Oui, vous devrez modifier la propriété de style de TreeViewItem. En voici un que j'utilise ...
<Style x:Key="TreeViewContainerStyle" TargetType="controls:TreeViewItem">
<Setter Property="Padding" Value="3"/>
<Setter Property="HorizontalContentAlignment" Value="Left"/>
<Setter Property="VerticalContentAlignment" Value="Top"/>
<Setter Property="Background" Value="Transparent"/>
<Setter Property="Foreground"
Value="Black" />
<Setter Property="BorderThickness" Value="1"/>
<Setter Property="Cursor" Value="Arrow"/>
<Setter Property="Margin" Value="-5,0,0,0"/>
<Setter Property="IsTabStop" Value="True"/>
<Setter Property="TabNavigation" Value="Once"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="controls:TreeViewItem">
<Grid Background="{x:Null}">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="25"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="25"/>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal"/>
<VisualState x:Name="Disabled">
<Storyboard>
<ObjectAnimationUsingKeyFrames Duration="0" Storyboard.TargetName="Header" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<SolidColorBrush Color="#FF999999"/>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="MouseOver">
<Storyboard>
<ColorAnimationUsingKeyFrames BeginTime="00:00:00" Duration="00:00:00.0010000" Storyboard.TargetName="ExpanderButton" Storyboard.TargetProperty="(Control.Background).(SolidColorBrush.Color)">
<EasingColorKeyFrame KeyTime="00:00:00" Value="#FF90B5D5"/>
</ColorAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="SelectionStates">
<VisualState x:Name="Unselected"/>
<VisualState x:Name="Selected">
<Storyboard>
<DoubleAnimation Duration="0" Storyboard.TargetName="select" Storyboard.TargetProperty="Opacity" To=".75"/>
</Storyboard>
</VisualState>
<VisualState x:Name="SelectedInactive">
<Storyboard>
<DoubleAnimation Duration="0" Storyboard.TargetName="inactive" Storyboard.TargetProperty="Opacity" To=".2"/>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="HasItemsStates">
<VisualState x:Name="HasItems"/>
<VisualState x:Name="NoItems">
<Storyboard>
<ObjectAnimationUsingKeyFrames Duration="0" Storyboard.TargetName="ExpanderButton" Storyboard.TargetProperty="Visibility">
<DiscreteObjectKeyFrame KeyTime="0" Value="Collapsed"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="ExpansionStates">
<VisualState x:Name="Collapsed"/>
<VisualState x:Name="Expanded">
<Storyboard>
<ObjectAnimationUsingKeyFrames Duration="0" Storyboard.TargetName="ItemsHost" Storyboard.TargetProperty="Visibility">
<DiscreteObjectKeyFrame KeyTime="0" Value="Visible"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<ToggleButton x:Name="ExpanderButton" IsTabStop="False" TabNavigation="Once" HorizontalAlignment="Stretch">
<ToggleButton.Template>
<ControlTemplate TargetType="ToggleButton">
<Grid x:Name="Root" Background="Transparent" Opacity=".6">
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal"/>
<VisualState x:Name="MouseOver">
<Storyboard>
<DoubleAnimation Duration="0" Storyboard.TargetName="Root" Storyboard.TargetProperty="Opacity" To="1.7"/>
</Storyboard>
</VisualState>
<VisualState x:Name="Disabled">
<Storyboard>
<DoubleAnimation Duration="0" Storyboard.TargetName="Root" Storyboard.TargetProperty="Opacity" To="1.7"/>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="CheckStates">
<VisualState x:Name="Unchecked"/>
<VisualState x:Name="Checked">
<Storyboard>
<DoubleAnimationUsingKeyFrames BeginTime="00:00:00" Duration="00:00:00.0010000" Storyboard.TargetName="MinusSign" Storyboard.TargetProperty="(UIElement.Opacity)">
<EasingDoubleKeyFrame KeyTime="00:00:00" Value="1"/>
</DoubleAnimationUsingKeyFrames>
<DoubleAnimationUsingKeyFrames BeginTime="00:00:00" Duration="00:00:00.0010000" Storyboard.TargetName="PlusSign" Storyboard.TargetProperty="(UIElement.Opacity)">
<EasingDoubleKeyFrame KeyTime="00:00:00" Value="0"/>
</DoubleAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Grid>
<Rectangle x:Name="PlusSign" HorizontalAlignment="Center" VerticalAlignment="Top" Width="20" Height="20" Visibility="Visible" Opacity="100">
<Rectangle.Fill>
<ImageBrush Stretch="Fill" ImageSource="./icon_expand_hover.png"/>
</Rectangle.Fill>
</Rectangle>
<Rectangle x:Name="MinusSign" HorizontalAlignment="Center" VerticalAlignment="Top" Width="20" Height="20" Visibility="Visible" Opacity="0">
<Rectangle.Fill>
<ImageBrush Stretch="Fill" ImageSource="./icon_collapse_hover.png"/>
</Rectangle.Fill>
</Rectangle>
</Grid>
</Grid>
</ControlTemplate>
</ToggleButton.Template>
</ToggleButton>
<Rectangle x:Name="select" StrokeThickness="1" RadiusX="2" RadiusY="2" IsHitTestVisible="False" Opacity="0" Grid.Column="1" HorizontalAlignment="Stretch" Width="Auto" Margin="0,0,5,0">
<Rectangle.Stroke>
<LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
<GradientStop Color="#00000000"/>
<GradientStop Color="#7F000000" Offset="1"/>
<GradientStop Color="#06000000" Offset="0.379"/>
</LinearGradientBrush>
</Rectangle.Stroke>
<Rectangle.Fill>
<LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
<GradientStop Color="#00000000"/>
<GradientStop Color="#7F000000" Offset="1"/>
<GradientStop Color="#06000000" Offset="0.659"/>
</LinearGradientBrush>
</Rectangle.Fill>
</Rectangle>
<Rectangle x:Name="inactive" Fill="#FF999999" Stroke="#FF333333" StrokeThickness="1" RadiusX="2" RadiusY="2" IsHitTestVisible="False" Opacity="0" Grid.Column="1" HorizontalAlignment="Left" Width="6"/>
<Button x:Name="Header" Cursor="{TemplateBinding Cursor}" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}" Background="{TemplateBinding Background}" Foreground="{TemplateBinding Foreground}" IsTabStop="False" TabNavigation="Once" ClickMode="Hover" Content="{TemplateBinding Header}" ContentTemplate="{TemplateBinding HeaderTemplate}" Grid.Column="1">
<Button.Template>
<ControlTemplate TargetType="Button">
<Grid Background="{TemplateBinding Background}">
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal"/>
<VisualState x:Name="Pressed">
<Storyboard>
<DoubleAnimation Duration="0" Storyboard.TargetName="hover" Storyboard.TargetProperty="Opacity" To=".5"/>
</Storyboard>
</VisualState>
<VisualState x:Name="Disabled">
<Storyboard>
<DoubleAnimation Duration="0" Storyboard.TargetName="content" Storyboard.TargetProperty="Opacity" To=".55"/>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Rectangle x:Name="hover" Fill="#FFBADDE9" Stroke="#FF6DBDD1" StrokeThickness="1" RadiusX="2" RadiusY="2" IsHitTestVisible="False" Opacity="0"/>
<ContentPresenter x:Name="content" Cursor="{TemplateBinding Cursor}" HorizontalAlignment="Left" Margin="{TemplateBinding Padding}" Content="{TemplateBinding Content}" ContentTemplate="{TemplateBinding ContentTemplate}"/>
</Grid>
</ControlTemplate>
</Button.Template>
</Button>
<Border Visibility="Collapsed" x:Name="ItemsHost" Grid.Column="1" Grid.Row="1" CornerRadius="1,4,8,4" BorderThickness="1,1,1,1" Padding="5,0,0,0" Margin="-27,0,0,0" HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
<Border.Background>
<SolidColorBrush Color="AntiqueWhite" />
</Border.Background>
<Border.BorderBrush>
<LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
<GradientStop Color="#FF0D0A45" Offset="0"/>
<GradientStop Color="#FF0D0A45" Offset="1"/>
<GradientStop Color="#FF38435B" Offset="0.2"/>
</LinearGradientBrush>
</Border.BorderBrush>
<Grid>
<Rectangle Fill="{x:Null}"
RadiusX="8" RadiusY="8"
VerticalAlignment="Bottom"
HorizontalAlignment="Right"
Width="100" Height="40"
Margin="0,0,3,3">
<Rectangle.Stroke>
<LinearGradientBrush EndPoint="1,1" StartPoint="0.7,0.7">
<GradientStop Color="#00FFFFFF" Offset="0"/>
<GradientStop Color="#7FFFFFFF" Offset="1"/>
</LinearGradientBrush>
</Rectangle.Stroke>
</Rectangle>
<ItemsPresenter />
</Grid>
</Border>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
Notez le bouton à bascule.
Alors dans le code derrière vous ferait ...
myDynamicTreeViewItem.Style = (Style)this.Resources["TreeViewContainerStyle"];
Merci pour la réponse. Dans mon cas, les TreeViewItems ne sont pas ajoutés via le code - ils sont créés dans les coulisses en utilisant le HierarchicalDataTemplate. Je n'ai donc pas d'accès immédiat à TreeViewItem pour définir son style. Si vous utilisez HierarchicalDataTemplate, comment accédez-vous aux TreeViewItems créés pour TreeView? –