if (!IsPostBack)
 {
if (Session["UserID"] == null)
 {
    Response.Redirect("Login.aspx");
 }
else
{
    Response.Redirect("xxx.aspx");
}
}

public ActionResult Register()
        {
            if (Session["UserID"] != null)
            {
              
            }
   return View();
        }

跳转到Url
public ActionResult rdurl(){return Redirect("http://www.baidu.com");}
跳转到Action
public ActionResult rdaction(){return RedirectToAction("Index","Eice");}