win10 uwp 如何判断一个对象被移除

        private object Foo { set; get; } = new object();

        private WeakReference<object> FooReference { set; get; }

        private void Button_OnClick(object sender, RoutedEventArgs e)
        {
            FooReference = new WeakReference<object>(Foo);
            Foo = null;
            GC.Collect();

            var timer = new DispatcherTimer();
            timer.Interval = new TimeSpan(0, 0, 0, 5);
            timer.Tick += (o, args) =>
            {
                Console.WriteLine(FooReference.TryGetTarget(out var t));
            };
             timer.Start();
        }