How to add panel in content page?

Question

I have to find a panel in content page and need to add a drop downlist on that panel.I have searched but i got only for adding controls to master page.Below is my code, Note:I have to add control from that page itself not from master page

<%@ Page Title="" Language="C#" MasterPageFile="~/MasterPage.master" AutoEventWireup="true" CodeFile="Salesorder.aspx.cs" Inherits="Salesorder" %>

<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="Server">
    <script src="assets/plugins/jquery-1.10.2.js"></script>
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="Server">
  <div class="row">
<asp:Panel ID="Panel1" runat="server"> 
</asp:Panel>             
        </div>
</asp:Content>

I need to add another panel to Panel1 from codebehind.

    protected void Page_PreInit(object sender, EventArgs e)
       {
          //Create a Dynamic Panel
          Panel pnlDropDownList; 
          pnlDropDownList = new Panel();
          pnlDropDownList.ID = "pnlDropDownList";
          pnlDropDownList.BorderWidth = 1;
          pnlDropDownList.Width = 300;
          ContentPlaceHolder cph = (ContentPlaceHolder)this.Page.FindControl("ContentPlaceHolder1");
          Panel panel = (Panel)cph.FindControl("Panel1");
          cph.Controls.Add(pnlDropDownList);
       }

Answer 1

Yes, you need to find your control in Master instead of Page.

Like below.

ContentPlaceHolder cph = (ContentPlaceHolder)this.Master.FindControl("ContentPlaceHolder1");

this should work.

Answer 2

Add a PlaceHolder control in the panel you want the new DropDownList panel to be added in the aspx page

<asp:Panel ID="Panel1" runat="server"> 
    <asp:PlaceHolder ID="placeholder1" runat="server"></PlaceHolder>
</asp:Panel>

In the code behind, to add the control to the page use:

placeholder1.Controls.add(pnlDropDownList);

Answer 3

I can't understand why don't you add controls like this

<asp:ContentPlaceHolder id="ContentPlaceHolder1" runat="server">
        <asp:Panel ID="Panel1" runat="server">
            <asp:DropDownList ID="DropDownList1" runat="server"></asp:DropDownList>
        </asp:Panel>
    </asp:ContentPlaceHolder>