ASP.NET

for(var i=0;i <list.options.length;>{
    alert(list.options[i].value);  
    window.opener.document.getElementById(Str3).value=list.options[i].value+";";
}

window.close();

function Child()
{
var Str1 = document.getElementById('txtCompCode').value; 
var Str2 = document.getElementById('txtCompName').value; 
var Str3 = document.etElementById('txtData').value;  // << This is my parent listbox id passed from parent page


var list = document.getElementById('lboxCompList'); //Childpage listbox id 
var flist= Str3;
var osel = document.createElement("option");  
var val=""

for(var i=0;i <list.options.length;>{
    alert(list.options[i].value);  
    val += list.options[i].value+",";
    
}

alert(flist);
flist.options.add(osel);
osel.text=val;
osel.value=val;
//window.opener.document.getElementById(Str3).value=val;
window.close(); 

}

<br />
//user control<br />
public partial class WebUserControl : System.Web.UI.UserControl<br />
{<br />
    public Button submitButton<br />
    {<br />
        get { return this.Button1; }<br />
        set { this.Button1 = value; }<br />
    }<br />
    protected void Page_Load(object sender, EventArgs e)<br />
    {<br />
<br />
    }<br />
<br />
    protected void Button1_Click(object sender, EventArgs e)<br />
    {<br />
<br />
    }<br />
}<br />
<br />
<br />
//page<br />
public partial class _Default : System.Web.UI.Page <br />
{<br />
    protected void Page_Init(object sender, EventArgs e)<br />
    {<br />
     this.WebUserControl1.submitButton.Click += new EventHandler(submitButton_Click);<br />
    }<br />
    protected void Page_Load(object sender, EventArgs e)<br />
    {<br />
      <br />
    }<br />
<br />
    protected void submitButton_Click(object sender, EventArgs e)<br />
    {<br />
        Response.Write("event handled");<br />
    }<br />
}<br />
<br />
<br />