可输入的DropDownList控件

具体代码如下：

<% @ Page language = " c# "  Codebehind = " ddlWriteControl.aspx.cs "  AutoEventWireup = " false "  Inherits = " WebDemo.ddlWriteControl "   %>
  <! DOCTYPE HTML PUBLIC  " -//W3C//DTD HTML 4.0 Transitional//EN "   >
  < HTML >
      < HEAD >
          < title > ddlWriteControl </ title >
          < meta name = " GENERATOR "  Content = " Microsoft Visual Studio .NET 7.1 " >
          < meta name = " CODE_LANGUAGE "  Content = " C# " >
          < meta name = " vs_defaultClientScript "  content = " JavaScript " >
          < meta name = " vs_targetSchema "  content = " http://schemas.microsoft.com/intellisense/ie5 " >
         < style >  #div1  {} { Z-INDEX: 1; LEFT: 250px; WIDTH: 216px; CLIP: rect(3px 280px 21px 110px); POSITION: absolute; TOP: 89px; HEIGHT: 72px }  #div2  {} { LEFT: 250px; WIDTH: 95px; POSITION: absolute; TOP: 89px; HEIGHT: 18px; 2: }   </ style >
     </ HEAD >
     < body MS_POSITIONING = " GridLayout " >
         < script language = " javascript " >
        function AddOption()
         {    
            var val = document.getElementById("txtValue").value;
            var option = document.getElementById("ddl");
            
            if(val == "")
            {
                alert("请输入项值");
                return;
            }
            if(option.length > 0)
           {
                for(var i=0;i<option.length;i++)
                {
                    if(option.options[i].value == val)
                    {
                       alert("该值已存在！");
                        return;
                    }
                }
            }
            
            var option_val = new Option(val,val);
                        
            option.add(option_val);
            option.selectedIndex = option.options.length-1;
            
        }
         </ script >
         < form id = " Form1 "  method = " post "  runat = " server " >
             < div id = " div1 "  style = " Z-INDEX: 101; WIDTH: 128px; HEIGHT: 72px " >                 
                 < asp:DropDownList id = " ddl "  onchange = " document.getElementById('txtValue').value = this.value "  runat = " server "
                    Width = " 127 " >
                     < asp:ListItem Value = " aaa1 " > aaa1 </ asp:ListItem >
                     < asp:ListItem Value = " bbb2 " > bbb2 </ asp:ListItem >
                     < asp:ListItem Value = " ccc3 " > ccc3 </ asp:ListItem >
                 </ asp:DropDownList >
             </ div >
             < FONT face = " 宋体 " ></ FONT >
             < div id = " div2 " >                 
                 < asp:TextBox id = " txtValue "  style = " PADDING-LEFT:2px;PADDING-TOP:2px "  runat = " server "  Width = " 127 " ></ asp:TextBox >
                 < input type = " button "  value = " OK "  id = " btn "  onclick = " AddOption(); " >
             </ div >
            
         </ form >
     </ body >
</ HTML >