Silverlight searcher

I'm trying to do something similar to a searcher in Windows Phone 7, and what I've done is the following, I've got a TextBox with a TextChanged event and a Listbox of HyperlinkButtons. What I try it's this:

private void searchFriend_TextChanged(object sender, TextChangedEventArgs e)
{
  int index = 0;
  foreach (Person person in lbFriends.Items)
  {
    ListBoxItem lbi = lbFriends.ItemContainerGenerator.ContainerFromItem(index) as ListBoxItem;
    lbi.Visibility = Visibility.Visible;

    if (!person.fullName.Contains((sender as TextBox).Text))
    {
      lbi.Background = new SolidColorBrush(Colors.Black);
    }
    index++;
  }
}

And here is the xaml:

<TextBox x:Name="searchFriend" TextChanged="searchFriend_TextChanged" />
<ListBox x:Name="lbFriends" Height="535" Margin="0,0,0,20">
  <ListBox.ItemTemplate>
    <DataTemplate>
      <StackPanel>
        <HyperlinkButton x:Name="{Binding id}" Content="{Binding fullName}" FontSize="24" Click="NavigateToFriend_Click" />
      </StackPanel>
    </DataTemplate>
  </ListBox.ItemTemplate>
</ListBox>

The problem here is when I've 68 or more elements, the ContainerFromItem just returns null ListBoxItems...

Any idea?

Thank you all

if the point is to filter the elements in the listbox then use a CollectionViewSource :

System.Windows.Data.CollectionViewSource cvs;
private void SetSource(IEnumerable<string> source)
{
     cvs = new System.Windows.Data.CollectionViewSource();
     cvs.Source=source;
     listBox1.ItemsSource = cvs.View;
}
private void TextBox_TextChanged(object sender, TextChangedEventArgs e)
{
     var box = (TextBox)sender;
     if (!string.IsNullOrEmpty(box.Text))
        cvs.View.Filter = o => ((string)o).Contains(box.Text);
     else
        cvs.View.Filter = null;
}

the problem using ItemContainer is that they are created only when the item has to be shown, that's why you have a null value.