Using VisualTreeHelper on listbox, can't get listboxitems

Question

I have the following XAML:

Answer 1

Silverlight is Asynchronous!

...the data in the listbox is reloaded...

When you change DataContext with some new data it doesn't mean that actual data will be loaded immediately. Just before you trying to get ListboxItem just simply call listBox1.UpdateLayout and everything should be fine.

public partial class MainPage : UserControl
{
    public MainPage()
    {
        InitializeComponent();

        Loaded += new RoutedEventHandler(MainPage_Loaded);
    }

    void MainPage_Loaded(object sender, RoutedEventArgs e)
    {
        ReloadData();
    }

    private void button1_Click(object sender, RoutedEventArgs e)
    {
        // 1 - Uncomment this line to crush your app
        // ReloadData();

        // 2 - Uncomment this line to fix everything
        // listBox1.UpdateLayout();

        // UpdateLayout - ensures that actual data is loaded to UI,             
        // all items are created and rendered 

        GetItemsRecursive(listBox1);
    }

    private void GetItemsRecursive(DependencyObject lb)
    {
        var childrenCount = VisualTreeHelper.GetChildrenCount(lb);

        for (int i = 0; i < childrenCount; i++)
        {
            var child = VisualTreeHelper.GetChild(lb, i);


            if (child is ListBoxItem)
            {
                MessageBox.Show(child.GetType().ToString());
                return;
            }

            GetItemsRecursive(child);
        }
    }

    private void ReloadData()
    {
        listBox1.DataContext = new List<Classs>() {
            new Classs{ Propty1 = "sasda", Propty2 = false, Propty3 = "asdasda"},
            new Classs{ Propty1 = "sasda", Propty2 = true, Propty3 = "asdasda"},
            new Classs{ Propty1 = "sasda", Propty2 = false, Propty3 = "asdasda"}
        };
    }
}

public class Classs
{
    public string Propty1 { get; set; }

    public bool Propty2 { get; set; }

    public string Propty3 { get; set; }
}

Answer 2

Please try this code snipped below to get the TextBox in ListBoxItem.

ListBox listbox = element as ListBox;
if (listbox != null && listbox.SelectedItem !=null)
{             
//find textbox and set focus here 

Textbox thisTextBox = (listbox.SelectedItem).Find("txt1") as Textbox;
if(thisTextBox !=null)
{
    thisTextBox.Focus();
    return;
}

}
int children = VisualTreeHelper.GetChildrenCount(element);         
for (int i = 0; i < children; i++)         
{
     DependencyObject child = VisualTreeHelper.GetChild(element, i);
     SetFocusOnTextBox(child);         
}