WPF密码框中禁止复制、粘贴

如题：

　　　　//前台代码
　　　　 <PasswordBox x:Name="txtPwd" Height="25" Width="222" FontSize="15" Margin="215,32,151,0" >
                <PasswordBox.CommandBindings>
                    <CommandBinding Command="ApplicationCommands.Paste" Executed="CommandBinding_Executed" CanExecute="CommandBinding_CanExecute"/>
                </PasswordBox.CommandBindings>
        </PasswordBox>
        //后台代码
        private void CommandBinding_Executed(object sender, ExecutedRoutedEventArgs e)
        {

        }

        private void CommandBinding_CanExecute(object sender, CanExecuteRoutedEventArgs e)
        {
            e.Handled = true;
        }