delegate void WillHide();
//
private WillHide willHide;
//
this.willHide = new WillHide(this.HideWin2);
//
private void HideWin2()
{
     this.win2.Hide();
}

        void win2_Closing(object sender, CancelEventArgs e)
        {
            e.Cancel = true;
            Dispatcher.BeginInvoke(System.Windows.Threading.DispatcherPriority.Normal, this.willHide);
        }

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
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;
using System.ComponentModel;

namespace ClosingDemo
{
    /// <summary>
    /// Interaction logic for Window1.xaml
    /// </summary>
    public partial class Window1 : Window
    {
        delegate void WillHide();

        private Window2 win2 = new Window2();
        private WillHide willHide;

        public Window1()
        {
            InitializeComponent();

            Test();
        }

        private void HideWin2()
        {
            this.win2.Hide();
        }


        private void Test()
        {
            App.Current.MainWindow = this;
            App.Current.ShutdownMode = ShutdownMode.OnMainWindowClose;

            this.willHide = new WillHide(this.HideWin2);

            this.win2.Closing += new CancelEventHandler(win2_Closing);

            this.btnShowWin2.Click += new RoutedEventHandler(btnShowWin2_Click);
           

            this.win2.Show();

        }

        void btnShowWin2_Click(object sender, RoutedEventArgs e)
        {
            this.win2.Show();
        }

        void win2_Closing(object sender, CancelEventArgs e)
        {
            e.Cancel = true;
            Dispatcher.BeginInvoke(System.Windows.Threading.DispatcherPriority.Normal, this.willHide);
        }
        
       
    }
}