利用JavaScript将DropDownList的选项值插入文本框

前台代码：
<asp:TextBox ID="txtExpression" runat="server" MaxLength="400" CssClass="TextBox400"
    ontentEditable="false" />
<br />
<asp:Label ID="lblAvaliableColumns" runat="server"></asp:Label>
<asp:DropDownList ID="ddlFields" runat="server" CssClass="DropDownList200">
</asp:DropDownList>
<asp:ImageButton ID="ibnAddField" runat="server" ImageUrl="~/Images/add.png" />
前台代码比较简单，就一个文本框和下拉列表框，还有一个按钮，点击后将下拉列表框的选项插入到文本框中。
后台代码：
    /// <summary>
    /// PreRender event handler
    /// </summary>
    /// <param name="e"></param>
    protected override void OnPreRender(EventArgs e)
    {
        base.OnPreRender(e);
        #region Register Client Script for Text treatment
        string txtScript, txtScriptName;
        #region All language usage
        txtScript = @"
<script type='text/javascript' language='javascript'>
    function insertInputText(input_obj, text){
        var prefix;
        var tail;
        if(typeof(input_obj.selectionStart)=='undefined'){
            prefix=input_obj.value;
            tail = '';
        }
        else{
          &n