如何在MFC中隐藏ComboBox的左右上边框，仅显示下边框？

下面针对不同技术场景给出具体实现方案：

WPF 实现方式 #

• 通过自定义ControlTemplate来精准控制边框显示，将左、上、右边框厚度设为0，仅保留蓝色下边框：

<ComboBox>
    <ComboBox.Template>
        <ControlTemplate TargetType="ComboBox">
            <Grid>
                <!-- 核心：仅保留下边框，设置蓝色 -->
                <Border x:Name="Border" 
                        BorderBrush="#0078D7" 
                        BorderThickness="0,0,0,1"
                        Background="White">
                    <Grid>
                        <ToggleButton x:Name="ToggleButton" 
                                      BorderBrush="Transparent"
                                      Background="Transparent"
                                      IsChecked="{Binding Path=IsDropDownOpen,Mode=TwoWay,RelativeSource={RelativeSource TemplatedParent}}">
                            <ToggleButton.Template>
                                <ControlTemplate TargetType="ToggleButton">
                                    <Grid>
                                        <Grid.ColumnDefinitions>
                                            <ColumnDefinition Width="*"/>
                                            <ColumnDefinition Width="20"/>
                                        </Grid.ColumnDefinitions>
                                        <ContentPresenter Grid.Column="0" Margin="5,0,0,0" VerticalAlignment="Center" HorizontalAlignment="Left" Content="{TemplateBinding Content}"/>
                                        <Path Grid.Column="1" HorizontalAlignment="Center" VerticalAlignment="Center" Data="M 0 0 L 4 4 L 8 0 Z" Fill="#888"/>
                                    </Grid>
                                </ControlTemplate>
                            </ToggleButton.Template>
                        </ToggleButton>
                        <ContentPresenter x:Name="ContentSite" IsHitTestVisible="False" Content="{TemplateBinding SelectionBoxItem}" ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}" Margin="5,0,20,0" VerticalAlignment="Center" HorizontalAlignment="Left"/>
                        <Popup x:Name="Popup" Placement="Bottom" IsOpen="{TemplateBinding IsDropDownOpen}" AllowsTransparency="True" Focusable="False" PopupAnimation="Slide">
                            <Grid x:Name="DropDown" SnapsToDevicePixels="True" MinWidth="{TemplateBinding ActualWidth}" MaxHeight="{TemplateBinding MaxDropDownHeight}">
                                <Border x:Name="DropDownBorder" BorderThickness="1" BorderBrush="#888" Background="White"/>
                                <ScrollViewer Margin="4,6,4,6" SnapsToDevicePixels="True">
                                    <ItemsPresenter KeyboardNavigation.DirectionalNavigation="Contained"/>
                                </ScrollViewer>
                            </Grid>
                        </Popup>
                    </Grid>
                </Border>
            </Grid>
            <ControlTemplate.Triggers>
                <Trigger Property="IsEnabled" Value="False">
                    <Setter TargetName="Border" Property="Background" Value="#EEE"/>
                </Trigger>
            </ControlTemplate.Triggers>
        </ControlTemplate>
    </ComboBox.Template>
</ComboBox>

• 关键操作：外层Border的BorderThickness设为0,0,0,1，指定蓝色BorderBrush，同时将内部ToggleButton的边框设为透明，避免多余边框干扰。

WinForms 实现方式 #

• 通过重绘ComboBox的方式，隐藏原有边框后手动绘制蓝色下边框：

private void comboBox1_Paint(object sender, PaintEventArgs e)
{
    ComboBox cb = sender as ComboBox;
    // 定义蓝色画笔，可根据需求调整色值和粗细
    using (Pen bluePen = new Pen(Color.FromArgb(0, 120, 215), 1))
    {
        // 绘制下边框
        e.Graphics.DrawLine(bluePen, 0, cb.Height - 1, cb.Width, cb.Height - 1);
    }
    // 隐藏原有边框
    cb.FlatStyle = FlatStyle.Flat;
    cb.BorderStyle = BorderStyle.None;
}

• 注意：需要在ComboBox的属性面板中绑定Paint事件，确保重绘逻辑生效。

HTML/CSS 实现方式 #

• 直接通过CSS样式清除多余边框，仅保留蓝色下边框：

.flat-combobox {
    border: none;
    border-bottom: 1px solid #0078D7;
    background-color: transparent;
    padding: 4px 0;
}
/* 可选：聚焦时修改下边框颜色增强交互 */
.flat-combobox:focus {
    outline: none;
    border-bottom-color: #005A9E;
}

• 对应HTML结构：

<select class="flat-combobox">
    <option>选项1</option>
    <option>选项2</option>
</select>

• 核心逻辑：先清除所有默认边框，再单独设置下边框的样式与颜色。

内容的提问来源于stack exchange，提问作者ssroreo