How to define a different style on a TreeViewItem based on Type

Question

I have my own treeview implementation based on this post, but I have problems defining a different style based on what object type is in the treeviewitem. I know the simple way is to define a templateselector, but I can't figure out how this would work when you have two HierarchicalDataTemplates.

 <HierarchicalDataTemplate DataType="{x:Type domainLayer:Folder}" ItemsSource="{Binding Converter={StaticResource BaseTypeConverter}}" />
 <HierarchicalDataTemplate DataType="{x:Type domainLayer:Document}" ItemsSource="{Binding Converter={StaticResource BaseTypeConverter}}" />

I use these templates to lazy load my tree, which works fine. There's also a Style defined for all TreeListViewItems. Maybe there's a solution too, but I can't figure out how to define if my TreeListViewItem is either a Folder or a Document.

Any help is much appreciated. If you need more code, just let me know!

Update 10:40am:

In a Style the row is defined for every TreeListViewItem:

<Style x:Key="cxc" TargetType="{x:Type local:TreeListViewItem}">
    <Setter Property="FontFamily" Value="TradeGothic LT" />
    <Setter Property="FontSize" Value="14px" />
    <Setter Property="Foreground" Value="{StaticResource TextBrush}" />
    <Setter Property="Background" Value="Transparent" />
    <EventSetter Event="MouseDoubleClick" Handler="OnItemMouseDoubleClick" />
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type local:TreeListViewItem}">
                <StackPanel>
                    <Border Name="Bd"
                            Background="{TemplateBinding Background}"
                            BorderBrush="{TemplateBinding BorderBrush}"
                            BorderThickness="{TemplateBinding BorderThickness}"
                            Padding="{TemplateBinding Padding}">
                        <GridViewRowPresenter x:Name="PART_Header"
                                                Content="{TemplateBinding Header}"
                                                Columns="{Binding Path=Columns,RelativeSource={RelativeSource AncestorType={x:Type local:TreeListView}}}"
                                                />
                    </Border>
                    <ItemsPresenter x:Name="ItemsHost" />
                </StackPanel>
                <ControlTemplate.Triggers>
<!-- ... -->

When I copy the template (everything in the StackPanel tag) to the HierarchicalDataTemplate I get an error: Cannot find the static member 'BackgroundProperty' on the type 'ContentPresenter'.

Answer 1

You seem to be a little bit confused by the different Templates that WPF provides. First, we can define a Style for the TreeViewItem (the ItemContainerStyle), the data item 'container'. Then we can define a DataTemplate for the data item that appears within the container (the ItemTemplate), or HierarchicalDataTemplate in your case.

So don't try to put your data item Binding in the ItemContainerStyle and don't put your UI element Binding in the DataTemplate and you should be fine. Note that you can supply one DataTemplate/HierarchicalDataTemplate without setting their x:Key property for each data type in your collection and there is no need to use a DataTemplateSelector... let WPF do the implicit selecting of DataTemplates based on the data item's types. Here is a simple example:

In code:

public class BaseClass { }

public class ClassA : BaseClass { }

public class ClassB : BaseClass { }

...

public Observablecollection<BaseClass> Items { get ; set; }

In XAML:

<ListBox ItemsSource="{Binding Items}">
    <ListBox.Resources>
        <DataTemplate DataType="{x:Type DataTypes:ClassA}">
            <Ellipse Fill="Red" />
        </DataTemplate>
        <DataTemplate DataType="{x:Type DataTypes:ClassB}">
            <Rectangle Fill="Blue" />
        </DataTemplate>
    </ListBox.Resources>
</ListBox>

This ListBox would display red circles for the ClassA items and blue rectangles for the ClassB items. You can do a similar thing for your folders and files

Answer 2

I've solved my problem as follows. In the aforementioned Style I've added a new MultiDataTrigger:

<MultiDataTrigger>
    <MultiDataTrigger.Conditions>
        <Condition
            Binding="{Binding RelativeSource={RelativeSource Self}, Path=Header, Converter={StaticResource IsFolder}}"
            Value="False" />
        <Condition
            Binding="{Binding RelativeSource={RelativeSource Self}, Path=IsSelected}"
            Value="True" />
    </MultiDataTrigger.Conditions>
    <Setter TargetName="Bd"
            Property="Background"
            Value="{StaticResource HighlightBackgroundBrush}" />
</MultiDataTrigger>

This seems to work and looks like the proper solution. Thanks, Sheridan for the background on this.