CheckBoxList的全选/反选

protected void Button3_Click(object sender, EventArgs e)
    {
        int i=0;
            foreach (ListItem li in CheckBoxList1.Items)
            {
                if (li.Selected)
                    i += 1;
            }
            if (i > CheckBoxList1.Items.Count / 2)
            {
                foreach (ListItem li in CheckBoxList1.Items)
                {
                    li.Selected = false;
                }
            }
            else
            {
                foreach (ListItem li in CheckBoxList1.Items)
                {
                    li.Selected = true;
                }
            }
       
       
    }