Adding a User Control to a Panel control fails

Question

This is my child page inside a master page:

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="doctoryab_shokri.Default" masterpagefile="~/web.Master" %>
<asp:Content ID="my_content" runat="server" contentplaceholderid="ContentPlaceHolder1">
<asp:Panel ID="pnl_controls" runat="server">
</asp:Panel>
</asp:Content>

And this is the code behind this page:

public partial class Default : System.Web.UI.Page
{

    protected void Page_Load(object sender, EventArgs e)
    {

        pnl_controls.Controls.Add(new sections.doctor_holder());
        //sectios is a folder and doctor_holder is the user control
    }
    protected void Page_Init(object sender, EventArgs e)
    {

        pnl_controls.Controls.Add(new sections.doctor_holder());
        //sectios is a folder and doctor_holder is the user control
    }
}

I try to add this user control to my panel but nothing is loaded. I cant use LoadControl method because I need to set a property to user control before loading it.

ADDED:

This is doctor_holder.cs:

public partial class doctor_holder : System.Web.UI.UserControl
{

    protected void Page_Load(object sender, EventArgs e)
    {

    }
}

And this is user control html code:

<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="doctor-holder.ascx.cs" Inherits="doctoryab_shokri.sections.doctor_holder" %>
<div class="doc-holder col-md-6">
<div class="doc-img">
    <asp:Image ID="img_doc_avatar" runat="server" Height="64px" Width="64px" ImageUrl="~/img/opt-doc-logo.png" />
</div>
        <div class="doc-info">
            <p class="doc-name">
                <asp:Label ID="lbl_name_lname" runat="server" Text="#" CssClass="doc-name"></asp:Label>
            </p>
            <p class="doc-pro">
                <asp:Label ID="lbl_proff" runat="server" Text="#"></asp:Label>
            </p>
            <p class="doc-addr">
                <asp:Label ID="lbl_address" runat="server" Text="#"></asp:Label>
            </p>
            <div style="float:left;">
                <p class="nobat-cap">امکان ثبت نوبت</p>
                <p class="nobat-type">
                    <asp:LinkButton ID="lnk_type" runat="server" ForeColor="White">#</asp:LinkButton>
                </p>
            </div>
            <br style="clear: both;">
        </div>

Answer 1

You can directly load use controls like that you have use the LoadControl method. The LoadControl method reads the file and instantiates it as a control that can be added to the page.
Example:

<%@ Page Language="C#" %>
<%@ Reference Control="~/Controls/Spinner.ascx" %>    
<script runat="server">
private ASP.Spinner Spinner1

protected void Page_Load(object sender, EventArgs e)
{
   Spinner1 = (ASP.Spinner)LoadControl("~/Controls/Spinner.ascx");
}

protected void Button1_Click(object sender, EventArgs e)
{
    PlaceHolder1.Controls.Add(Spinner1);
}
</script>

Refer the below links:
How to: Create Instances of ASP.NET User Controls Programmatically
Dynamic Loading of ASP.NET User Controls

Hope this help..