限制CheckBoxList选中的数量

        function limit(cbl) {            var cbs = document.getElementById("CheckBoxList的客户端ID").getElementsByTagName("input");              var count = 0;            for (var i = 0; i < cbs.length; i++) {                if (cbs[i].type == "checkbox") {                   if (cbs[i].checked) {                        count++;                    }                }            }            if (count > 3) {                cbl.checked = false;                alert("不能多于3项！");                return;            }        }

    protected void Page_Load(object sender, EventArgs e)    {   foreach (ListItem li in CheckBoxList1.Items)        {            li.Attributes.Add("onclick", "limit(this)");        }    }