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>