public class SignInRequired : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        // User is verified, continue executing action

        if (Acme.Web.CurrentUser != null)
        {
            return;
        }

        // End response with 401 Unauthorized

        var response = HttpContext.Current.Response;
        response.StatusCode = (int)HttpStatusCode.Unauthorized;
        response.End();

        // Prevent the action from actually being executed

        filterContext.Result = new EmptyResult();
    }
}

public class SignInRequiredAttribute : AuthorizeAttribute
{
    protected override bool AuthorizeCore(HttpContextBase httpContext)
    {
        return !Acme.Web.CurrentUser != null;
    }
}

public override void OnActionExecuting(HttpActionContext actionContext)

{
 actionContext.Response = new HttpResponseMessage(HttpStatusCode.Unauthorized);
}

filterContext.Result = new RedirectResult("~/Error/Unauthorized");