ASP.NET Drop Down list , the button function not firing after the first click
<p>ASP.Net Dropdownlist , on changing the value of drop down list and clicking the button it is not sending an upadted value to the New.aspx</p>

<blockquote>
  <p>&lt;%@ Page Language="C#"
  AutoEventWireup="true"
  CodeFile="Test_DropDown.aspx.cs"
  Inherits="Test_DropDown" %></p>
</blockquote>

<p></p>

<pre><code>&lt;html xmlns="http://www.w3.org/1999/xhtml" &gt;
&lt;head runat="server"&gt;
    &lt;title&gt;Untitled Page&lt;/title&gt;
&lt;/head&gt;
&lt;body&gt;
    &lt;form id="form1" runat="server"&gt;
    &lt;div&gt;
        &lt;asp:DropDownList ID="ddl_test" runat="server" OnSelectedIndexChanged="ddl_test_SelectedIndexChanged" AutoPostBack="true"&gt;
        &lt;asp:ListItem&gt;First Item&lt;/asp:ListItem&gt;
            &lt;asp:ListItem&gt;Second Item&lt;/asp:ListItem&gt;
            &lt;asp:ListItem&gt;Third Item&lt;/asp:ListItem&gt;

        &lt;/asp:DropDownList&gt;
        &lt;asp:HiddenField ID="hdf_ddl" runat="server" /&gt;
        &lt;asp:Button ID="btn_test" runat="server" Text="Button" OnClick="Button1_Click" /&gt;
    &lt;/div&gt;
    &lt;/form&gt;
&lt;/body&gt;
&lt;/html&gt;



using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

public partial class Test_DropDown : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        string passValue = ddl_test.SelectedValue.ToString();
        string val = hdf_ddl.Value.ToString();
        btn_test.Attributes.Add("onclick", "window.open('New.aspx?ddlValue=" + val + "', 'OpenPopWindow','left=250,top=100,width=500,height=500,toolbar=1,resizable=0,status=0,menubar=0,scrollbars=1');return false;");
        //btn_test.OnClientClick = "window.open('New.aspx?ddlValue=" + val + "', 'OpenPopWindow','left=250,top=100,width=500,height=500,toolbar=1,resizable=0,status=0,menubar=0,scrollbars=1');return false;";


    }
    protected void ddl_test_SelectedIndexChanged(object sender, EventArgs e)
    {

        hdf_ddl.Value = ddl_test.SelectedValue.ToString();
    }
}
</code></pre>
