Check.aspx code is
Code:
<%@ Register TagName="TopBar" TagPrefix="CRS" Src="~/Demo.ascx" %>
---
<div>
<CRS:TopBar ID="Hide" runat="server"/>
</div>
Check.aspx.cs code is
Code:
protected void Page_Load(object sender, EventArgs e)
{
  Demo uc = (Demo)Page.LoadControl("Demo.ascx");
  HiddenField hide = (HiddenField)uc.FindControl("hidden1");
...