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)");
}
}