在C＃WPF应用程序中关闭所有Windows

我正在VS2013Express中创建一个小的WPF应用程序，但遇到了一个小问题。你看，有三个窗口，MainWindow，LatAndLongDialog，TimeCitiesDialog。

LatAndLongDialog并TimeCitiesDialog从MainWindow中打开（单击按钮）。我希望在Closed()调用该事件时关闭所有其他窗口MainWindow。MainWindow.xaml.cs上的代码：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace GlobalTime_ELITE_for_WPF
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();


            UserDescText.Content = "Select a TimeCity or enter the latitude and longitude in \n" +
                                 "to view the World Time there. Or, select another one of the\n" +
                                 "options below to do that. Go to Help by clicking on the link\n" +
                                 "on the upper-right corner of the window to view everything you\n" +
                                 "can do.";
            this.Closed += CloseOff;
        }

        private void OpenTimeCitiesDialog(Object Sender, EventArgs E)
        {
            TimeCitiesDialog ObjectReference = new TimeCitiesDialog();
            ObjectReference.Show();
        }

        private void OpenLatAndLongDialog(Object Sender, EventArgs E)
        {
            LatAndLongDialog ObjectReference = new LatAndLongDialog();
            ObjectReference.Show();
        }

        private void CloseOff(Object Sender, EventArgs E)
        {
            this.Close();

            TimeCitiesDialog tcdor = new TimeCitiesDialog();
            LatAndLongDialog laldor = new LatAndLongDialog();


        }
    }
}

我该如何全部关闭？请帮忙！