问如何在ComboBox中根据XAML值改变字体大小

EN

<Window x:Class="DataBinding.UsingCommandsSample"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:DataBinding"
        mc:Ignorable="d"
        Title="MainWindow" WindowState="Maximized" >
    <DockPanel>

        <Menu  DockPanel.Dock="Top" Margin="0 0 0 10">
            <MenuItem Header="_File" Name="file">
                <MenuItem Header="Open File" Click="btnOpenFile_Click"/>
                <MenuItem Header="Save File" Click="btnSaveFile_Click">
                    <MenuItem Header="Save As" Click="btnSaveAs_Click" />
                </MenuItem>

                <MenuItem Header="Save As" Click="btnSaveAs_Click" />
                <MenuItem Command="Print" />
            </MenuItem>
            <MenuItem Header="_Edit" Name="edit">
                <MenuItem Command="Copy" />
                <MenuItem Command="Cut" />
                <MenuItem Command="Paste" />
                <MenuItem Command="SelectAll" />
                <MenuItem Command="Undo" />
                <MenuItem Command="AlignLeft" />
                <MenuItem Command="EditingCommands.AlignCenter" />
                <MenuItem Command="AlignRight" />
                <MenuItem Command="AlignJustify" />
            </MenuItem>
        </Menu>

        <ToolBarTray DockPanel.Dock="Top" Height="auto">
            <ToolBar>
                <Button Command="Cut" Content="Cut" />
                <Button Command="Copy" Content="Copy"  />
                <Button Command="Paste" Content="Paste" />
                <ComboBox x:Name="fontBox" SelectedValue="selected" SelectionChanged="fontBox_SelectionChanged">
                    <ComboBoxItem Content="12" IsSelected="True"/>
                    <ComboBoxItem Content="16" />
                    <ComboBoxItem Content="18" />
                    <ComboBoxItem Content="20" />
                </ComboBox>
            </ToolBar>

        </ToolBarTray>
        <TextBox Name="txtEditor" AcceptsReturn="True" />
    </DockPanel>
</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;
using System.ComponentModel;
using System.Collections.ObjectModel;
using Microsoft.Win32;
using System.IO;

namespace DataBinding
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class UsingCommandsSample : Window
    {
        public UsingCommandsSample()
        {
            InitializeComponent();
        }
        public string path;

        private void btnOpenFile_Click(object sender, RoutedEventArgs e)
        {
            OpenFileDialog openFileDialog = new OpenFileDialog();
            if (openFileDialog.ShowDialog() == true)
                txtEditor.Text = File.ReadAllText(openFileDialog.FileName);
            path = openFileDialog.FileName;
        }

        private void btnSaveFile_Click(object sender, RoutedEventArgs e)
        {
            if (path != null)
                File.WriteAllText(path, txtEditor.Text);
            else
                MessageBox.Show("You have not specified a location to which the file should be saved. Click 'OK' then File >> Save As.", "Cannot find path", MessageBoxButton.OK, MessageBoxImage.Warning);       
        }



        private void btnSaveAs_Click(object sender, RoutedEventArgs e)
        {
            SaveFileDialog saveFileDialog = new SaveFileDialog();
            saveFileDialog.Filter = "Text file (*.txt)|*txt | Banana (*.banana)|*.cs";
            if (saveFileDialog.ShowDialog() == true)
                File.WriteAllText(saveFileDialog.FileName, txtEditor.Text);
        }

        private void fontBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            int temp;
            if (Int32.TryParse(fontBox.Text, out temp))
            {
                txtEditor.FontSize = temp;
            }
        }
    }
}