I have a comboxbox that is databound to an observablecollection from my viewmodel. I can get my list to populate with the data but I also would like to add a default item like "--All Models--". The code below displays "--All Models--" as the default item but it is not selectable if you select another item.

```
<ContentControl Content="{Binding Items}">
    <ContentControl.ContentTemplate>
        <DataTemplate>
            <Grid>
                <ComboBox x:Name="cb" ItemsSource="{Binding}"/>
                <TextBlock x:Name="tb" Text="--Choose One--" IsHitTestVisible="False" Visibility="Hidden"/>
            </Grid>
            <DataTemplate.Triggers>
                <Trigger SourceName="cb" Property="SelectedItem" Value="{x:Null}">
                    <Setter TargetName="tb" Property="Visibility" Value="Visible"/>
                </Trigger>
            </DataTemplate.Triggers>
        </DataTemplate>
    </ContentControl.ContentTemplate>
</ContentControl>

```

I have tried with a compositecollection but that does not seem to to work. Is there a way to accomplish this?

Thanks in advance!