<location path="MyWebForm.aspx">
    <system.web>
     <authorization>
         <deny users="?" />
        </authorization>
    </system.web>
</location>

public class SecureUserControl : System.Web.UI.UserControl
{

    private void Page_Load(object sender, System.EventArgs e)
    {
        if (this.Page.User.Identity.Name=="")
        {
            Server.Transfer("default.aspx?View=Login&RETURNURL="+Server.UrlPathEncode(Request.RawUrl));
        }
    }
}