dropdownlist中既可以输入又可以下拉列表

前台页面源码：
div id="selDiv"
                                style=" z-index:100; visibility:visible; clip:rect(0px 110px 80px 92px); position:absolute; top: 95px; left: 862px;" ><%--left:279px; top:167px"--%>
                               <asp:DropDownList ID="ddlType_Field" runat="server" style="z-index:-1" Width="110px" DataSource="<%#  getTypeFieldInfo() %>"  DataValueField="Type_Field" DataTextField="Type_Name"  OnSelectedIndexChanged="ddlType_Field_Selected_Index_Changed"  AutoPostBack= "true"></asp:DropDownList>
                          </div>
                          <asp:TextBox ID="txtType_Field_Text" runat="server"
                                style=" z-index:103px; position:absolute; top: 95px; left: 862px;"
                                Font-Size="10" Width="93px" MaxLength ="50" Height="22px" ></asp:TextBox>
                          <asp:TextBox ID="txtType_Field_value" runat="server" style=" z-index:103px; position:absolute" Font-Size="10" Width="93px" MaxLength ="50" Height="22px" Visible="false"></asp:TextBox>

javascript代码：
<script type="text/javascript" language="javascript">
        function addTxtTanto(val) {
            document.getElementById("txtType_Field_value").value = val;
            document.getElementById("txtType_Field_value").select();
        }
    </script>