控件制作

控件代码如下：
using System;
using System.Collections.Generic;
using System.Text;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Xml;
using System.Xml.XPath;namespace CbCuzon
{
    public class DoubleDropDownList:Control, INamingContainer
    {
        protected override void CreateChildControls()
        {
            Controls.Add(new LiteralControl("<b>Job Search</b>"));  
            DropDownList ddlSkills = new DropDownList();
            ddlSkills.AutoPostBack = true;
            ddlSkills.TextChanged += new EventHandler(DDLTextChanged);
            ddlSkills.Items.Add("---------");
            PopuldateList(ref ddlSkills, "jobCats.xml", "jobCategory");
            Controls.Add(ddlSkills);  
        }        private void DDLTextChanged(object sender, EventArgs e)
        { 
            DropDownList ddlSkill = (DropDownList)Controls[2];
            System.Web.HttpContext.Current.Response.Write("textchanged" + ddlSkill.SelectedItem.Text);
        }        private void PopuldateList(ref DropDownList ddl, string xmlFileName, string node)
        {
            string currentPath = System.Web.HttpContext.Current.Request.PhysicalApplicationPath;
            int lastPos = currentPath.LastIndexOf("\\");
            int totalChars = currentPath.Length;
            int removeTotal = totalChars - lastPos - 1;
            string strXmlPath = currentPath.Remove(lastPos + 1, removeTotal) + xmlFileName;
            XmlDataDocument xmlDoc = new XmlDataDocument();
            xmlDoc.DataSet.ReadXml(strXmlPath);
            XmlNodeList col = xmlDoc.GetElementsByTagName(node);            foreach(XmlNode xmlNode in col)
            {
                ddl.Items.Add(xmlNode.FirstChild.FirstChild.Value);
            }
        }        protected override void OnPreRender(EventArgs e)
        {
        }
    }
}请问DDLTextChanged事件怎么没有执行？