Expander with Virtualization inside WPF Datagrid - wpf

Question

I have a performance issue in a prototype I am working on. The requirement is to build a datagrid with multiple synchronized frozen panes, supporting grouping and sorting etc... For more details about the grid I am building, see this previous question.
Now, I have a question related to Grouping and in particular Expanders. I have a GroupStyle defined by the following Xaml, and taken from this blog post.
<!--Default GroupStyle-->
<GroupStyle x:Key="gs_Default">
<GroupStyle.Panel>
<ItemsPanelTemplate>
<DataGridRowsPresenter/>
</ItemsPanelTemplate>
</GroupStyle.Panel>
<GroupStyle.HeaderTemplate>
<DataTemplate>
<StackPanel>
<TextBlock Text="{Binding Path=Name}" FontWeight="Bold" Padding="3"/>
</StackPanel>
</DataTemplate>
</GroupStyle.HeaderTemplate>
<GroupStyle.ContainerStyle>
<Style TargetType="{x:Type GroupItem}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type GroupItem}">
<Expander x:Name="exp"
BorderBrush="#FFA4B97F"
BorderThickness="0,0,0,1"
IsExpanded="{Binding Path=Items[0].IsExpanded}">
<Expander.Header>
<DockPanel TextBlock.FontWeight="Bold">
<TextBlock Text="{Binding Path=Name}" Margin="5,0,5,0" />
<TextBlock Text="{Binding Path=ItemCount}"/>
</DockPanel>
</Expander.Header>
<ItemsPresenter/>
</Expander>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</GroupStyle.ContainerStyle>
</GroupStyle>
The Expander is not virtualized and we are experiencing a performance issue when there are several hundred rows in a group.
Has anyone encoutered this before and have a fix? I am ideally looking for a Virtualizing Expander, and have seen workarounds such as this (which doesn't solve the problem).

Answer 1

If you're using .NET 4 then get the ItemsPresenters visibility to track/be dependent on the IsExpanded state.
WPF DataGrid Virtualization with Grouping
In .NET 4.5 there's a new attached property VirtualizingPanel.IsVirtualizingWhenGrouping.
Another alternative is to push the grouping into the ViewModel rather than making the DataGrid/CollectionView be responsible for that. See here:
http://blog.smoura.com/wpf-toolkit-datagrid-part-iv-templatecolumns-and-row-grouping/

Answer 2

In order to solve this, we did the following.
We used DataTemplates to present a different row view for different row viewmodels. We had a GroupRowViewModel and ItemRowViewModel. Also a parent ViewModel which had a sorted list of Group/Item viewmodels.
When the grid was instantiated, the parent ViewModel would sort all child viewmodels into the following:
Group
Item
Item
Item
Group
Item
Item
Item
When a GroupRow is clicked you want to execute some code where the parent (which contains a sorted list of group+item rows) will remove or include the items. E.g. say the second gropu was clicked, your list of rowviewmodels you bind to now becomes
Group
Item
Item
Item
Group (Collapsed)
That's it. So no magic at all, you manually remove or include the rows you want depending on what was clicked. It worked with virtualization and hundreds of thousands of rows at an acceptible speed.
Sorry I can't post any code (due to NDA) but I hope that helps you. Also - I would suggest looking at Telerik Grid as this is awesomely fast for large datasets