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<Window x:Class="WpfApplication2.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="281" Width="461">
    <DockPanel LastChildFill="True">
        <Menu DockPanel.Dock="Top">
            <MenuItem Header="Editar">
                <MenuItem Header="Cortar" Command="ApplicationCommands.Cut"/>
                <MenuItem Header="Copiar" Command="ApplicationCommands.Copy"/>
                <MenuItem Header="Pegar" Command="ApplicationCommands.Paste"/>
                <Separator/>
                <MenuItem Header="Seleccionar Todos" Click="Seleccionar"/>
                <MenuItem Header="Destacar" Click="Destacar" FontWeight="Bold" Name="miBold"/>
                <MenuItem Header="Italico" Click="Estilo" FontStyle="Italic" Name="miItalic"/>
            </MenuItem>
        </Menu>
        <TextBox Name="txtTextbox" TextWrapping="Wrap">Algun Texto que se te ocurra</TextBox>
    </DockPanel>
</Window>


VENTANA DE CODIGO XAML.CS
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
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 WpfApplication2
{
    /// <summary>
    /// Lógica de interacción para MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Destacar(object sender, RoutedEventArgs e)
        {
            txtTextbox.FontWeight = FontWeights.Bold;
        }

        private void Estilo(object sender, RoutedEventArgs e)
        {
            txtTextbox.FontStyle = FontStyles.Italic;
        }

        private void Seleccionar(object sender, RoutedEventArgs e)
        {
            txtTextbox.SelectAll();
        }
    }
}