GridView问题

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;
public partial class admin_gridview : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
    }
    protected void Button2_Click(object sender, EventArgs e)
    {
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
    }
    protected void CheckBox1_CheckedChanged(object sender, EventArgs e)
    {
       foreach(GridView gr in GridView1.Rows)
       {
           CheckBox chk = (CheckBox)gr.Cells[5].FindControl["chk"];
           if (chk.Checked)
           {
               chk.Checked = false;
           }
           else
           {
               chk.Checked = true;
           }
       }
    }
}

[bold]编译器错误信息: [/bold]CS0117: “System.Web.UI.WebControls.GridView”并不包含“Cells”的定义