<Window x:Class="WpfApplication1.Window2"

        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

        Title="Window2" Height="345" Width="478">

    <Grid>

        <TextBox Height="23" HorizontalAlignment="Left" Margin="152,50,0,0" Name="textBox1" VerticalAlignment="Top" Width="158" Text="ABCDE" />

        <TextBox Height="23" HorizontalAlignment="Left" Margin="152,105,0,0" Name="textBox2" Text="{Binding Path=Text.[3],ElementName=textBox1,Mode=OneWay}" VerticalAlignment="Top" Width="158"/>

    </Grid>

</Window>

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.Shapes;

namespace WpfApplication1

{

    /// <summary>

    /// Window2.xaml 的交互逻辑

    /// </summary>

    public partial class Window2 : Window

    {

        public Window2()

        {

            InitializeComponent();

            //this.textBox2.SetBinding(TextBox.TextProperty, new Binding("Text.Length") {Source = textBox1, Mode= BindingMode.OneWay });

            this.textBox2.SetBinding(TextBox.TextProperty, new Binding("Text.[3]") { Source=textBox1,Mode= BindingMode.OneWay});

        }

    }

}