ComboBox with fixed Header

问题

I need to show a default text in my ComboBox, this text must not changed also when the user select an item of the Combobox, actually for do this I've created this structure:

<ComboBox ItemsSource="{Binding AvailableNations}" Width="160" Height="55" Margin="0, 0, 0, 15" 
           Text="Select Countries" IsEditable="True">
     <ComboBox.ItemTemplate>
         <DataTemplate>
             <CheckBox IsChecked="{Binding IsChecked}" Content="{Binding Item.Name}" />
         </DataTemplate>
     </ComboBox.ItemTemplate>
</ComboBox>

this display as default text Select Countries but if I select an item the default text will disappear and the item selected will be displayed, how can I fix this?

回答1:

You could use a Combined Template (ref post)

<Window.Resources>
    <ResourceDictionary>
        <DataTemplate x:Key="NormalItemTemplate" >
            <CheckBox IsChecked="{Binding IsChecked}" Content="{Binding Item.Name}" />
        </DataTemplate>
        <DataTemplate x:Key="SelectionBoxTemplate" >
            <TextBlock>Select Countries</TextBlock>
        </DataTemplate>
        <DataTemplate x:Key="CombinedTemplate">
            <ContentPresenter x:Name="Presenter"
                   Content="{Binding}"
                   ContentTemplate="{StaticResource NormalItemTemplate}" />
            <DataTemplate.Triggers>
                <DataTrigger
                        Binding="{Binding RelativeSource={RelativeSource FindAncestor,ComboBoxItem,1}}"
                        Value="{x:Null}">
                    <Setter TargetName="Presenter" Property="ContentTemplate"
                            Value="{StaticResource SelectionBoxTemplate}" />
                </DataTrigger>
            </DataTemplate.Triggers>
        </DataTemplate>
    </ResourceDictionary>
</Window.Resources>
<Grid>
    <ComboBox ItemsSource="{Binding AvailableNations}"                  
              SelectedItem="{Binding SelectedNation}"
              ItemTemplate="{StaticResource CombinedTemplate}"
              Width="160" Height="55" Margin="0, 0, 0, 15" >
    </ComboBox>
</Grid>

The way it works is described in the original answer. Note that the suggested solution will only work when IsEditable is set to false, I assume that won't be a problem in your case. Second, to get the text displayed at start up I bound SelectedItem (e.g. to the first item in the collection).

回答2:

From the comments, it sounds like you just want the Select Countries text to display at all times, even when an item is selected.

Personally I would just go the simple route and place a TextBox on top of the ComboBox, and hide the Display Text of the ComboBox by using a Transparent Foreground color.

Here's a quick example demonstrating it :

<Grid>
    <ComboBox SelectedIndex="1" Foreground="Transparent">
        <ComboBox.Resources>
            <Style TargetType="{x:Type ComboBoxItem}">
                <!-- Make sure ComboBoxItems don't have transparent text -->
                <Setter Property="Foreground" Value="{StaticResource {x:Static SystemColors.ControlTextBrushKey}}" />
            </Style>
        </ComboBox.Resources>
        <ComboBoxItem>Test 1</ComboBoxItem>
        <ComboBoxItem>Test 2</ComboBoxItem>
        <ComboBoxItem>Test 3</ComboBoxItem>
    </ComboBox>

    <TextBlock Text="Select Countries" Margin="4,3" IsHitTestVisible="False" />
</Grid>

And the results (note that SelectedIndex = 1)

I'm sure there's other ways too, such as overwriting the way it paints the display text, or changing the control template, but this seemed like the easiest to me.

来源：https://stackoverflow.com/questions/39390123/combobox-with-fixed-header

标签

wpf

xaml

combobox