ASP.NET / C#：DropDownList SelectedIndexChanged事件未触发

using System;
using System.Collections;
using System.Collections.Generic;
using System.Data;
using System.Data.SqlClient;
using System.Diagnostics;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using ADONET_namespace;

namespace AddFileToSQL
{
    public partial class DataMatch : _Default
    {
        protected System.Web.UI.WebControls.PlaceHolder phTextBoxes;
        protected System.Web.UI.WebControls.PlaceHolder phDropDownLists;
        protected System.Web.UI.WebControls.Button btnAnotherRequest;
        protected System.Web.UI.WebControls.Panel pnlCreateData;
        protected System.Web.UI.WebControls.Literal lTextData;
        protected System.Web.UI.WebControls.Panel pnlDisplayData;
        //Panel pnlDropDownList;

        protected static string inputfile2;
        static string[] headers = null;
        static string[] data = null;
        static string[] data2 = null;
        static DataTable myInputFile = new DataTable("MyInputFile");
        static string[] myUserSelections;

        // a Property that manages a counter stored in ViewState
        protected int NumberOfControls
        {
            get { return (int)ViewState["NumControls"]; }
            set { ViewState["NumControls"] = value; }
        }

        public void EditRecord(object recordID)
        {
            SelectedRecordID = recordID;
            // Load record from database and show in control
        }

        protected object SelectedRecordID
        {
            get
            {
                return ViewState["SelectedRecordID"];
            }
            set
            {
                ViewState["SelectedRecordID"] = value;
            }
        }

        // Page Load 
        private void Page_Load(object sender,System.EventArgs e)
        {
            if (!Page.IsPostBack)
            {
                this.NumberOfControls = 0;
            }
        }

        // Add DropDownList Control to Placeholder
        private void CreateDropDownLists()
        {
            for (int counter = 0; counter < NumberOfControls; counter++)
            {
                DropDownList ddl = new DropDownList();
                SqlDataReader dr = ADONET_methods.DisplayTableColumns(targettable);
                ddl.ID = "DropDownListID" + (counter + 1).ToString();
                ddl.DataTextField = "COLUMN_NAME";
                ddl.DataValueField = "COLUMN_NAME";
                ddl.DataSource = dr;
                ddl.DataBind();

                //myUserSelections[counter] = "";

                ddl.AutoPostBack = true;
                ddl.EnableViewState = true; //Preserves View State info on Postbacks
                ddl.Style["position"] = "absolute";
                ddl.Style["top"] = 100 * counter + 80 + "px";
                ddl.Style["left"] = 250 + "px";
                ddl.SelectedIndexChanged += new EventHandler(SelectedIndexChanged);

                pnlDisplayData.Controls.Add(ddl);
                pnlDisplayData.Controls.Add(new LiteralControl("<br><br><br>"));
                pnlDisplayData.Visible = true;

               // pnlDropDownList.FindControl(ddl.ID);
                dr.Close();
            }
        }

        protected void SelectedIndexChanged(object sender,EventArgs e)
        {
            DropDownList ddl = (DropDownList)sender; 
            string ID = ddl.ID;
        }

        // Add TextBoxes Control to Placeholder
        private void RecreateDropDownLists()
        {
            for (int counter = 0; counter < NumberOfControls; counter++)
            {
                DropDownList ddl = new DropDownList();
                SqlDataReader dr = ADONET_methods.DisplayTableColumns(targettable);

                ddl.ID = "DropDownListID" + (counter + 1).ToString();
                ddl.DataTextField = "COLUMN_NAME";
                ddl.DataValueField = "COLUMN_NAME";
                ddl.DataSource = dr;
                ddl.DataBind();
                myUserSelections[counter] = "";
                dr.Close();

                ddl.AutoPostBack = true;
                ddl.EnableViewState = false; //Preserves View State info on Postbacks
                ddl.Style["position"] = "absolute";
                ddl.Style["top"] = 100 * counter + 80 + "px";
                ddl.Style["left"] = 250 + "px";
                pnlDisplayData.Controls.Add(ddl);
                pnlDisplayData.Controls.Add(new LiteralControl("<br><br><br>"));
            }
        }

        // Create TextBoxes and DropDownList data here on postback.
        protected override void CreateChildControls()
        {
            // create the child controls if the server control does not contains child controls
            this.EnsureChildControls();

            // Creates a new ControlCollection. 
            this.CreateControlCollection();

            // Here we are recreating controls to persist the ViewState on every post back
            if (Page.IsPostBack)
            {
                RecreateDropDownLists();
                RecreateLabels();
            }
            // Create these conrols when asp.net page is created
            else
            {
                PopulateFileInputTable();
                CreateDropDownLists();
                CreateLabels();
            }

            // Prevent child controls from being created again.
            this.ChildControlsCreated = true;
        }

        // Read all the data from TextBoxes and DropDownLists 
        protected void btnSubmit_Click(object sender,System.EventArgs e)
        {
            int cnt = FindOccurence("DropDownListID");
            EditRecord("DropDownListID" + Convert.ToString(cnt + 1));
            AppendRecords();
            pnlDisplayData.Visible = false;
        }

        private int FindOccurence(string substr)
        {
            string reqstr = Request.Form.ToString();
            return ((reqstr.Length - reqstr.Replace(substr,"").Length) / substr.Length);
        }
    }
}