WPF RichTextBox Simple word editor

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.NET

WPF

.NET4.5

RichTextBox

Hi, I'm trying to make a simple wordpad application. But it still doesn't work as I expected. Can anyone try the code below and tell me what I do wrong?

Thanks for advices.

<Window x:Class="WPFRtBox.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="447" Width="395">
    <Grid>
        <ComboBox x:Name="fontSizeComBox" HorizontalAlignment="Left" Margin="10,10,0,0" VerticalAlignment="Top" Width="50" SelectionChanged="fontSizeComBox_SelectionChanged"/>
        <RichTextBox x:Name="rtBox" HorizontalAlignment="Left" Height="369" Margin="10,37,0,0" VerticalAlignment="Top" Width="367" SelectionChanged="rtBox_SelectionChanged">
            <RichTextBox.Resources>
                <Style TargetType="{x:Type Paragraph}">
                    <Setter Property="Margin" Value="0"/>
                </Style>
            </RichTextBox.Resources>
        </RichTextBox>
    </Grid>
</Window>

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 WPFRtBox
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            fontSizeComBox.ItemsSource = new List<double>() { 8, 9, 10, 11, 12, 14, 16, 18, 20, 22, 24, 26, 28, 36, 48, 72 };
            fontSizeComBox.SelectedIndex = 4;
            rtBox.AcceptsTab = true;
            rtBox.AcceptsReturn = true;
            rtBox.VerticalScrollBarVisibility = ScrollBarVisibility.Visible;
        }

        private void fontSizeComBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            if (rtBox.Selection.IsEmpty)
            {
                Run r = new Run() { FontSize = (double)fontSizeComBox.SelectedValue };
                rtBox.CaretPosition.Paragraph.Inlines.Add(r);
                rtBox.CaretPosition = r.ElementStart;
            }
            else
            {
                rtBox.Selection.ApplyPropertyValue(Inline.FontSizeProperty, fontSizeComBox.SelectedItem);
            }
            rtBox.Focus();
        }

        private void rtBox_SelectionChanged(object sender, RoutedEventArgs e)
        {
            object selChanged = rtBox.Selection.GetPropertyValue(Inline.FontSizeProperty);
            fontSizeComBox.Text = selChanged.ToString();
        }
    }
}