Can I use a different Template for the selected item in a WPF ComboBox than for the items in the dropdown part?

I have a WPF Combobox which is filled with, say, Customer objects. I have a DataTemplate:

<DataTemplate DataType="{x:Type MyAssembly:Customer}">
    <StackPanel>
        <TextBlock Text="{Binding Name}" />
        <TextBlock Text="{Binding Address}" />
    </StackPanel>
</DataTemplate>

This way, when I open my ComboBox, I can see the different Customers with their Name and, below that, the Address.

But when I select a Customer, I only want to display the Name in the ComboBox. Something like:

<DataTemplate DataType="{x:Type MyAssembly:Customer}">
    <StackPanel>
        <TextBlock Text="{Binding Name}" />
    </StackPanel>
</DataTemplate>

Can I select another Template for the selected item in a ComboBox?

Solution

With help from the answers, I solved it like this:

<UserControl.Resources>
    <ControlTemplate x:Key="SimpleTemplate">
        <StackPanel>
            <TextBlock Text="{Binding Name}" />
        </StackPanel>
    </ControlTemplate>
    <ControlTemplate x:Key="ExtendedTemplate">
        <StackPanel>
            <TextBlock Text="{Binding Name}" />
            <TextBlock Text="{Binding Address}" />
        </StackPanel>
    </ControlTemplate>
    <DataTemplate x:Key="CustomerTemplate">
        <Control x:Name="theControl" Focusable="False" Template="{StaticResource ExtendedTemplate}" />
        <DataTemplate.Triggers>
            <DataTrigger Binding="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type ComboBoxItem}}, Path=IsSelected}" Value="{x:Null}">
                <Setter TargetName="theControl" Property="Template" Value="{StaticResource SimpleTemplate}" />
            </DataTrigger>
        </DataTemplate.Triggers>
    </DataTemplate>
</UserControl.Resources>

Then, my ComboBox:

<ComboBox ItemsSource="{Binding Customers}" 
                SelectedItem="{Binding SelectedCustomer}"
                ItemTemplate="{StaticResource CustomerTemplate}" />

The important part to get it to work was Binding="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type ComboBoxItem}}, Path=IsSelected}" Value="{x:Null}" (the part where value should be x:Null, not True).

The issue with using the DataTrigger/Binding solution mentioned above are two-fold. The first is you actually end up with a binding warning that you can't find the relative source for the selected item. The bigger issue however is that you've cluttered up your data templates and made them specific to a ComboBox.

The solution I present better follows WPF designs in that it uses a DataTemplateSelector on which you can specify separate templates using its SelectedItemTemplate and DropDownItemsTemplate properties as well as ‘selector’ variants for both.

Note: Updated for C#9 with nullability enabled and using pattern matching during the search

public class ComboBoxTemplateSelector : DataTemplateSelector {

	public DataTemplate?         SelectedItemTemplate          { get; set; }
	public DataTemplateSelector? SelectedItemTemplateSelector  { get; set; }
	public DataTemplate?         DropdownItemsTemplate         { get; set; }
	public DataTemplateSelector? DropdownItemsTemplateSelector { get; set; }

	public override DataTemplate SelectTemplate(object item, DependencyObject container) {

		var itemToCheck = container;

        // Search up the visual tree, stopping at either a ComboBox or
        // a ComboBoxItem (or null). This will determine which template to use
		while(itemToCheck is not null
        and not ComboBox
        and not ComboBoxItem)
			itemToCheck = VisualTreeHelper.GetParent(itemToCheck);

        // If you stopped at a ComboBoxItem, you're in the dropdown
		var inDropDown = itemToCheck is ComboBoxItem;

		return inDropDown
			? DropdownItemsTemplate ?? DropdownItemsTemplateSelector?.SelectTemplate(item, container)
			: SelectedItemTemplate  ?? SelectedItemTemplateSelector?.SelectTemplate(item, container); 
	}
}

To make it easier to use in XAML, I've also included a markup extension that simply creates and returns the above class in its ProvideValue function.

public class ComboBoxTemplateSelectorExtension : MarkupExtension {

	public DataTemplate?         SelectedItemTemplate          { get; set; }
	public DataTemplateSelector? SelectedItemTemplateSelector  { get; set; }
	public DataTemplate?         DropdownItemsTemplate         { get; set; }
	public DataTemplateSelector? DropdownItemsTemplateSelector { get; set; }

	public override object ProvideValue(IServiceProvider serviceProvider)
        => new ComboBoxTemplateSelector(){
			SelectedItemTemplate          = SelectedItemTemplate,
			SelectedItemTemplateSelector  = SelectedItemTemplateSelector,
			DropdownItemsTemplate         = DropdownItemsTemplate,
			DropdownItemsTemplateSelector = DropdownItemsTemplateSelector
		};
}

And here's how you use it. Nice, clean and clear and your templates stay 'pure' >Note: 'is:' here is my xmlns mapping for where I put the class in code. Make sure to import your own namespace and change 'is:' as appropriate.

<ComboBox x:Name="MyComboBox"
	ItemsSource="{Binding Items}"
	ItemTemplateSelector="{is:ComboBoxTemplateSelector
		SelectedItemTemplate={StaticResource MySelectedItemTemplate},
		DropdownItemsTemplate={StaticResource MyDropDownItemTemplate}}" />

You can also use DataTemplateSelectors if you prefer...

<ComboBox x:Name="MyComboBox"
	ItemsSource="{Binding Items}"
	ItemTemplateSelector="{is:ComboBoxTemplateSelector
		SelectedItemTemplateSelector={StaticResource MySelectedItemTemplateSelector},
		DropdownItemsTemplateSelector={StaticResource MyDropDownItemTemplateSelector}}" />

Or mix and match! Here I'm using a template for the selected item, but a template selector for the DropDown items.

<ComboBox x:Name="MyComboBox"
	ItemsSource="{Binding Items}"
	ItemTemplateSelector="{is:ComboBoxTemplateSelector
		SelectedItemTemplate={StaticResource MySelectedItemTemplate},
		DropdownItemsTemplateSelector={StaticResource MyDropDownItemTemplateSelector}}" />

Additionally, if you don't specify a Template or a TemplateSelector for the selected or dropdown items, it simply falls back to the regular resolving of data templates based on data types, again, as you would expect. So, for instance, in the below case, the selected item has its template explicitly set, but the dropdown will inherit whichever data template applies for the DataType of the object in the data context.

<ComboBox x:Name="MyComboBox"
	ItemsSource="{Binding Items}"
	ItemTemplateSelector="{is:ComboBoxTemplateSelector
		SelectedItemTemplate={StaticResource MyTemplate} />

Enjoy!

Simple solution:

<DataTemplate>
    <StackPanel>
        <TextBlock Text="{Binding Name}"/>
        <TextBlock Text="{Binding Address}">
            <TextBlock.Style>
                <Style TargetType="TextBlock">
                    <Style.Triggers>
                        <DataTrigger Binding="{Binding RelativeSource={RelativeSource AncestorType=ComboBoxItem}}" Value="{x:Null}">
                            <Setter Property="Visibility" Value="Collapsed"/>
                        </DataTrigger>
                    </Style.Triggers>
                </Style>
            </TextBlock.Style>
        </TextBlock>
    </StackPanel>
</DataTemplate>

(Note that the element that is selected and displayed in the box and not the list is not inside a ComboBoxItem hence the trigger on Null)

If you want to switch out the whole template you can do that as well by using the trigger to e.g. apply a different ContentTemplate to a ContentControl. This also allows you to retain a default DataType-based template selection if you just change the template for this selective case, e.g.:

<ComboBox.ItemTemplate>
    <DataTemplate>
        <ContentControl Content="{Binding}">
            <ContentControl.Style>
                <Style TargetType="ContentControl">
                    <Style.Triggers>
                        <DataTrigger Binding="{Binding RelativeSource={RelativeSource AncestorType=ComboBoxItem}}"
                                        Value="{x:Null}">
                            <Setter Property="ContentTemplate">
                                <Setter.Value>
                                    <DataTemplate>
                                        <!-- ... -->
                                    </DataTemplate>
                                </Setter.Value>
                            </Setter>
                        </DataTrigger>
                    </Style.Triggers>
                </Style>
            </ContentControl.Style>
        </ContentControl>
    </DataTemplate>
</ComboBox.ItemTemplate>

Note that this method will cause binding errors as the relative source is not found for the selected item. For an alternate approach see MarqueIV's answer.

I was going to suggest using the combination of an ItemTemplate for the combo items, with the Text parameter as the title selection, but I see that ComboBox doesn't respect the Text parameter.

I dealt with something similar by overriding the ComboBox ControlTemplate. Here's the MSDN website with a sample for .NET 4.0.

In my solution, I change the ContentPresenter in the ComboBox template to have it bind to Text, with its ContentTemplate bound to a simple DataTemplate that contains a TextBlock like so:

<DataTemplate x:Uid="DataTemplate_1" x:Key="ComboSelectionBoxTemplate">
	<TextBlock x:Uid="TextBlock_1" Text="{Binding}" />
</DataTemplate>

with this in the ControlTemplate:

<ContentPresenter Name="ContentSite" IsHitTestVisible="False" Content="{TemplateBinding Text}" ContentTemplate="{StaticResource ComboSelectionBoxTemplate}" Margin="3,3,23,3" VerticalAlignment="Center" HorizontalAlignment="Left"/>

With this binding link, I am able to control the Combo selection display directly via the Text parameter on the control (which I bind to an appropriate value on my ViewModel).

I used next approach

 <UserControl.Resources>
    <DataTemplate x:Key="SelectedItemTemplate" DataType="{x:Type statusBar:OffsetItem}">
        <TextBlock Text="{Binding Path=ShortName}" />
    </DataTemplate>
</UserControl.Resources>
<StackPanel Orientation="Horizontal">
    <ComboBox DisplayMemberPath="FullName"
              ItemsSource="{Binding Path=Offsets}"
              behaviors:SelectedItemTemplateBehavior.SelectedItemDataTemplate="{StaticResource SelectedItemTemplate}"
              SelectedItem="{Binding Path=Selected}" />
    <TextBlock Text="User Time" />
    <TextBlock Text="" />
</StackPanel>

And the behavior

public static class SelectedItemTemplateBehavior
{
    public static readonly DependencyProperty SelectedItemDataTemplateProperty =
        DependencyProperty.RegisterAttached("SelectedItemDataTemplate", typeof(DataTemplate), typeof(SelectedItemTemplateBehavior), new PropertyMetadata(default(DataTemplate), PropertyChangedCallback));

    public static void SetSelectedItemDataTemplate(this UIElement element, DataTemplate value)
    {
        element.SetValue(SelectedItemDataTemplateProperty, value);
    }

    public static DataTemplate GetSelectedItemDataTemplate(this ComboBox element)
    {
        return (DataTemplate)element.GetValue(SelectedItemDataTemplateProperty);
    }

    private static void PropertyChangedCallback(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var uiElement = d as ComboBox;
        if (e.Property == SelectedItemDataTemplateProperty && uiElement != null)
        {
            uiElement.Loaded -= UiElementLoaded;
            UpdateSelectionTemplate(uiElement);
            uiElement.Loaded += UiElementLoaded;

        }
    }

    static void UiElementLoaded(object sender, RoutedEventArgs e)
    {
        UpdateSelectionTemplate((ComboBox)sender);
    }

    private static void UpdateSelectionTemplate(ComboBox uiElement)
    {
        var contentPresenter = GetChildOfType<ContentPresenter>(uiElement);
        if (contentPresenter == null)
            return;
        var template = uiElement.GetSelectedItemDataTemplate();
        contentPresenter.ContentTemplate = template;
    }


    public static T GetChildOfType<T>(DependencyObject depObj)
        where T : DependencyObject
    {
        if (depObj == null) return null;

        for (int i = 0; i < VisualTreeHelper.GetChildrenCount(depObj); i++)
        {
            var child = VisualTreeHelper.GetChild(depObj, i);

            var result = (child as T) ?? GetChildOfType<T>(child);
            if (result != null) return result;
        }
        return null;
    }
}

worked like a charm. Don't like pretty much Loaded event here but you can fix it if you want

In addition to what said by H.B. answer, the Binding Error can be avoided with a Converter. The following example is based from the Solution edited by the OP himself.

The idea is very simple: bind to something that alway exists (Control) and do the relevant check inside the converter. The relevant part of the modified XAML is the following. Please note that Path=IsSelected was never really needed and ComboBoxItem is replaced with Control to avoid the binding errors.

<DataTrigger Binding="{Binding 
    RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type Control}},
    Converter={StaticResource ComboBoxItemIsSelectedConverter}}"
    Value="{x:Null}">
  <Setter TargetName="theControl" Property="Template" Value="{StaticResource SimpleTemplate}" />
</DataTrigger>

The C# Converter code is the following:

public class ComboBoxItemIsSelectedConverter : IValueConverter
{
    private static object _notNull = new object();
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        // value is ComboBox when the item is the one in the closed combo
        if (value is ComboBox) return null; 

        // all the other items inside the dropdown will go here
        return _notNull;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

Yes. You use a Template Selector to determine which template to bind at run-time. Thus if IsSelected = False then Use this template, if IsSelected = True, use this other template.

Of Note: Once you implement your template selector, you will need to give the templates keynames.

I propose this solution without DataTemplateSelector, Trigger, binding nor behavior.

The first step is to put the ItemTemplate (of the selected element) in the ComboBox resources and the ItemTemplate (of the item in the drop down menu) in the ComboBox.ItemsPanel resources and give both resources the same key.

The second step is to postpone the ItemTemplate resolution at run time by using both a ContentPresenter and a DynamicResource in the actual ComboBox.ItemTemplate implementation.

<ComboBox ItemsSource="{Binding Items, Mode=OneWay}">

    <ComboBox.Resources>
        <!-- Define ItemTemplate resource -->
        <DataTemplate x:Key="ItemTemplate" DataType="viewModel:ItemType">
            <TextBlock Text="{Binding FieldOne, Mode=OneWay}" />
        </DataTemplate>
    </ComboBox.Resources>

    <ComboBox.ItemsPanel>
        <ItemsPanelTemplate>
            <StackPanel Grid.IsSharedSizeScope="True"
                        IsItemsHost="True">
                <StackPanel.Resources>
                    <!-- Redefine ItemTemplate resource -->
                    <DataTemplate x:Key="ItemTemplate" DataType="viewModel:ItemType">
                        <Grid>
                            <Grid.ColumnDefinitions>
                                <ColumnDefinition Width="Auto" SharedSizeGroup="GroupOne" />
                                <ColumnDefinition Width="10" SharedSizeGroup="GroupSpace" />
                                <ColumnDefinition Width="Auto" SharedSizeGroup="GroupTwo" />
                            </Grid.ColumnDefinitions>
                
                            <TextBlock Grid.Column="0" Text="{Binding FieldOne, Mode=OneWay}" />
                            <TextBlock Grid.Column="2" Text="{Binding FieldTwo, Mode=OneWay}" />
                        </Grid>
                    </DataTemplate>
                </StackPanel.Resources>
            </StackPanel>
        </ItemsPanelTemplate>
    </ComboBox.ItemsPanel>

    <ComboBox.ItemTemplate>
        <DataTemplate>
            <ContentPresenter ContentTemplate="{DynamicResource ItemTemplate}" />
        </DataTemplate>
    </ComboBox.ItemTemplate>
</ComboBox>