Create a simple application to change the font of text in a RichTextBox control by using controls in a toolbar.

Sample code:

XAML

<Window x:Class="SimpleApp.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <DockPanel>
        <ToolBar DockPanel.Dock="Top" Height="26" Name="toolBar1" Width="276">
            <Button Margin="0,0,0,-0.2" VerticalAlignment="Top" Click="Button_Click_1">Bold

             </Button>
            <Button Click="Button_Click_2">Italic</Button>
            <Slider Name="Slider1" Minimum="2" Maximum="72" Width="100" 

             ValueChanged="Slider1_ValueChanged"/>
        </ToolBar>
        <Grid Name="grid1">
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="100"/>
                <ColumnDefinition Width="5"/>
                <ColumnDefinition Width="*"/>
            </Grid.ColumnDefinitions>
 
            <ListBox Grid.Column="0" Name="listBox1" 

            SelectionChanged="listBox1_SelectionChanged"></ListBox>
            <GridSplitter Name="gridSplitter1" Margin="0" Width="5"
            Grid.Column="1" HorizontalAlignment="Left"/>
            <RichTextBox Grid.Column="2" Name="richTextBox1"/>
        </Grid>       
    </DockPanel>
</Window>

Event handling codes

using System.Windows;
using System.Windows.Controls;
using System.Windows.Media;
 
namespace SimpleApp
{
  /// <summary>
  /// Interaction logic for MainWindow.xaml
  /// </summary>
  public partial class MainWindow : Window
  {
    public MainWindow()
    {
      InitializeComponent();
      foreach (FontFamily F in Fonts.SystemFontFamilies)
      {
        ListBoxItem l = new ListBoxItem();
        l.Content = F.ToString();
        l.FontFamily = F;
        listBox1.Items.Add(l);
      }
    }
 
    private void Button_Click_1(object sender, RoutedEventArgs e)
    {
      richTextBox1.Selection.ApplyPropertyValue(
        FontWeightProperty,
        FontWeights.Bold);
    }
 
    private void Button_Click_2(object sender, RoutedEventArgs e)
    {
      richTextBox1.Selection.ApplyPropertyValue(
        FontStyleProperty,
        FontStyles.Italic);
    }
 
    private void Slider1_ValueChanged(object sender, RoutedPropertyChangedEventArgs<double>)
    {
      try
      {
        richTextBox1.Selection.ApplyPropertyValue(FontSizeProperty,
        Slider1.Value.ToString());
      }
      catch { }
    }
 
    private void listBox1_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
      richTextBox1.Selection.ApplyPropertyValue(
        FontFamilyProperty,
        ((ListBoxItem)listBox1.SelectedItem).FontFamily);
    }
  }
}