How to define a TreeViewItem Triggers in a TreeView user control - wpf

Question

I have created TreeView user control that allows multiselection. In order to visually show the selected items I use triggers in a Style for TreeViewItem. This Style is defined in the user control resources as follows:
<TreeView x:Class="SchoolsExample.MultiSelectionTreeView"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
mc:Ignorable="d"
xmlns:local="clr-namespace:SchoolsExample"
d:DesignHeight="300" d:DesignWidth="300" SelectedItemChanged="TreeView_SelectedItemChanged">
<TreeView.Resources>
<Style TargetType="TreeViewItem">
<EventSetter Event="PreviewMouseLeftButtonDown" Handler="TreeViewItem_MouseLeftButtonDown"/>
<EventSetter Event="MouseLeftButtonUp" Handler="TreeViewItem_MouseLeftButtonUp"/>
<Style.Triggers>
<Trigger Property="local:MultiSelectionTreeViewItemBehaviour.IsMultiSelected" Value="True">
<Setter Property="Background" Value="{DynamicResource {x:Static SystemColors.HighlightBrushKey}}"/>
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.HighlightTextBrushKey}}"/>
</Trigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding Path=(local:MultiSelectionTreeViewItemBehaviour.IsMultiSelected), RelativeSource={RelativeSource Self}}" Value="True"/>
<Condition Binding="{Binding IsSelectionActive, RelativeSource={RelativeSource Self}}" Value="False"/>
</MultiDataTrigger.Conditions>
<MultiDataTrigger.Setters>
<Setter Property="Background" Value="{DynamicResource {x:Static SystemColors.InactiveSelectionHighlightBrushKey}}"/>
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.InactiveSelectionHighlightTextBrushKey}}"/>
</MultiDataTrigger.Setters>
</MultiDataTrigger>
</Style.Triggers>
</Style>
</TreeView.Resources>
</TreeView>
But when I want to use this control I want to style my TreeViewItems something like this:
<local:MultiSelectionTreeView x:Name="MultiSelectionTreeView" ItemsSource="{Binding Schools}">
<local:MultiSelectionTreeView.Resources>
<Style TargetType="TreeViewItem">
<Setter Property="local:MultiSelectionTreeViewItemBehaviour.IsMultiSelected" Value="{Binding IsSelected, Mode=TwoWay}"/>
</Style>
</local:MultiSelectionTreeView.Resources>
<local:MultiSelectionTreeView.ItemTemplate>
<HierarchicalDataTemplate ItemsSource="{Binding Pupils}">
<TextBlock Text="{Binding Name}"/>
<HierarchicalDataTemplate.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding FullName}"/>
</DataTemplate>
</HierarchicalDataTemplate.ItemTemplate>
</HierarchicalDataTemplate>
</local:MultiSelectionTreeView.ItemTemplate>
</local:MultiSelectionTreeView>
Then I get an exception saying that I can't set the Resource twice. Even if I manage to set the style for the TreeViewItems somehow, then the visual behaviour that I want for the TreeViewItems is overwriten.
What I want to know is if there is some way to set a default visual behaviour for the TreeViewItems I use in my TreeView user control and yet allow me to set an Style for those TreeViewItems when I use the user control.
Thanks in advance.
Update:
Exception message:
System.Windows.Markup.XamlParseException: ''Set property 'System.Windows.ResourceDictionary.DeferrableContent' threw an exception.' Line number '22' and line position '14'.'
Inner exception:
InvalidOperationException: Cannot re-initialize ResourceDictionary instance.

Answer 1

The initial problem is because you cannot have two resources sections for a control. They collide.
Since the idea is to expose the treeview itself as the control then you need some other way to pass in extra styling.
One way to do that is with a dependency property.
Code in the control itself can then grab whatever you pass in and add it to the styling it already has. Not a very nice solution but it has the benefits of being reasonably straight forward without forcing you to redo the way the control works.