WPF combobox with image in case of null value

I would like to have a combobox in WPF whose items are of type string (like the default). The only difference is that I want to display an image (error) if the string is x:null. Anyone an idea how I can create an itemtemplate or style to display an image in the case of a null (or empty) string and the string in all other cases in the co开发者_开发问答mbobox.

Thanks!

-- Carsten

Here is an example that uses an ItemTemplate and a DataTrigger to only show an image if the item is null:

<Grid>
    <Grid.Resources>
        <x:Array x:Key="sampleData" Type="sys:String">
            <sys:String>abc</sys:String>
            <sys:String>def</sys:String>
            <sys:String>ghi</sys:String>
            <x:Null/>
        </x:Array>
    </Grid.Resources>
    <StackPanel>
        <ComboBox ItemsSource="{StaticResource sampleData}" Text="abc">
            <ComboBox.ItemTemplate>
                <DataTemplate>
                    <Grid>
                        <TextBlock Text="{Binding}"/>
                        <Image x:Name="errorImage" Source="http://thecybershadow.net/misc/stackoverflow.png" Visibility="Collapsed"/>
                    </Grid>
                    <DataTemplate.Triggers>
                        <DataTrigger Binding="{Binding}" Value="{x:Null}">
                            <Setter TargetName="errorImage" Property="Visibility" Value="Visible"/>
                        </DataTrigger>
                    </DataTemplate.Triggers>
                </DataTemplate>
            </ComboBox.ItemTemplate>
        </ComboBox>
    </StackPanel>
</Grid>

and it looks like this where the fourth null item is displayed as a stack overflow image:

Ok, here is the remaining piece in the style combobox (edit a copy of it and look for ContenPresenter and replace with this):

                        <Grid>                         
        <ContentPresenter HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}" IsHitTestVisible="false" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" Content="{TemplateBinding SelectionBoxItem}" ContentStringFormat="{TemplateBinding SelectionBoxItemStringFormat}" ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}" ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}"/>
                                <Image x:Name="errorImage" Source="/Autodesk.UtilityDesign.Electric.UI;component/Themes/Images/Error.ico" Visibility="Collapsed" Width="16"/>                     
                            </Grid>                     
                        </Grid>
                        <ControlTemplate.Triggers>
                            <Trigger Property="SelectedItem" Value="{x:Null}">
                                <Setter TargetName="errorImage" Property="Visibility" Value="Visible"/>                     
                            </Trigger>