GridView中的CheckBox的全选,全不选

//GridView中的CheckBox的全选,全不选
    protected void Button2_Click(object sender, EventArgs e)
    {
        foreach (GridViewRow gr in GridView1.Rows)
        {
            CheckBox chk = (CheckBox)gr.Cells[0].FindControl("checkbox1");
            if (!chk.Checked)
            {
                chk.Checked = true;
                Button2.Text = "取消全选";
            }
            else
            {
                chk.Checked = false;
                Button2.Text = "全选";
            }
        }
    }