dropdownlist加Js 实现联动

1.在前天添加两个控件DropDownList和TextBox，在后台给DropDownList赋值和属性的值

            DropDownList1.Items.Add(new ListItem(" ", " "));
            ListItem item = new ListItem("1+1", "1");//绑定value
            item.Attributes.Add("key", "2");//绑定属性的value
            DropDownList1.Items.Add(item);
            ListItem item2 = new ListItem("2+2", "2");
            item2.Attributes.Add("key", "4");
            DropDownList1.Items.Add(item2);

2.在前台

//DropDownList1后面添加onchange="ChangeDDL(this)"

<script type="text/javascript">
    function ChangeDDL(obj) {
        var DropDownList1 = document.getElementById("<%=DropDownList1.ClientID %>");
        var TextBox1 = document.getElementById("<%=TextBox1.ClientID %>");
        if (obj.value == " ") {
            TextBox3.value = "";
        }
        else {
            var text = obj.options[obj.selectedIndex].getAttribute("key");
            TextBox1.value = text ;
        }
    }
</script>