asp.net gridview多页时的批量删除

<asp:gridview id="grwbook" runat="server" cellpadding="4" forecolor="#333333" gridlines="none" autogeneratecolumns="false" allowpaging="true" datakeynames="id" onpageindexchanging="grwbook_pageindexchanging" pagesize="12">
    <footerstyle backcolor="#507cd1" font-bold="true" forecolor="white" />
    <rowstyle backcolor="#eff3fb" />
    <editrowstyle backcolor="#2461bf" />
    <selectedrowstyle backcolor="#d1ddf1" font-bold="true" forecolor="#333333" />
    <pagerstyle backcolor="#2461bf" forecolor="white" horizontalalign="center" />
    <headerstyle backcolor="#507cd1" font-bold="true" forecolor="white" />
    <alternatingrowstyle backcolor="white" />
    <columns>
    <asp:boundfield datafield="id" headertext = "id" />
    <asp:boundfield datafield="username" headertext="姓名">
    <itemstyle width="60px" />
    </asp:boundfield>
    <asp:boundfield datafield="comments" headertext="评论">
    <itemstyle width="500px"  />
    </asp:boundfield>
    <asp:boundfield datafield="postdate" headertext="日期" />
    <asp:hyperlinkfield datanavigateurlfields="newsid" datanavigateurlformatstring="../news_zi.asp?id={0}" headertext="文章"  datatextfield="newsid" target="_blank" />
    <asp:templatefield headertext="操作" >
        <itemtemplate>
        <asp:checkbox runat="server" id="cbxid" />
        </itemtemplate>
    </asp:templatefield>
    </columns>
    <pagersettings mode="nextpreviousfirstlast" firstpagetext="第一页" lastpagetext="末页" nextpagetext="下一页" previouspagetext="上一页" />
</asp:gridview>

using system;
using system.data;
using system.configuration;
using system.collections;
using system.web;
using system.web.security;
using system.web.ui;
using system.web.ui.webcontrols;
using system.web.ui.webcontrols.webparts;
using system.web.ui.htmlcontrols;
using system.data.oledb;

public partial class admin_book_admin : system.web.ui.page
{

    
    protected void page_load(object sender, eventargs e)
    {
        //判断管理员是否已经登陆
        admin.checkadmin();

        btndel.attributes.add("onclick", "return confirm('确定删除吗？')");

        if (!page.ispostback)
        {
            datainit();
        }
    }

    protected void grwbook_pageindexchanging(object sender, gridviewpageeventargs e)
    {
        this.grwbook.pageindex = e.newpageindex;
        session["curretnpage"] = e.newpageindex;
        datainit();
    }

    //数据绑定
    private void datainit()
    {
        dataset ds = db.dataset("select id,username,comments,postdate,newsid from feedback order by postdate desc,newsid desc");
        if (viewstate["currentpage"] != null)
        {
            this.grwbook.pageindex = convert.toint32(session["currentpage"]);
        }
        this.grwbook.datasource = ds;
        this.grwbook.databind();

    }

    //执行删除操作
    protected void btndel_click(object sender, eventargs e)
    {
        string sqltext = "(";
        for (int i = 0; i < grwbook.rows.count; i++)
        {
            //搜索第n行3列
            checkbox cbx = (checkbox)grwbook.rows[i].findcontrol("cbxid");
            if (cbx.checked == true)
            {
               sqltext = sqltext + convert.toint32(grwbook.datakeys[i].value) + ",";
            }
        }

        //判断是否有选中
        if (sqltext != "(")
        {
            //去掉最后的逗号，并且加上右括号
            sqltext = sqltext.substring(0, sqltext.length - 1) + ")";
            sqltext = "delete from feedback where id in" + sqltext;
            try
            {
                //执行删除语句
                db.excutesql(sqltext);
                //重新绑定数据
                common.salert("删除成功");
                datainit();
                //response.redirect("book_admin.aspx");
            }
            catch (exception ex)
            {
                //若有错误发生，输出错误信息
                common.salert(ex.message);
            }
            finally
            {

            }
        }
        else
        {
            common.salert("您还没有选中有删除的项");
        }
    }


}