public class DefaultController : Controller
  {
    protected override void OnActionExecuting(ActionExecutingContext filterContext)
    {
      base.OnActionExecuting(filterContext);
      var controllerName = filterContext.ActionDescriptor.ControllerDescriptor.ControllerName;

      var userName = Session["UserName"] as String;
      if (String.IsNullOrEmpty(userName))
      {
        //重定向至登录页面
        filterContext.Result = RedirectToAction("Index", "Login", new { url = Request.RawUrl});
        return;
      }

    }
  }

public class LoginController : Controller
  {
    // GET: Login
    public ActionResult Index(string ReturnUrl)
    {
      if (Session["UserName"] != null)
      {
        return RedirectToAction("Index", "Home");
      }
      ViewBag.Url = ReturnUrl;
      return View();
    }

    [HttpPost]
    public ActionResult Index(string name, string password, string returnUrl)
    {
      /*
        添加验证用户名密码代码
      */
      Session["UserName"] = name;
      if (Url.IsLocalUrl(returnUrl) && returnUrl.Length > 1 && returnUrl.StartsWith("/") && !returnUrl.StartsWith("//") && !returnUrl.StartsWith("/\\"))
      {
        return Redirect(returnUrl);
      }
      else
      {
        return RedirectToAction("Index", "Home");
      }
    }

    // POST: /Account/LogOff
    [HttpPost]
    public ActionResult LogOff()
    {
      Session["UserName"] = null;
      return RedirectToAction("Index", "Home");
    }
  }

public class HomeController : DefaultController
  {
    public ActionResult Index()
    {
      return View();
    }
  }