if (HttpContext.Current.User.Identity.IsAuthenticated)
{
Control c = LoadControl("b.ascx");
//cast to the right class if you need to set some properties
placeholder1.Controls.Add(c);
}
web.config does not work on ascx files, just do, in Page_Load, for example
if (HttpContext.Current.User.Identity.IsAuthenticated)
{
Control c = LoadControl("b.ascx");
//cast to the right class if you need to set some properties
form1.Controls.Add(c);
}