wpf binding not work on custom list object

Question

in THIS code , binding not work:

 

<Window x:Class="WpfApplication6.Window2"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:l="clr-namespace:WpfApplication6"
        Title="Window2" Height="300" Width="300">
    <Grid>
        <StackPanel>
            <Button Height="50" Width="100" Click="Button_Click_1">
                <l:Item val="{Binding ElementName=aaa,Path=Text,Mode=TwoWay}" /> <!--work!!-->
            </Button>
            <Button Height="50" Margin="30" Click="Button_Click">
                <l:ItemList>
                    <l:ItemList.MyList>
                        <l:Item val="{Binding ElementName=aaa, Path=Text, Mode=TwoWay}" /><!--not work!!-->
                        <l:Item val="{Binding ElementName=bbb, Path=Text, Mode=TwoWay}" /><!--not work!!-->
                    </l:ItemList.MyList>
                </l:ItemList>
            </Button>
            <TextBlock Text="aaa" Name="aaa" />
            <TextBlock Text="bbb" Name="bbb"/>
        </StackPanel>
    </Grid>
</Window>

 c#:

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;
using System.Collections.ObjectModel;
using System.ComponentModel;

namespace WpfApplication6
{
    /// <summary>
    /// Interaction logic for Window2.xaml
    /// </summary>
    public partial class Window2 : Window
    {
        public Window2()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            ItemList il = (((Button)sender).Content) as ItemList;
            MessageBox.Show(il.MyList[0].val);  //not work!
        }

        private void Button_Click_1(object sender, RoutedEventArgs e)
        {
            Item i = (((Button)sender).Content) as Item;
            MessageBox.Show(i.val.ToString()); //work!
        }


    }

    public class ItemList : Control
    {
        public ObservableCollection<Item> MyList
        {
            get { return (ObservableCollection<Item>)GetValue(MyListProperty); }
            set { SetValue(MyListProperty, value); }
        }
        public static readonly DependencyProperty MyListProperty =
            DependencyProperty.Register("MyList", typeof(ObservableCollection<Item>), typeof(ItemList), new UIPropertyMetadata(new ObservableCollection<Item>()));
    }

    public class Item : FrameworkElement
    {
        public string val
        {
            get { return (string)GetValue(valProperty); }
            set { SetValue(valProperty, value); }
        }
        public static readonly DependencyProperty valProperty =
            DependencyProperty.Register("val", typeof(string), typeof(Item), new UIPropertyMetadata(null,onChange));

        private static void onChange(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
        }
    }

}

 

error msg:

System.Windows.Data Error: 4 :
Cannot find source for binding with reference 'ElementName=aaa'.
BindingExpression:Path=Text; DataItem=null; target element is 'Item' (Name=''); target property is 'val' (type 'String')

Does anyone know why it does not work?

Thanks