Different template for items in ComboBox's drop-down list and for selected item

Question

I have a ComboBox with fairly complex template for individual items, which includes two images and several lines of text:

Answer 1

The selected item (in the ComboBox itself, not the dropdown) is not inside a ComboBoxItem so you can do something like this:

<ComboBox.ItemTemplate>
    <DataTemplate>
        <ContentControl Content="{Binding}">
            <ContentControl.Style>
                <Style TargetType="{x:Type ContentControl}">
                    <!-- Complex default template -->
                    <Setter Property="ContentTemplate">
                        <Setter.Value>
                            <DataTemplate>
                                <Image Source="{Binding XPath=media:thumbnail/@url}" Width="100" Height="100" />
                            </DataTemplate>
                        </Setter.Value>
                    </Setter>
                    <Style.Triggers>
                        <!-- Simple selection box template -->
                        <DataTrigger
                                Binding="{Binding RelativeSource={RelativeSource AncestorType=ComboBoxItem}}"
                                Value="{x:Null}">
                            <Setter Property="ContentTemplate">
                                <Setter.Value>
                                    <DataTemplate>
                                        <TextBlock Text="{Binding XPath=title}" />
                                    </DataTemplate>
                                </Setter.Value>
                            </Setter>
                        </DataTrigger>
                    </Style.Triggers>
                </Style>
            </ContentControl.Style>
        </ContentControl>
    </DataTemplate>
</ComboBox.ItemTemplate>

(Edit: Note that the binding in the for the selection box will throw errors because the RelativeSource is not found. There are various options of circumventing this, one being a custom value converter that returns true or false depending on whether the ancestor exists (manual tree walking).)

Answer 2

I was searching for a standard (not hacky and without binding errors) solution to this problem. And I found it here: using DataTemplateSelector.

It's the same idea as per @H.B. answer: check whenever there is a ComboBoxItem as a parent in visual tree.

public class ComboBoxItemTemplateSelector : DataTemplateSelector
{
    public DataTemplate SelectedTemplate { get; set; }
    public DataTemplate DropDownTemplate { get; set; }

    public override DataTemplate SelectTemplate(object item, DependencyObject container)
    {
        while (container != null)
        {
            container = VisualTreeHelper.GetParent(container);
            if (container is ComboBoxItem)
                return DropDownTemplate;
        }
        return SelectedTemplate;
    }
}

Usage:

<ComboBox.ItemTemplateSelector>
    <local:ComboBoxItemTemplateSelector>
        <local:ComboBoxItemTemplateSelector.SelectedTemplate>
            <DataTemplate>
                ... simple template for selected item
            </DataTemplate>
        </local:ComboBoxItemTemplateSelector.SelectedTemplate>
        <local:ComboBoxItemTemplateSelector.DropDownTemplate>
            <DataTemplate>
                ... complex template used by dropdown items
            </DataTemplate>
        </local:ComboBoxItemTemplateSelector.DropDownTemplate>
    </local:ComboBoxItemTemplateSelector>
</ComboBox.ItemTemplateSelector>