wpf datatrigger未在预期时触发

public class CountToSelectedTextConverter : IValueConverter
{
    #region IValueConverter Members

    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        if ((int)value == 1)
            return "item selected";
        else
            return "items selected";
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }

    #endregion
}

xmlns:local="clr-namespace:ValueConverterExample"

<Window.Resources>
    <local:CountToSelectedTextConverter x:Key="CountToSelectedTextConverter"/>
</Window.Resources>

    <TextBlock Text="{Binding ElementName=EditListBox, Path=SelectedItems.Count, Converter={StaticResource CountToSelectedTextConverter}}"/>

<TextBlock Margin="0,0,5,0" Text="{Binding ElementName=EditListBox, Path=SelectedItems.Count}"/>
<TextBlock>
    <TextBlock.Style>
        <Style TargetType="{x:Type TextBlock}">
            <Setter Property="Text" Value="items selected"/>
            <Style.Triggers>
                <DataTrigger Binding="{Binding ElementName=EditListBox, Path=SelectedItems.Count}" Value="1">
                    <Setter Property="Text" Value="item selected"/>
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </TextBlock.Style>
</TextBlock>