Use TextBox.CommandBindingst to bind command


   
  
<Window x:Class="Commands.TwoDocument"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="TwoDocument" Height="300" Width="300">

  <Window.Resources>
    <CommandBinding x:Key="binding" Command="ApplicationCommands.Save" Executed="SaveCommand" CanExecute="SaveCommand_CanExecute" />
  </Window.Resources>

  <Grid>
    <Grid.RowDefinitions>
      <RowDefinition Height="Auto"></RowDefinition>
      <RowDefinition Height="Auto"></RowDefinition>
      <RowDefinition></RowDefinition>
      <RowDefinition></RowDefinition>
    </Grid.RowDefinitions>
    <Menu Grid.Row="0">
      <MenuItem Header="File">
        <MenuItem Command="New"></MenuItem>
        <MenuItem Command="Open"></MenuItem>
        <MenuItem Command="Save"></MenuItem>
        <MenuItem Command="SaveAs"></MenuItem>
        <Separator></Separator>
        <MenuItem Command="Close"></MenuItem>
      </MenuItem>
    </Menu>

    <TextBox Margin="5" Grid.Row="3" TextWrapping="Wrap" AcceptsReturn="True" TextChanged="txt_TextChanged">
      <TextBox.CommandBindings>
        <StaticResource ResourceKey="binding"></StaticResource>
        <CommandBinding Command="ApplicationCommands.Save" Executed="SaveCommand" />
      </TextBox.CommandBindings>
    </TextBox>

  </Grid>
</Window>
//File:Window.xaml.cs
using System;
using System.Collections.Generic;
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.Shapes;

namespace Commands
{
    public partial class TwoDocument : System.Windows.Window
    {
        public TwoDocument()
        {
            InitializeComponent();
        }

        private void SaveCommand(object sender, ExecutedRoutedEventArgs e)
        {            
        }
        private void txt_TextChanged(object sender, RoutedEventArgs e)
        {
        }
        private void SaveCommand_CanExecute(object sender, CanExecuteRoutedEventArgs e)
        {
            e.CanExecute = false;
        }
    }
}