aspx: 

<div id="selDiv" style=" z-index:100; visibility:visible; clip:rect(0px 110px 80px 92px); position:absolute"><%--left:279px; top:167px"--%> 
<asp:DropDownList ID="workerno_list" runat="server" style="z-index:-1" Width="110px" ></asp:DropDownList> 
</div> 
<asp:TextBox ID="workerno_value" runat="server" style=" z-index:103px; position:absolute" Font-Size="10" Width="93px" MaxLength ="50" Height="22px"></asp:TextBox> 
<script> 
function addTxtTanto(va) 
{ 
document.getElementById("workerno_value").value=va; 
document.getElementById("workerno_value").select(); 
} 
</script> 

aspx.cs: 

page_load() 
{ 
if (!IsPostBack) 
{ 
workerno_list.Attributes["onchange"] = "addTxtTanto(this.options[selectedIndex].innerText)"; 
} 
}