Code

var tabDoc = dockControl.Items.OfType<TabbedDocument>().FirstOrDefault();
if (tabDoc != null)
{
    SetBinding(
            SelectedItemProperty,
            new Binding
            {
                Source = tabDoc,
                Path = new PropertyPath(DockWindowBase.SelectedItemProperty),
                Mode = BindingMode.OneWay
            });
}

DependencyProperty SelectedItemProperty = DependencyProperty.Register(
    "SelectedItem", typeof(DockItem), typeof(Window1),
    new PropertyMetadata(OnSelectedItemChanged));

static void OnSelectedItemChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    var dockItem = (DockItem)e.NewValue;
    Debug.WriteLine("OnSelectedItemChanged: " + dockItem.Content);
} 

Code

void dockControl_ItemClosing(object sender, ItemValidationEventArgs e)
{
    if (MessageBox.Show(
        "Are you sure you want to close unsaved document?",
        "Document changed", MessageBoxButton.YesNo) != MessageBoxResult.Yes)
    {
        e.Cancel = true;
    }
}