如何判断WPF窗口是否已关闭？

using System;
using System.Windows;

namespace WpfApp
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            if (IsClosed)
            {
                MessageBox.Show("窗口已关闭");
            }
            else
            {
                MessageBox.Show("窗口未关闭");
            }
        }
    }
}