public ActionResult Login()
{
    HttpCookie cookie = new HttpCookie("userid", "12312");

    System.Web.HttpContext.Current.Response.Cookies.Add(cookie);

    return Redirect("/Home/Catagory");
}

@{
    var cookie = System.Web.HttpContext.Current.Request.Cookies.Get("userid");
    if (cookie == null || string.IsNullOrWhiteSpace(cookie.Value))
    {
        <a href="#">登录</a>
    }
    else
    {
        <p>
            <span>@(cookie.Value)</span>
        </p>
    }
}