组合框的显示应该为空

Question

I've a simple combobox:

 <StackPanel>
        <ComboBox>
            <ComboBoxItem>ComboBox Item #1</ComboBoxItem>
            <ComboBoxItem>ComboBox Item #2</ComboBoxItem>
            <ComboBoxItem>ComboBox Item #3</ComboBoxItem>
        </ComboBox>
 </StackPanel>

I don't know the name of the element, so I marked it with a red circle. I don't want that the selected item is displayed in this circle. It should be empty even if I select an item because I have a comboBox.Background image and that looks terrible. How can I do that?

Answer 1

The part you want to hide is actually called SelectionBoxItem . I've tried using Reflection to get the private static field SelectionBoxItemTemplatePropertyKey so that we can use SetValue method to change the SelectionBoxItemTemplate (which is publicly read-only). However not sure why it did not work (there's nothing changing).

Using Blend for Visual Studio , you will see that the part you need to remove here is the ContentPresenter inside the MainGrid (the root visual of the ComboBox's ControlTemplate ). The default style of a ComboBox is fairly complicated. So you should copy all the template into a file (to use in your project) and delete the ContentPresenter to achieve what you want. That way all the ComboBoxes using that same template will have the effect you want.

In case you just want to empty the SelectionBoxItem of some specific comboboxes and copying the default template is something overkill, you can use the following code to remove the ContentPresenter (hence empty the SelectionBoxItem):

//be sure you run this code when the Window has already been loaded.
var grid = yourComboBox.Template.FindName("MainGrid", yourComboBox) as Grid;
grid.Children.Remove(grid.Children.OfType<ContentPresenter>().First());

Here is the whole default ComboBox's Template:

<ResourceDictionary
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:Themes="clr-namespace:Microsoft.Windows.Themes;assembly=PresentationFramework.Aero">
<Style x:Key="ComboBoxFocusVisual">
    <Setter Property="Control.Template">
        <Setter.Value>
            <ControlTemplate>
                <Rectangle Margin="4,4,21,4" SnapsToDevicePixels="true" Stroke="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}" StrokeThickness="1" StrokeDashArray="1 2"/>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>
<!-- Resource dictionary entries should be defined here. -->
<Geometry x:Key="DownArrowGeometry">M 0 0 L 3.5 4 L 7 0 Z</Geometry>
<Style x:Key="ComboBoxReadonlyToggleButton" TargetType="{x:Type ToggleButton}">
    <Setter Property="OverridesDefaultStyle" Value="true"/>
    <Setter Property="IsTabStop" Value="false"/>
    <Setter Property="Focusable" Value="false"/>
    <Setter Property="ClickMode" Value="Press"/>
    <Setter Property="Background" Value="Transparent"/>
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type ToggleButton}">
                <Themes:ButtonChrome x:Name="Chrome" BorderBrush="{TemplateBinding BorderBrush}" Background="{TemplateBinding Background}" RenderMouseOver="{TemplateBinding IsMouseOver}" RenderPressed="{TemplateBinding IsPressed}" SnapsToDevicePixels="true">
                    <Grid HorizontalAlignment="Right" Width="{DynamicResource {x:Static SystemParameters.VerticalScrollBarWidthKey}}">
                        <Path x:Name="Arrow" Data="{StaticResource DownArrowGeometry}" Fill="Black" HorizontalAlignment="Center" Margin="3,1,0,0" VerticalAlignment="Center"/>
                    </Grid>
                </Themes:ButtonChrome>
                <ControlTemplate.Triggers>
                    <Trigger Property="IsChecked" Value="true">
                        <Setter Property="RenderPressed" TargetName="Chrome" Value="true"/>
                    </Trigger>
                    <Trigger Property="IsEnabled" Value="false">
                        <Setter Property="Fill" TargetName="Arrow" Value="#AFAFAF"/>
                    </Trigger>
                </ControlTemplate.Triggers>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>
<Style x:Key="ComboBoxEditableTextBox" TargetType="{x:Type TextBox}">
    <Setter Property="OverridesDefaultStyle" Value="true"/>
    <Setter Property="AllowDrop" Value="true"/>
    <Setter Property="MinWidth" Value="0"/>
    <Setter Property="MinHeight" Value="0"/>
    <Setter Property="FocusVisualStyle" Value="{x:Null}"/>
    <Setter Property="ScrollViewer.PanningMode" Value="VerticalFirst"/>
    <Setter Property="Stylus.IsFlicksEnabled" Value="False"/>
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type TextBox}">
                <ScrollViewer x:Name="PART_ContentHost" Background="Transparent" Focusable="false" HorizontalScrollBarVisibility="Hidden" VerticalScrollBarVisibility="Hidden"/>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>
<Style x:Key="ComboBoxToggleButton2" TargetType="{x:Type ToggleButton}">
    <Setter Property="OverridesDefaultStyle" Value="true"/>
    <Setter Property="IsTabStop" Value="false"/>
    <Setter Property="Focusable" Value="false"/>
    <Setter Property="ClickMode" Value="Press"/>
    <Setter Property="Background" Value="Transparent"/>
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type ToggleButton}">
                <Themes:ButtonChrome x:Name="Chrome" BorderBrush="{TemplateBinding BorderBrush}" Background="{TemplateBinding Background}" RenderMouseOver="{TemplateBinding IsMouseOver}" RenderPressed="{TemplateBinding IsPressed}" RoundCorners="false" SnapsToDevicePixels="true" Width="{DynamicResource {x:Static SystemParameters.VerticalScrollBarWidthKey}}">
                    <Path x:Name="Arrow" Data="{StaticResource DownArrowGeometry}" Fill="Black" HorizontalAlignment="Center" Margin="0,1,0,0" VerticalAlignment="Center"/>
                </Themes:ButtonChrome>
                <ControlTemplate.Triggers>
                    <Trigger Property="IsChecked" Value="true">
                        <Setter Property="RenderPressed" TargetName="Chrome" Value="true"/>
                    </Trigger>
                    <Trigger Property="IsEnabled" Value="false">
                        <Setter Property="Fill" TargetName="Arrow" Value="#AFAFAF"/>
                    </Trigger>
                </ControlTemplate.Triggers>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>
<ControlTemplate x:Key="ComboBoxEditableTemplate" TargetType="{x:Type ComboBox}">
    <Grid x:Name="Placement" SnapsToDevicePixels="true">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*"/>
            <ColumnDefinition Width="Auto"/>
        </Grid.ColumnDefinitions>
        <Popup x:Name="PART_Popup" AllowsTransparency="true" Grid.ColumnSpan="2" IsOpen="{Binding IsDropDownOpen, RelativeSource={RelativeSource TemplatedParent}}" PopupAnimation="{DynamicResource {x:Static SystemParameters.ComboBoxPopupAnimationKey}}" Placement="Bottom">
            <Themes:SystemDropShadowChrome x:Name="Shdw" Color="Transparent" MaxHeight="{TemplateBinding MaxDropDownHeight}" MinWidth="{Binding ActualWidth, ElementName=Placement}">
                <Border x:Name="DropDownBorder" BorderBrush="{DynamicResource {x:Static SystemColors.WindowFrameBrushKey}}" BorderThickness="1" Background="{DynamicResource {x:Static SystemColors.WindowBrushKey}}">
                    <ScrollViewer x:Name="DropDownScrollViewer">
                        <Grid RenderOptions.ClearTypeHint="Enabled">
                            <Canvas HorizontalAlignment="Left" Height="0" VerticalAlignment="Top" Width="0">
                                <Rectangle x:Name="OpaqueRect" Fill="{Binding Background, ElementName=DropDownBorder}" Height="{Binding ActualHeight, ElementName=DropDownBorder}" Width="{Binding ActualWidth, ElementName=DropDownBorder}"/>
                            </Canvas>
                            <ItemsPresenter x:Name="ItemsPresenter" KeyboardNavigation.DirectionalNavigation="Contained" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
                        </Grid>
                    </ScrollViewer>
                </Border>
            </Themes:SystemDropShadowChrome>
        </Popup>
        <Themes:ListBoxChrome x:Name="Border" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Grid.ColumnSpan="2" RenderMouseOver="{TemplateBinding IsMouseOver}" RenderFocused="{TemplateBinding IsKeyboardFocusWithin}"/>
        <TextBox x:Name="PART_EditableTextBox" HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}" IsReadOnly="{Binding IsReadOnly, RelativeSource={RelativeSource TemplatedParent}}" Margin="{TemplateBinding Padding}" Style="{StaticResource ComboBoxEditableTextBox}" VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"/>
        <ToggleButton Grid.Column="1" IsChecked="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}" Style="{StaticResource ComboBoxToggleButton2}"/>
    </Grid>
    <ControlTemplate.Triggers>
        <Trigger Property="IsKeyboardFocusWithin" Value="true">
            <Setter Property="Foreground" Value="Black"/>
        </Trigger>
        <Trigger Property="IsDropDownOpen" Value="true">
            <Setter Property="RenderFocused" TargetName="Border" Value="true"/>
        </Trigger>
        <Trigger Property="HasItems" Value="false">
            <Setter Property="Height" TargetName="DropDownBorder" Value="95"/>
        </Trigger>
        <Trigger Property="IsEnabled" Value="false">
            <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
            <Setter Property="Background" Value="#FFF4F4F4"/>
        </Trigger>
        <MultiTrigger>
            <MultiTrigger.Conditions>
                <Condition Property="IsGrouping" Value="true"/>
                <Condition Property="VirtualizingPanel.IsVirtualizingWhenGrouping" Value="false"/>
            </MultiTrigger.Conditions>
            <Setter Property="ScrollViewer.CanContentScroll" Value="false"/>
        </MultiTrigger>
        <Trigger Property="HasDropShadow" SourceName="PART_Popup" Value="true">
            <Setter Property="Margin" TargetName="Shdw" Value="0,0,5,5"/>
            <Setter Property="Color" TargetName="Shdw" Value="#71000000"/>
        </Trigger>
        <Trigger Property="ScrollViewer.CanContentScroll" SourceName="DropDownScrollViewer" Value="false">
            <Setter Property="Canvas.Top" TargetName="OpaqueRect" Value="{Binding VerticalOffset, ElementName=DropDownScrollViewer}"/>
            <Setter Property="Canvas.Left" TargetName="OpaqueRect" Value="{Binding HorizontalOffset, ElementName=DropDownScrollViewer}"/>
        </Trigger>
    </ControlTemplate.Triggers>
</ControlTemplate>
<Style TargetType="{x:Type ComboBox}">
    <Setter Property="FocusVisualStyle" Value="{StaticResource ComboBoxFocusVisual}"/>
    <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.WindowTextBrushKey}}"/>
    <Setter Property="Background" Value="{StaticResource ButtonNormalBackground}"/>
    <Setter Property="BorderBrush" Value="{StaticResource ButtonNormalBorder}"/>
    <Setter Property="BorderThickness" Value="1"/>
    <Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Auto"/>
    <Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto"/>
    <Setter Property="Padding" Value="4,3"/>
    <Setter Property="ScrollViewer.CanContentScroll" Value="true"/>
    <Setter Property="ScrollViewer.PanningMode" Value="Both"/>
    <Setter Property="Stylus.IsFlicksEnabled" Value="False"/>
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type ComboBox}">
                <Grid x:Name="MainGrid" SnapsToDevicePixels="true">
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="*"/>
                        <ColumnDefinition MinWidth="{DynamicResource {x:Static SystemParameters.VerticalScrollBarWidthKey}}" Width="0"/>
                    </Grid.ColumnDefinitions>
                    <Popup x:Name="PART_Popup" AllowsTransparency="true" Grid.ColumnSpan="2" IsOpen="{Binding IsDropDownOpen, RelativeSource={RelativeSource TemplatedParent}}" Margin="1" PopupAnimation="{DynamicResource {x:Static SystemParameters.ComboBoxPopupAnimationKey}}" Placement="Bottom">
                        <Themes:SystemDropShadowChrome x:Name="Shdw" Color="Transparent" MaxHeight="{TemplateBinding MaxDropDownHeight}" MinWidth="{Binding ActualWidth, ElementName=MainGrid}">
                            <Border x:Name="DropDownBorder" BorderBrush="{DynamicResource {x:Static SystemColors.WindowFrameBrushKey}}" BorderThickness="1" Background="{DynamicResource {x:Static SystemColors.WindowBrushKey}}">
                                <ScrollViewer x:Name="DropDownScrollViewer">
                                    <Grid RenderOptions.ClearTypeHint="Enabled">
                                        <Canvas HorizontalAlignment="Left" Height="0" VerticalAlignment="Top" Width="0">
                                            <Rectangle x:Name="OpaqueRect" Fill="{Binding Background, ElementName=DropDownBorder}" Height="{Binding ActualHeight, ElementName=DropDownBorder}" Width="{Binding ActualWidth, ElementName=DropDownBorder}"/>
                                        </Canvas>
                                        <ItemsPresenter x:Name="ItemsPresenter" KeyboardNavigation.DirectionalNavigation="Contained" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
                                    </Grid>
                                </ScrollViewer>
                            </Border>
                        </Themes:SystemDropShadowChrome>
                    </Popup>
                    <ToggleButton BorderBrush="{TemplateBinding BorderBrush}" Background="{TemplateBinding Background}" Grid.ColumnSpan="2" IsChecked="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}" Style="{StaticResource ComboBoxReadonlyToggleButton}"/>
                    <!--ContentPresenter ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}" ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}" Content="{TemplateBinding SelectionBoxItem}" ContentStringFormat="{TemplateBinding SelectionBoxItemStringFormat}" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" IsHitTestVisible="false" Margin="{TemplateBinding Padding}" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/-->
                </Grid>
                <ControlTemplate.Triggers>
                    <Trigger Property="HasDropShadow" SourceName="PART_Popup" Value="true">
                        <Setter Property="Margin" TargetName="Shdw" Value="0,0,5,5"/>
                        <Setter Property="Color" TargetName="Shdw" Value="#71000000"/>
                    </Trigger>
                    <Trigger Property="HasItems" Value="false">
                        <Setter Property="Height" TargetName="DropDownBorder" Value="95"/>
                    </Trigger>
                    <Trigger Property="IsEnabled" Value="false">
                        <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
                        <Setter Property="Background" Value="#FFF4F4F4"/>
                    </Trigger>
                    <MultiTrigger>
                        <MultiTrigger.Conditions>
                            <Condition Property="IsGrouping" Value="true"/>
                            <Condition Property="VirtualizingPanel.IsVirtualizingWhenGrouping" Value="false"/>
                        </MultiTrigger.Conditions>
                        <Setter Property="ScrollViewer.CanContentScroll" Value="false"/>
                    </MultiTrigger>
                    <Trigger Property="ScrollViewer.CanContentScroll" SourceName="DropDownScrollViewer" Value="false">
                        <Setter Property="Canvas.Top" TargetName="OpaqueRect" Value="{Binding VerticalOffset, ElementName=DropDownScrollViewer}"/>
                        <Setter Property="Canvas.Left" TargetName="OpaqueRect" Value="{Binding HorizontalOffset, ElementName=DropDownScrollViewer}"/>
                    </Trigger>
                </ControlTemplate.Triggers>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
    <Style.Triggers>
        <Trigger Property="IsEditable" Value="true">
            <Setter Property="BorderBrush" Value="{StaticResource TextBoxBorder}"/>
            <Setter Property="Background" Value="{DynamicResource {x:Static SystemColors.WindowBrushKey}}"/>
            <Setter Property="IsTabStop" Value="false"/>
            <Setter Property="Padding" Value="3"/>
            <Setter Property="Template" Value="{StaticResource ComboBoxEditableTemplate}"/>
        </Trigger>
    </Style.Triggers>
</Style>
</ResourceDictionary>

You can see that it's very long. That's why if you target just some specific ComboBoxes, using the code above might be better (although it involves some codebehind).