WPF14：绑定中数据模型必须为public问题

不久前遇到一个问题，在绑定的时候打算将数据模型类全部设置为internal类型，进行模块的封装。不过当设置为internal之后绑定居然不起作用了。代码如下：
数据模型部分：

public abstract class NotifyBind : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;


        public void OnPropertyChanged(string propname)
        {
            if (this.PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propname));
            }
        }
    }

    public class MainModel:NotifyBind
    {
        private string ishowTest = string.Empty;


        private string pshowTest = string.Empty;


        internal string IShowTest
        {
            get { return this.ishowTest; }


            set
            {
                this.ishowTest = value;


                this.OnPropertyChanged("IShowTest");
            }
        }


        public string PShowTest
        {
            get { return this.pshowTest; }


            set
            {
                this.pshowTest = value;


                this.OnPropertyChanged("PShowTest");
            }
        }
    }

页面部分：

<Window x:Class="TestInternalBinding.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="436" Width="803">
    <Grid>
        <TextBox Height="30" HorizontalAlignment="Left" Margin="130,120,0,0" Name="textBox1" VerticalAlignment="Top" Width="186"
                 Text="{Binding PShowTest,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"/>
        <TextBox Height="30" HorizontalAlignment="Left" Margin="130,170,0,0" Name="textBox2" VerticalAlignment="Top" Width="186"
                 Text="{Binding IShowTest,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"/>
        <Button Content="public绑定" Height="23" HorizontalAlignment="Left" Margin="333,127,0,0" Name="button1" 
        VerticalAlignment="Top" Width="85" Click="button1_Click" />
        <Button Content="internal绑定" Height="23" HorizontalAlignment="Left" Margin="333,174,0,0" Name="button2" 
        VerticalAlignment="Top" Width="85" Click="button2_Click" />
    </Grid>
</Window>

后台部分：

 public partial class MainWindow : Window
    {
        private MainModel data = new MainModel();


        public MainWindow()
        {
            InitializeComponent();


            this.DataContext = data;
        }


        private void button1_Click(object sender, RoutedEventArgs e)
        {
            string showstring = string.IsNullOrEmpty(data.PShowTest) ? "空" : data.PShowTest;


            MessageBox.Show("当前为public绑定，输入内容为：" + showstring);
        }


        private void button2_Click(object sender, RoutedEventArgs e)
        {
            string showstring = string.IsNullOrEmpty(data.IShowTest) ? "空" : data.IShowTest;


            MessageBox.Show("当前为internal绑定，输入内容为：" + showstring);
        }
    }

当输入文字到文本框后，分别触发按钮点击时间后，如下图：
1、字段为public的，


2、字段为internal的，

可以看出字段为internal后，绑定是没有作用的。所以在绑定的时候数据模型类，必须为public类型才可以。

代码下载： http://download.csdn.net/detail/yysyangyangyangshan/5238192