[求助]wpf中TreeView的显示问题
想做一个基于WPF的目录树
具体的效果是这样的

每个TreeViewItem显示文件夹
分别显示图标、文件夹名称和当前文件夹的子文件夹个数

现在问题就是第三项显示“当前文件夹的子文件夹个数”这一项总是为0。（调试有数值，但界面显示0）
前台代码

<Window x:Class="WpfApplication2.MainWindow"

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

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

        xmlns:loc="clr-namespace:WpfApplication2"

        Title="MainWindow" Height="350" Width="525">

    <Grid>

        <TreeView>

            <loc:MyTreeViewItem x:Name="MyFiles">

                <loc:MyTreeViewItem.Header>

                    <StackPanel Orientation="Horizontal">

                        <Image Width="16" Height="16" Source="/WpfApplication2;component/Images/Folder.png" />

                        <TextBlock Text="我的文件"></TextBlock>

                        <TextBlock Text="{Binding Count}" Margin="5,0,0,0"></TextBlock>

                    </StackPanel>

                </loc:MyTreeViewItem.Header>

                <loc:MyTreeViewItem>

                    <loc:MyTreeViewItem.Header>

                        <StackPanel Orientation="Horizontal">

                            <Image Width="16" Height="16" Source="/WpfApplication2;component/Images/Folder.png" />