wpf combobox with custom itemtemplate text

Question

I have ComboBox with custom ItemTemplate.

<ComboBox Height="20" Width="200" 
          SelectedItem="{Binding Path=SelectedDesign}"
          ItemsSource="{Binding Path=Designs}" HorizontalAlignment="Left" 
          ScrollViewer.CanContentScroll="False">

    <ComboBox.ItemTemplate>
        <DataTemplate DataType="{x:Type formdesign:FormDesignContainer}">
            <Rectangle Width="200" Height="100">
                <Rectangle.Fill>
                    <ImageBrush ImageSource="{Binding Path=ImageThumb}" Stretch="Uniform" />
                </Rectangle.Fill>
            </Rectangle>
        </DataTemplate>
    </ComboBox.ItemTemplate>

</ComboBox>

This works well. However WPF tries to draw rectangle as Combobox Text. How can I set "text" for this template. By "text" I mean string or control which represent selected item and write into combobox when item is selected

In other words I'd like to do this:

But now I got this

Answer 1

Try setting SelectionBoxItemTemplate with a TextBlock. Appears that SelectionBoxItemTemplate is read-only. So another approach is to override ItemContainerStyle.Template. Example