How do add values in dynamically created drop downlist

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C#

ASP.NET

Can any body help me to add values in drop down list which is created dynamically. I have written some code for it. But when i click on button to add the value in drop down list, panel having dynamic controls shows no value in it due to post back.Here is the code:

ASPX:

<form id="form1" runat="server">

        <asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>
        <div>
            <asp:DropDownList ID="ddlIds" runat="server" AutoPostBack="True" OnSelectedIndexChanged="ddlIds_SelectedIndexChanged">
                <asp:ListItem>Textbox</asp:ListItem>
                <asp:ListItem>Button</asp:ListItem>
                <asp:ListItem>CheckBox</asp:ListItem>
                <asp:ListItem>DropdownList</asp:ListItem>
            </asp:DropDownList>

            <asp:Panel ID="Panel1" runat="server" EnableViewState="True"></asp:Panel>
            <asp:HiddenField ID="hdnValue" runat="server" />

            <asp:UpdatePanel runat="server" ID="updpnl_pnl_controls">

                <ContentTemplate>
                    <asp:Panel runat="server" ID="pnl_items">
                        <asp:TextBox runat="server" ID="txtvalue"></asp:TextBox>
                        <asp:Button runat="server" ID="addbtn" Text="Add Item" OnClick="addbtn_OnClick" />
                        <asp:Button runat="server" ID="doneBtn" Text="Done" OnClick="doneBtn_OnClick"/>
                        <asp:HiddenField ID="hdnId" runat="server"></asp:HiddenField>
                    </asp:Panel>
                </ContentTemplate>
                <Triggers>
                    <asp:AsyncPostBackTrigger runat="server" ControlID="addbtn" />
                </Triggers>
            </asp:UpdatePanel>
        </div>
    </form>

ASPX.CS:

protected void Page_Load(object sender, EventArgs e)
   {
       pnl_items.Visible = false;
       if (!IsPostBack)
       {
           create_controls();
       }
   }
   protected void ddlIds_SelectedIndexChanged(object sender, EventArgs e)
   {

       create_controls();

   }

   protected void create_controls()
   {
       switch (ddlIds.SelectedItem.Text)
       {
           case "Textbox":
               String txt = DateTime.Now.Ticks.ToString(CultureInfo.InvariantCulture);
               string txtid = "txtBox" + txt;
               hdnValue.Value = hdnValue.Value + "^" + txtid;
               break;
           case "CheckBox":
               String chkbox = DateTime.Now.Ticks.ToString(CultureInfo.InvariantCulture);
               string chkid = "chkBox" + chkbox;
               hdnValue.Value = hdnValue.Value + "^" + chkid;
               break;
           case "Button":
               string count = DateTime.Now.Ticks.ToString(CultureInfo.InvariantCulture);
               string id = "BttnBox" + count;
               hdnValue.Value = hdnValue.Value + "^" + id;
               break;
           case "DropdownList":
               string ddcount = DateTime.Now.Ticks.ToString(CultureInfo.InvariantCulture);
               string ddid = "Dropdown" + ddcount;
               hdnValue.Value = hdnValue.Value + "^" + ddid;
               break;
       }


       if (hdnValue.Value == "") return;
       var val = hdnValue.Value.Split('^');
       foreach (var ds in val)
       {
           if (ds.Contains("chk"))
           {
               var chkBox = new CheckBox { ID = ds, Text = "Test" };
               Panel1.Controls.Add(chkBox);
               var lt = new Literal { Text = "<br />" };
               Panel1.Controls.Add(lt);
           }

           else if (ds.Contains("txt"))
           {
               var txtBox = new TextBox { ID = ds };
               Panel1.Controls.Add(txtBox);
               var lt = new Literal { Text = "<br />" };
               Panel1.Controls.Add(lt);
           }
           else if (ds.Contains("Bttn"))
           {
               var btn = new Button { ID = ds, Text = "Test" };
               Panel1.Controls.Add(btn);
               var lt = new Literal { Text = "<br />" };
               Panel1.Controls.Add(lt);
           }
           else if (ds.Contains("Drop"))
           {
               var ddl = new DropDownList { ID = ds };
               pnl_items.Visible = true;
               hdnId.Value = ds;
               Panel1.Controls.Add(ddl);
               var lt = new Literal { Text = "<br />" };
               Panel1.Controls.Add(lt);
           }
       }
   }

   protected void addbtn_OnClick(object sender, EventArgs e)
   {
       var i = Panel1.Controls.Count;
       var ddl = (DropDownList)Panel1.FindControl(hdnId.Value);
       ddl.Items.Add(new ListItem(txtvalue.Text, txtvalue.Text));
       pnl_items.Visible = true;
       //throw new NotImplementedException();
   }

   protected void doneBtn_OnClick(object sender, EventArgs e)
   {
       pnl_items.Visible = false;
       hdnId.Value = string.Empty;
   }

Thanks...