview.rows)
            {
                index = (string)gridview.datakeys[row.rowindex].value;
                bool result = ((checkbox)row.findcontrol("deletethis")).checked;

                // check in the session
                if (session["id"] != null)
                    categoryidlist = (arraylist)session["id"];
                if (result)
                {
                    if (!categoryidlist.contains(index))
                        categoryidlist.add(index);
                }
                else
                    categoryidlist.remove(index);
            }
            if (categoryidlist != null && categoryidlist.count > 0)
                session["id"] = categoryidlist;
        }

        /// <summary>
        /// session分頁時之前多選框為true
        /// </summary>
        private void repopulatevalues()
        {
            arraylist categoryidlist = (arraylist)session["id"];
            if (categoryidlist != null && categoryidlist.count > 0)
            {
                foreach (gridviewrow row in gridview.rows)
                {
                    string index = (string)gridview.datakeys[row.rowindex].value;
                    if (categoryidlist.contains(index))
                    {
                        checkbox mycheckbox = (checkbox)row.findcontrol("deletethis");
                        mycheckbox.checked = true;
                    }
                }
            }
        }
        #endregion

protected void gridview_pageindexchanging(object sender, gridviewpageeventargs e)
        {
            rememberoldvalues();
            gridview.pageindex = e.newpageindex;
            binddata();
            repopulatevalues();
        }

protected void btnselect_click(object sender, eventargs e)
        {
            string items = "";
            arraylist categoryidlist = new arraylist();
            string index ="";
            foreach (gridviewrow row in gridview.rows)
            {
                index = (string)gridview.datakeys[row.rowindex].value;
                bool result = ((checkbox)row.findcontrol("deletethis")).checked;

                // check in the session
                if (session["id"] != null)
                    categoryidlist = (arraylist)session["id"];
                if (result)
                {
                    if (!categoryidlist.contains(index))
                        categoryidlist.add(index);
                }
                else
                    categoryidlist.remove(index);
            }
            if (categoryidlist != null && categoryidlist.count > 0)
                for (int i = 0; i < categoryidlist.count; i++)
                {
                    items += categoryidlist[i] + ",";
                }
            items = items.substring(0, items.length - 1);
            scriptmanager.registerstartupscript(this, this.gettype(), "", "check('" + items + "');", true);
            session.remove("id");
        }