How to make the first(primary) selected item in listbox multiple mode different in style?

[CodingOctocat]

Like this, I selected "CCC" first, then "AAA" and "DDD", and i want "CCC" is Bold style

I've actually done this by adding a bool IsFirstSelected property to the Model and then setting the DataTrigger in the Style, but is there any way to avoid adding this "unnecessary" property?

I've also tried MultiBinding with IMultiValueConverter to determine if the current item is the first item in the SelectedItems, But when I make multiple selections, IMultiValueConverter not executed, and all the SelectedItems are now in Bold.

[CodingOctocat]

I did a different approach and it worked fine, I added a PrimarySelectedItem (impl INotifyPropertyChange) to the VM:

<TextBlock.Style>
    <Style TargetType="{x:Type TextBlock}">
        <Setter Property="FontWeight" Value="Normal" />

        <Style.Triggers>
            <DataTrigger Value="{StaticResource True}">
                <DataTrigger.Binding>
                    <MultiBinding Converter="{StaticResource ValueEqualsMultiConverter}">
                        <Binding Path="." />
                        <Binding Path="DataContext.PrimarySelectedItem"
                                 RelativeSource="{RelativeSource AncestorType={x:Type ListBox}}" />
                    </MultiBinding>
                </DataTrigger.Binding>

                <Setter Property="FontWeight" Value="Bold" />
            </DataTrigger>
        </Style.Triggers>
    </Style>
</TextBlock.Style>

public class ValueEqualsMultiConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        try
        {
            return values.All(x => x == values[0]);
        }
        catch
        {
            return Binding.DoNothing;
        }
    }

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