能否自定义TreeView层级展开/折叠按钮的样式？

默认TreeView的展开/折叠按钮确实会跟随系统主题走，但完全可以通过自定义控件模板或自绘的方式替换成你想要的三角形、图标或图片，不用受系统环境限制。

WPF 实现方案 #

重写TreeViewItem的控件模板，替换默认的ToggleButton为自定义样式：

<!-- 自定义TreeViewItem模板 -->
<Style TargetType="{x:Type TreeViewItem}">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type TreeViewItem}">
                <Grid>
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="Auto"/>
                        <ColumnDefinition Width="*"/>
                    </Grid.ColumnDefinitions>
                    <Grid.RowDefinitions>
                        <RowDefinition Height="Auto"/>
                        <RowDefinition/>
                    </Grid.RowDefinitions>
                    <!-- 自定义展开/折叠按钮 -->
                    <ToggleButton x:Name="Expander" 
                                  IsChecked="{Binding IsExpanded, RelativeSource={RelativeSource TemplatedParent}}"
                                  Style="{StaticResource TreeViewExpanderStyle}"/>
                    <!-- 节点标题 -->
                    <ContentPresenter x:Name="PART_Header" Grid.Column="1" 
                                      ContentSource="Header"
                                      HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"/>
                    <!-- 子节点容器 -->
                    <ItemsPresenter x:Name="ItemsHost" Grid.Row="1" Grid.Column="1"/>
                </Grid>
                <ControlTemplate.Triggers>
                    <Trigger Property="IsExpanded" Value="False">
                        <Setter TargetName="ItemsHost" Property="Visibility" Value="Collapsed"/>
                    </Trigger>
                    <Trigger Property="HasItems" Value="False">
                        <Setter TargetName="Expander" Property="Visibility" Value="Collapsed"/>
                    </Trigger>
                </ControlTemplate.Triggers>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

<!-- 展开按钮的自定义样式，用Path绘制三角形 -->
<Style x:Key="TreeViewExpanderStyle" TargetType="{x:Type ToggleButton}">
    <Setter Property="Width" Value="16"/>
    <Setter Property="Height" Value="16"/>
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type ToggleButton}">
                <Grid Background="Transparent">
                    <!-- 折叠状态三角形 -->
                    <Path x:Name="CollapsedPath" 
                          Data="M3,6 L9,6 L6,12 Z" 
                          Fill="#666" 
                          Visibility="Visible"/>
                    <!-- 展开状态三角形 -->
                    <Path x:Name="ExpandedPath" 
                          Data="M6,3 L12,9 L6,15 Z" 
                          Fill="#666" 
                          Visibility="Collapsed"/>
                </Grid>
                <ControlTemplate.Triggers>
                    <Trigger Property="IsChecked" Value="True">
                        <Setter TargetName="CollapsedPath" Property="Visibility" Value="Collapsed"/>
                        <Setter TargetName="ExpandedPath" Property="Visibility" Value="Visible"/>
                    </Trigger>
                    <Trigger Property="IsMouseOver" Value="True">
                        <Setter TargetName="CollapsedPath" Property="Fill" Value="#333"/>
                        <Setter TargetName="ExpandedPath" Property="Fill" Value="#333"/>
                    </Trigger>
                </ControlTemplate.Triggers>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

如果想用图片代替三角形，把Path控件换成Image，绑定对应图片资源即可。

WinForms 实现方案 #

开启TreeView的OwnerDrawMode为OwnerDrawAll，然后重写DrawNode事件来自绘按钮：

private void treeView1_DrawNode(object sender, DrawTreeNodeEventArgs e)
{
    // 先绘制节点文本
    e.DrawDefault = false;
    e.Graphics.DrawString(e.Node.Text, treeView1.Font, Brushes.Black, e.Bounds.Left + 20, e.Bounds.Top);

    // 只有存在子节点时才绘制展开/折叠按钮
    if (e.Node.Nodes.Count == 0) return;

    Point btnPosition = new Point(e.Bounds.Left, e.Bounds.Top + 2);
    Brush btnBrush = Brushes.Gray;

    // 鼠标悬停时高亮按钮
    if (e.Node.IsMouseHover)
        btnBrush = Brushes.DarkGray;

    if (e.Node.IsExpanded)
    {
        // 绘制展开状态的三角形
        e.Graphics.FillPolygon(btnBrush, new Point[] {
            new Point(btnPosition.X + 4, btnPosition.Y + 8),
            new Point(btnPosition.X + 8, btnPosition.Y + 4),
            new Point(btnPosition.X + 12, btnPosition.Y + 8)
        });
    }
    else
    {
        // 绘制折叠状态的三角形
        e.Graphics.FillPolygon(btnBrush, new Point[] {
            new Point(btnPosition.X + 8, btnPosition.Y + 4),
            new Point(btnPosition.X + 4, btnPosition.Y + 8),
            new Point(btnPosition.X + 8, btnPosition.Y + 12)
        });
    }
}

这样就能完全摆脱系统主题的限制，实现你想要的按钮样式。

内容的提问来源于stack exchange，提问作者Danya K