ICollectionView collView = null; 
datagrid.ItemsSource = observerItems; 
collView = CollectionViewSource.GetDefaultView(datagrid.ItemsSource);
collView.SortDescriptions.Add(new SortDescription("Lastname", ListSortDirection.Ascending));
collView.SortDescriptions.Add(new SortDescription("Firstname", ListSortDirection.Ascending));