Элемент RichEditBox в WinRT

using DirectXWrapper;
using System;
using System.Collections.Generic;
using Windows.ApplicationModel;
using Windows.Foundation.Collections;
using Windows.Storage;
using Windows.Storage.Pickers;
using Windows.Storage.Streams;
using Windows.UI.Popups;
using Windows.UI.Text;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;

namespace WinRTTestApp
{
    public sealed partial class MainPage : Page
    {
        public MainPage()
        {
            this.InitializeComponent();
            Loaded += OnLoaded;
            Application.Current.Suspending += OnAppSuspending;
        }

        private async void OnLoaded(object sender, RoutedEventArgs e)
        {
            // Получение списка шрифтов от библиотеки DirectXWrapper
            WriteFactory writeFactory = new WriteFactory();
            WriteFontCollection writeFontCollection =
                        writeFactory.GetSystemFontCollection();

            int count = writeFontCollection.GetFontFamilyCount();
            string[] fonts = new string[count];

            for (int i = 0; i < count; i++)
            {
                WriteFontFamily writeFontFamily =
                                    writeFontCollection.GetFontFamily(i);

                WriteLocalizedStrings writeLocalizedStrings =
                                    writeFontFamily.GetFamilyNames();
                int index;

                if (writeLocalizedStrings.FindLocaleName("en-us", out index))
                    fonts[i] = writeLocalizedStrings.GetString(index);
                else
                    fonts[i] = writeLocalizedStrings.GetString(0);
            }

            Array.Sort<string>(fonts);
            fontFamilyComboBox.ItemsSource = fonts;

            // Загрузка текущего документа
            StorageFolder localFolder = ApplicationData.Current.LocalFolder;

            try
            {
                StorageFile storageFile = await localFolder.CreateFileAsync("RichTextEditor.rtf",
                                                       CreationCollisionOption.OpenIfExists);
                IRandomAccessStream stream = await storageFile.OpenAsync(FileAccessMode.Read);
                richEditBox.Document.LoadFromStream(TextSetOptions.FormatRtf, stream);
            }
            catch
            {
                // Исключения игнорируются
            }

            // Загрузка параметров выделения
            IPropertySet propertySet = ApplicationData.Current.LocalSettings.Values;

            if (propertySet.ContainsKey("SelectionStart"))
                richEditBox.Document.Selection.StartPosition = (int)propertySet["SelectionStart"];

            if (propertySet.ContainsKey("SelectionEnd"))
                richEditBox.Document.Selection.EndPosition = (int)propertySet["SelectionEnd"];
        }

        private async void OnAppSuspending(object sender, SuspendingEventArgs e)
        {
            SuspendingDeferral deferral = e.SuspendingOperation.GetDeferral();

            // Сохранение текущего документа
            StorageFolder localFolder = ApplicationData.Current.LocalFolder;

            try
            {
                StorageFile storageFile = await localFolder.CreateFileAsync("RichTextEditor.rtf",
                                                       CreationCollisionOption.ReplaceExisting);
                IRandomAccessStream stream = await storageFile.OpenAsync(FileAccessMode.ReadWrite);
                richEditBox.Document.SaveToStream(TextGetOptions.FormatRtf, stream);
            }
            catch
            {
                // Исключения игнорируются
            }

            // Сохранение параметров выделения
            IPropertySet propertySet = ApplicationData.Current.LocalSettings.Values;
            propertySet["SelectionStart"] = richEditBox.Document.Selection.StartPosition;
            propertySet["SelectionEnd"] = richEditBox.Document.Selection.EndPosition;

            deferral.Complete();
        }
        
        // ...

        
    }
}

private void AppBar_Opened(object sender, object e)
{
    // Получение форматирования символов для текущего выделения
    ITextCharacterFormat charFormat = richEditBox.Document.Selection.CharacterFormat;

    // Задание состояния кнопок AppBarToggleButton в строке приложения
    boldAppBarCheckBox.IsChecked = charFormat.Bold == FormatEffect.On;
    italicAppBarCheckBox.IsChecked = charFormat.Italic == FormatEffect.On;
    underlineAppBarCheckBox.IsChecked = charFormat.Underline == UnderlineType.Single;

    // Заполнение двух полей ComboBox's
    fontSizeComboBox.SelectedItem = (int)charFormat.Size;
    fontFamilyComboBox.SelectedItem = charFormat.Name;
}

private void boldAppBarCheckBox_Checked(object sender, RoutedEventArgs e)
{
    richEditBox.Document.Selection.CharacterFormat.Bold =
        (sender as AppBarToggleButton).IsChecked.Value ? FormatEffect.On : FormatEffect.Off;
}

private void italicAppBarCheckBox_Checked(object sender, RoutedEventArgs e)
{
    richEditBox.Document.Selection.CharacterFormat.Italic =
        (sender as AppBarToggleButton).IsChecked.Value ? FormatEffect.On : FormatEffect.Off;
}

private void underlineAppBarCheckBox_Checked(object sender, RoutedEventArgs e)
{
    richEditBox.Document.Selection.CharacterFormat.Underline =
        (sender as AppBarToggleButton).IsChecked.Value ? UnderlineType.Single : UnderlineType.None;
}

private void fontSizeComboBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    ComboBox comboBox = sender as ComboBox;
    if (comboBox.SelectedItem != null)
    {
        richEditBox.Document.Selection.CharacterFormat.Size = (int)comboBox.SelectedItem;
    }
}

private void fontFamilyComboBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    ComboBox comboBox = sender as ComboBox;
    if (comboBox.SelectedItem != null)
    {
        richEditBox.Document.Selection.CharacterFormat.Name = (string)comboBox.SelectedItem;
    }
}

private void alignAppBarRadioButton_Click(object sender, RoutedEventArgs e)
{
    ParagraphAlignment paragraphAlign = ParagraphAlignment.Undefined;

    if (sender == alignLeftAppBarRadioButton)
        paragraphAlign = ParagraphAlignment.Left;

    else if (sender == alignCenterAppBarRadioButton)
        paragraphAlign = ParagraphAlignment.Center;

    else if (sender == alignRightAppBarRadioButton)
        paragraphAlign = ParagraphAlignment.Right;

    richEditBox.Document.Selection.ParagraphFormat.Alignment = paragraphAlign;
}

private async void open_Click(object sender, RoutedEventArgs e)
{
    FileOpenPicker picker = new FileOpenPicker();
    picker.FileTypeFilter.Add(".txt");
    picker.FileTypeFilter.Add(".rtf");
    StorageFile storageFile = await picker.PickSingleFileAsync();

    // If user presses Cancel, result is null
    if (storageFile == null)
        return;

    TextSetOptions textOptions = TextSetOptions.None;

    if (storageFile.ContentType != "text/plain")
        textOptions = TextSetOptions.FormatRtf;

    string message = null;

    try
    {
        IRandomAccessStream stream = await storageFile.OpenAsync(FileAccessMode.Read);
        richEditBox.Document.LoadFromStream(textOptions, stream);
    }
    catch (Exception exc)
    {
        message = exc.Message;
    }

    if (message != null)
    {
        MessageDialog msgdlg = new MessageDialog("Файл не может быть открыт. " +
                         "Windows сообщила об ошибке: " +
                         message, "RichTextEditor");
        await msgdlg.ShowAsync();
    }
}

private async void save_Click(object sender, RoutedEventArgs e)
{
    FileSavePicker picker = new FileSavePicker();
    picker.DefaultFileExtension = ".rtf";
    picker.FileTypeChoices.Add("Документ RTF", new List<string> { ".rtf" });
    picker.FileTypeChoices.Add("Текстовый документ", new List<string> { ".txt" });
    StorageFile storageFile = await picker.PickSaveFileAsync();

    if (storageFile == null)
        return;

    TextGetOptions textOptions = TextGetOptions.None;

    if (storageFile.ContentType != "text/plain")
        textOptions = TextGetOptions.FormatRtf;

    string message = null;

    try
    {
        IRandomAccessStream stream = await storageFile.OpenAsync(FileAccessMode.ReadWrite);
        richEditBox.Document.SaveToStream(textOptions, stream);
    }
    catch (Exception exc)
    {
        message = exc.Message;
    }

    if (message != null)
    {
        MessageDialog msgdlg = new MessageDialog("Файл не может быть сохранен. " +
                         "Windows сообщила об ошибке: " +
                         message, "RichTextEditor");
        await msgdlg.ShowAsync();
    }
}