WPF技术问询：如何让ComboBox的选项列表显示在控件上方？

要让ComboBox的下拉列表显示在控件按钮上方，核心是修改其内部Popup控件的Placement属性——默认该属性为Bottom，改为Top即可实现需求。

直接重写ComboBox模板实现 #

下面是完整的XAML示例，通过自定义ControlTemplate设置Popup的放置位置：

<ComboBox Width="150" Height="30">
    <ComboBox.Style>
        <Style TargetType="ComboBox">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="ComboBox">
                        <Grid>
                            <!-- 保留基础下拉按钮逻辑 -->
                            <ToggleButton x:Name="ToggleBtn"
                                          IsChecked="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"
                                          Background="{TemplateBinding Background}"
                                          BorderBrush="{TemplateBinding BorderBrush}"
                                          BorderThickness="{TemplateBinding BorderThickness}">
                                <Grid>
                                    <Grid.ColumnDefinitions>
                                        <ColumnDefinition Width="*"/>
                                        <ColumnDefinition Width="20"/>
                                    </Grid.ColumnDefinitions>
                                    <ContentPresenter Content="{TemplateBinding SelectionBoxItem}"
                                                      Margin="5,0,0,0"
                                                      VerticalAlignment="Center"/>
                                    <Path Grid.Column="1"
                                          Data="M0,0L4,4L8,0Z"
                                          Fill="Black"
                                          HorizontalAlignment="Center"
                                          VerticalAlignment="Center"/>
                                </Grid>
                            </ToggleButton>
                            
                            <!-- 关键设置：让Popup显示在控件上方 -->
                            <Popup x:Name="Popup"
                                   Placement="Top"
                                   IsOpen="{TemplateBinding IsDropDownOpen}"
                                   AllowsTransparency="True"
                                   MinWidth="{TemplateBinding ActualWidth}"
                                   MaxHeight="{TemplateBinding MaxDropDownHeight}">
                                <Border Background="White"
                                        BorderBrush="#DDDDDD"
                                        BorderThickness="1">
                                    <ScrollViewer>
                                        <ItemsPresenter/>
                                    </ScrollViewer>
                                </Border>
                            </Popup>
                        </Grid>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </ComboBox.Style>
    
    <ComboBoxItem>选项A</ComboBoxItem>
    <ComboBoxItem>选项B</ComboBoxItem>
    <ComboBoxItem>选项C</ComboBoxItem>
</ComboBox>

细节微调 #

如果下拉列表和控件出现重叠，可以给Popup添加VerticalOffset属性调整位置，比如：

<Popup x:Name="Popup"
       Placement="Top"
       VerticalOffset="-2"
       ...>

附上你提供的默认效果截图：

内容的提问来源于stack exchange，提问作者Can Le