Отображение выпадающий список должен быть пустым

Question

Я простой выпадающий:

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

я не знаю имя элемента, так что я пометил его красным кружком. Я не хочу, чтобы выбранный элемент отображался в этом круге. Он должен быть пустым, даже если я выбираю элемент, потому что у меня есть изображение comboBox.Background, и это выглядит ужасно. Как я могу это сделать?

Answer 1

Часть, которую вы хотите скрыть, на самом деле называется SelectionBoxItem. Я попытался использовать Reflection, чтобы получить личное статическое поле SelectionBoxItemTemplatePropertyKey, чтобы мы могли использовать метод SetValue для изменения SelectionBoxItemTemplate (который является общедоступным для чтения). Однако не уверен, почему это не сработало (ничего не меняется).

Использование смесь для Visual Studio, вы увидите, что часть, которую нужно удалить вот ContentPresenter внутри MainGrid (корень визуальный ControlTemplate в ComboBox в). Стиль по умолчанию для ComboBox довольно сложный. Поэтому вы должны скопировать весь шаблон в файл (использовать в своем проекте) и удалить ContentPresenter для достижения того, чего вы хотите. Таким образом, все ComboBox, использующие тот же шаблон, будут иметь нужный эффект.

В случае, если вы просто хотите, чтобы очистить SelectionBoxItem некоторых конкретного Наримера и копирование шаблона по умолчанию является то излишеством, вы можете использовать следующий код, чтобы удалить ContentPresenter (отсюда опорожнить 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()); 

Вот шаблон шаблона ComboBox по умолчанию:

<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> 

Вы можете видеть, что это очень долго. Вот почему, если вы нацеливаетесь только на некоторые специальные ComboBox, использование вышеприведенного кода может быть лучше (хотя это связано с некоторым кодом).