Sort Data in ComboBox with TextSearch

Hello, I have a question about sorting Lastname and Firstname in an Combobox with TextSearch.TextPath=Lastname.

If I type a Lastname in the combobox I see in the drop-down Listbox and all Lastnames are sorted. But all Firstnames are unsorted!!

<ComboBox Name="cbOrderCustomer" ItemsSource="{Binding Path={}}" TextSearch.TextPath="Lastname" IsTextSearchEnabled="True" Height="30" Width="130" IsEditable="True" IsEnabled="True" SelectionChanged="cbOrderCustomer_SelectionChanged" ItemsPanel="{StaticResource Template_ComboPanel}"><ComboBox.ItemTemplate><DataTemplate><StackPanel Orientation="Horizontal"><TextBlock Text="{Binding Path=Lastname}" Width="120"></TextBlock><TextBlock Text="{Binding Path=Firstname}" Width="120"></TextBlock><TextBlock Text="{Binding Path=Street}" Width="100"></TextBlock><TextBlock Text="{Binding Path=City}" Width="80"></TextBlock><TextBlock Text="{Binding Path=PostalCode}" Width="50"></TextBlock></StackPanel></DataTemplate></ComboBox.ItemTemplate></ComboBox>

I tried .Select("", "Lastname, Firstname);

(CollectionViewSource.GetDefaultView(this.cbOrderCustomer.ItemsSource) as ICollectionView).SortDescriptions.Clear();

(CollectionViewSource.GetDefaultView(this.cbOrderCustomer.ItemsSource) as ICollectionView).SortDescriptions.Add(new SortDescription { PropertyName = "Lastname", Direction = ListSortDirection.Ascending });
(CollectionViewSource.GetDefaultView(this.cbOrderCustomer.ItemsSource) as ICollectionView).SortDescriptions.Add(new SortDescription { PropertyName = "Firstname", Direction = ListSortDirection.Ascending });

But nothing works!

What can I do?

Greetings from Germany

Michael Burkhardt

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Dipl. Inform. Michael Burkhardt NRW, Dortmund