how to get the value of the control loaded dynamically in asp.net

Question

In my page,I load some control dynamically.

In Page

<asp:DropDownList runat="server" OnSelectedIndexChanged="onchanged"></asp:DropdownList>
<asp:PlaceHolder id="placeholder" runat="server">

In Page.aspx:

public void onchanged(object sender,EventArgs e){
  Control c=loadControl('part.ascx');
  this.placeholder.Controls.Add(c);
}

Now,then this page is submitted,I want to get the value of the controls in the 'part.ascx',but I can not get it .

What is the problem?