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"); 
        } 
    } 
}