Catch the silverlight closing event

Recently, I must catch the closing event of a silverlight application when users have not save their modifications. For this functionnality, I make a class with a boolean indicate pending changes.

public class ClosingHandler

{

    #region Properties

    private const string ScriptableObjectName = "OnBeforeUnload";

    /// <summary>

    /// Confirmation message

    /// </summary>

    public String Message { get; set; }

    private Boolean mHasPendingChange;

    /// <summary>

    /// Indicate pending change

    /// </summary>

    public Boolean HasPendingChange

    {

        get { return mHasPendingChange; }

        set

        {

            mHasPendingChange = value;

            if (value)

                Initialize();

            else

                Clear();

        }

    }

    #endregion

    #region Methods

    /// <summary>

    /// Initialization of the Hander

    /// </summary>

    public void Initialize()

    {

        //Out of Browser application

        if (Application.Current.IsRunningOutOfBrowser)

        {

            Application.Current.MainWindow.Closing +=

                new EventHandler<System.ComponentModel.ClosingEventArgs>(

                    MainWindow_Closing);

        }

        else    //In Browser application

        {

            HtmlPage.RegisterScriptableObject(ScriptableObjectName, this);

            string pluginName = HtmlPage.Plugin.Parent.Id;

            HtmlPage.Window.Eval(string.Format(

                @"window.onbeforeunload = function () {{

                var slApp = document.getElementById('{0}').getElementsByTagName('object')[0];

                var result = slApp.Content.{1}.OnBeforeUnload();

                if(result.length > 0)

                    return result;

                }}",

                pluginName, ScriptableObjectName)

                );

        }

    }

    /// <summary>

    /// Clear event handler and message

    /// </summary>

    public void Clear()

    {

        //Out of Browser application

        if (Application.Current.IsRunningOutOfBrowser)

        {

            Application.Current.MainWindow.Closing -=

                new EventHandler<System.ComponentModel.ClosingEventArgs>(

                    MainWindow_Closing);

        }

        else    //In Browser application

        {

            HtmlPage.RegisterScriptableObject(ScriptableObjectName, this);

            string pluginName = HtmlPage.Plugin.Parent.Id;

            HtmlPage.Window.Eval(@"window.onbeforeunload = ''");

        }

    }

    #endregion

    #region EventHandler

    /// <summary>

    /// MainWindow Closing event handler

    /// </summary>

    /// <param name="sender"></param>

    /// <param name="e"></param>

    void MainWindow_Closing(object sender, System.ComponentModel.ClosingEventArgs e)

    {

        MessageBoxResult boxResult = MessageBox.Show(

                    Message, "Closing confirmation",

                    MessageBoxButton.OKCancel);

        if (boxResult == MessageBoxResult.Cancel)

            e.Cancel = true;

    }

    [ScriptableMember]

    public string OnBeforeUnload()

    {

        return Message;

    }

    #endregion

}

Source : http://mark.mymonster.nl/2011/01/30/how-to-cancel-the-closing-of-your-silverlight-application-in-browser-and-out-of-browser