Selected Item getting lost on post back (Sharepoint wss3.0 webpart)

I am new Sharepoint developer and trying to create search webpart in window Sharepoint service 3.0 [Free edition], so far I can select multiple item from list box but when i trying to display selected item into textbox it getting lost.

Below is my code:

using System;
using System.Runtime.InteropServices;
using System.Data;
using System.Collections.ObjectModel;
using System.Collections.Generic;
using System.ComponentModel;
using System.Text;
using System.Web;
using System.Drawing;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using Microsoft.SharePoint;
using Microsoft.SharePoint.Utilities;
using Microsoft.SharePoint.WebControls;
using Microsoft.SharePoint.WebPartPages;
using System.Xml.Serialization;
using System.Collections;


namespace Filter_WebPart  
{
    [Guid("6641a7a3-d2c4-4fda-9ef5-89596845bd6e")]

    public class Filter_WebPart : System.Web.UI.WebControls.WebParts.WebPart
    {
        protected DataSet _dataset;        
        protected ListBox lstRegion;
        protected ListBox lstMaterial;        
        protected HtmlButton btnSubmit;
        protected HtmlInputText txtDisplay;        

        private string myExceptions = "";                
        //private int[] Index1;


        protected override void CreateChildControls()
        {                                                
            try
            {                                                                
                //Region Table / DataSet
                lstRegion = new ListBox();                                

                lstRegion.ID="lstRegion";                
                lstRegion.SelectionMode = ListSelectionMode.Multiple;                                  
                //lstRegion.EnableViewState = true;                                
                //lstRegion.SelectedIndex = 0;                                


                //Material Table / DataSet
                lstMaterial = new ListBox();                
                lstMaterial.SelectionMode = ListSelectionMode.Multiple;                
                lstMaterial.EnableViewState = true;

                btnSubmit = new HtmlButton();                
                btnSubmit.InnerText = "Filter";                                                
                btnSubmit.ServerClick += new EventHandler(btnSubmit_Click);

                txtDisplay = new HtmlInputText();                                


                //CommandButton

                this.Controls.Add(lstRegion);
                this.Controls.Add(lstMaterial);
                this.Controls.Add(btnSubmit);
                this.Controls.Add(txtDisplay);               

            }
            catch(Exception ChildControlException)
            {
                myExceptions += "ChildControlException:" + ChildControlException.Message;
            }
            finally
            { 
                //base.CreateChildControls();
            }

        }        

        protected override void OnPreRender(EventArgs e)
        {            
           if(!this.Page.IsPostBack)
           {                
                lstMaterial.DataSource = GetMaterialList();
                lstMaterial.DataTextField = "Material Name";
                lstMaterial.DataValueField = "Material Name";
                lstMaterial.DataBind();                                                              

                lstRegion.DataSource = GetRegionList();                                
           开发者_开发知识库     lstRegion.DataTextField = "Region Name";
                lstRegion.DataValueField = "Region Name";
                lstRegion.DataBind();
                txtDisplay.Value="1 time";
            }
            base.OnPreRender(e);            
        }

        void btnSubmit_Click(object sender, EventArgs e)
        {                        
            string tmpStr="";                                   
            int k=0;    
            //int i=0; 


            lstMaterial.DataBind(); 
            lstRegion.DataBind();

            //int[] indx = lstRegion.GetSelectedIndices();
            //for(i=0;i<indx.Length;i++)
            //{
            //    tmpStr = tmpStr+","+lstRegion.Items[indx[i]].Text;
            //}


            //if(lstRegion.SelectedIndex >=0 )
            //{
               //for(i=0;i < lstRegion.Items.Count;i++)
               //{
               //    //if(i==5 || i==10)
               //    //{
               //    //    lstRegion.Items[i].Selected = true;
               //    //}
               //    if(lstRegion.Items[i].Selected)
               //    {
               //        tmpStr = lstRegion.Items[i].Text;                       
               //    }
               //    k=k+1;
               //}
            //}

            foreach(ListItem RgItem in lstRegion.Items)
            {                                    
                if(RgItem.Selected == true)
                {                                                
                    tmpStr = tmpStr +","+RgItem.Text;                    
                    k=k+1;
                }                                
            }                

            //for(i=0;i<lstRegion.Items.Count;i++)
            //{
            //    if(lstRegion.Items[i].Selected == true)
            //    {
            //        txtDisplay.Value = txtDisplay.Value +","+lstRegion.Items[i].Text;     
            //        k=k+1;
            //    }                
            //}

            if(tmpStr != "" )
            {txtDisplay.Value = tmpStr;}
            else{                                   
                txtDisplay.Value = k.ToString();
                btnSubmit.InnerText = "Done";} 


        }                


        protected override void OnInit(EventArgs e)
        {                               
            base.OnInit(e);
            EnsureChildControls();                                                     
        }

        private DataSet GetRegionList()
        {            
            _dataset = new DataSet();
            DataTable _tbl = new DataTable();
            DataColumn _tblcol = new DataColumn("Region Name");
            _tbl.Columns.Add(_tblcol);            

            SPWeb web = SPContext.Current.Site.RootWeb;
            SPList myList = web.Lists["Service Area"];
            SPQuery query = new SPQuery();
            query.Query = "";
            SPListItemCollection items = myList.GetItems(query);

                foreach (SPListItem item in items)
                {
                    DataRow _row = _tbl.NewRow();                     
                    _row[0] = SPEncode.HtmlEncode(item["Region Name"].ToString());
                    _tbl.Rows.Add(_row);
                }

            _dataset.Tables.Add(_tbl);
            return _dataset;
        }

        private DataSet GetMaterialList()
        {
            _dataset = new DataSet();
            DataTable _tbl = new DataTable();
            DataColumn _tblcol = new DataColumn("Material Name");
            _tbl.Columns.Add(_tblcol);
            SPWeb web = SPContext.Current.Site.RootWeb;

                SPList myList = web.Lists["Material Master"];
                SPQuery query = new SPQuery();
                query.Query = "";
                SPListItemCollection items = myList.GetItems(query);
                foreach (SPListItem item in items)
                {
                    DataRow _row = _tbl.NewRow();
                    _row[0] = SPEncode.HtmlEncode(item["Material Name"].ToString());
                    _tbl.Rows.Add(_row);
                }

            _dataset.Tables.Add(_tbl);
            return _dataset;            
        }

        protected override void RenderContents(HtmlTextWriter output)
        {
            try
            {                
                this.EnsureChildControls();
                lstRegion.RenderControl(output);
                lstMaterial.RenderControl(output);
                btnSubmit.RenderControl(output);
                output.Write("<br>");
                txtDisplay.RenderControl(output);     

                //base.RenderContents(output);
            }
            catch (Exception RenderContentsException)
            {
                myExceptions += "RenderException:" + RenderContentsException.Message;
            }
            finally
            {
                if (myExceptions.Length > 0)
                {
                    output.WriteLine(myExceptions);
                }
            }
        }


    }
}

This should be working. I've tried your code on my 2010 farm and it works fine (should also work for WSS3).

Did you forget to do an iisreset after updating the assembly ?

Your best chance to finding a solution is to use the debugger. You can attach the debugger to the right w3wp.exe process. If you don't know which one, select them all. Then set a breakpoint on your event handler and check where you lose your selection.

You don't need to override OnInit, you can put EnsureChildControls() in btnSubmit_Click.