HttpContext.Current.Request.Browser.IsMobileDevice

public ActionResult Index()
        {
            if (IsMobile())
            {
                return Redirect("mobile.xxx.com");
            }
            return View();
        }
       
        public static bool IsMobile()
        {
            if (System.Web.HttpContext.Current.Request.Browser.IsMobileDevice)
                return true;

            return false;
        }

public class RedirectMobileDevicesToMobileAreaAttribute : AuthorizeAttribute
    {
        protected override bool AuthorizeCore(System.Web.HttpContextBase httpContext)
        {
            RouteData rData = httpContext.Request.RequestContext.RouteData;
            //判断，如果是页面手工点击的【电脑版】，则不进行跳转
            string customChange = (rData.Values["id"] ?? "").ToString();
            if (!string.IsNullOrWhiteSpace(customChange))
            {
                return true;

            }
            if (!httpContext.Session.IsNewSession)
                return true;

            if (!httpContext.Request.Browser.IsMobileDevice)
                return true;

            if (Regex.IsMatch(httpContext.Request.Url.PathAndQuery, "/Mobile($|/)"))
                return true;
            return false;
        }

        protected override void HandleUnauthorizedRequest(AuthorizationContext filterContext)
        {
            var redirectionRouteValues = GetRedirectionRouteValues(filterContext.RequestContext);
            filterContext.Result = new RedirectToRouteResult(redirectionRouteValues);
        }
        
        protected virtual RouteValueDictionary GetRedirectionRouteValues(RequestContext requestContext)
        {
            return new RouteValueDictionary(new { area = "Mobile", controller = requestContext.RouteData.Values["controller"], action = requestContext.RouteData.Values["action"] });
        }
    }

[RedirectMobileDevicesToMobileArea]

[RedirectMobileDevicesToMobileArea]
public ActionResult Index()
        {
            //if (IsMobile())
            //{
            //    return Redirect("mobile.xxx.com");
            //}
            return View();
        }