Datagrid expander (from grouping) header - wpf

Question

Following this tutorial I had an idea to put in the Expander Header more data.
I have 2 tables (Document 1 - * Entry).
I'm displaying the Entries grouped by Documents and I don't want some data to be repeated in the
datagrid so I thought to place it in the expander header.
<DataGrid.GroupStyle>
<GroupStyle>
<GroupStyle.HeaderTemplate>
<DataTemplate>
<StackPanel>
<TextBlock Text="{Binding Path=Name}" />
</StackPanel>
</DataTemplate>
</GroupStyle.HeaderTemplate>
<GroupStyle.ContainerStyle>
<Style TargetType="{x:Type GroupItem}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type GroupItem}">
<Expander IsExpanded="True">
<Expander.Header>
<StackPanel Orientation="Horizontal">
<TextBlock Text="{Binding Path=Name}" />
<TextBlock Text=" - "/>
**<TextBlock Text="{Binding Path=Document.Number or Name2}"/>**
</StackPanel>
...

Answer 1

You can do this:
<Expander.Header>
<StackPanel Orientation="Horizontal">
<TextBlock Text="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type GroupItem}}, Converter={StaticResource ResourceKey=groupToTitleConverter}}" />
</StackPanel> </Expander.Header>
Converter:
public class GroupToTitleConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
GroupItem groupItem = value as GroupItem;
CollectionViewGroup collectionViewGroup = groupItem.Content as CollectionViewGroup;
EntryViewModel entryViewModel = collectionViewGroup.Items[0] as EntryViewModel;
string title = string.Format("{0} - {1} {2}", entryViewModel.Id, entryViewModel.Numar, entryViewModel.Obiect);
return title;
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
throw new NotImplementedException();
}
}
Taking first item from collection in group to form header title might not be the most elegant solution but it will serve the purpose.
Full code is available here: ExpanderHeadersInDataGridGroupStyle.zip