Создание элементов управления динамически при нажатии кнопки

I need to create dynamic controls on button click.  

HTML:

<asp:LinkButton ID="lnkSrt" runat="server" Text="Multi Sort" ForeColor="Black" Font-Names="Calibri" Font-Size="10pt" />
<ajax:ModalPopupExtender runat="server" ID="mpeSrt"
 TargetControlID="lnkSrt" PopupControlID="pnlSrt" CancelControlID="btnClose">
     </ajax:ModalPopupExtender>
         <asp:UpdatePanel ID="upPanel" runat="server">
             <ContentTemplate>
                  <asp:Panel ID="pnlSrt" runat="server" align="center" Style="display: none" BackColor="LightGray">
                  <asp:Label runat="server" Text="Sort By Column" />
                  <asp:DropDownList ID="ddlColumnSort" runat="server" AutoPostBack="true" />
                  <asp:Label runat="server" Text="Order By" />
                  <asp:DropDownList ID="ddlOrder" runat="server" AutoPostBack="true">
                  <asp:ListItem Text="Ascending" Value="0"></asp:ListItem>
                  <asp:ListItem Text="Descending" Value="1"></asp:ListItem>
                  </asp:DropDownList>
                  <asp:LinkButton ID="lnkAdd" runat="server" Text="Add Sorting" Font-Underline="true" OnClick="lnkAddOrder_Click" Font-Names="Calibri" Font-Size="10pt" ForeColor="Black" />
                  <asp:Button ID="btnSorting" runat="server" Text="Sort" OnClick="btnSorting_Click" />
                  <asp:Button ID="btnClose" runat="server" Text="Close" />
                </asp:Panel>
             </ContentTemplate>
          </asp:UpdatePanel>

CODE: 

When clicked on AddSorting I am trying to create dropdown control using below code and the controls doesn't show. 
 
protected void lnkAddSort_Click(object sender, EventArgs e)
{
int index = pnlSort.Controls.OfType<DropDownList>().ToList().Count + 1;
this.Createddl("ddldyn" + index);
}
public void Createddl(string id)
{
DropDownList ddl = new DropDownList();
ddl.ID = id;
pnlSort.Controls.Add(ddl);
Literal lt = new Literal();
lt.Text = "<br />";
pnlSort.Controls.Add(lt);
}

Can anyone please provide suggestions on how to handle this?