WPF: l'Alternance des couleurs sur un ItemsControl?

Comment puis-je obtenir l'alternance des couleurs sur un ItemsControl? J'ai AlternationCount à 2, mais le ItemsControl.AlternationIndex propriété renvoie toujours 0.

        <ItemsControl ItemsSource="{Binding}" AlternationCount="2">
<ItemsControl.Resources>
<Style x:Key="FooBar" TargetType="Grid">
<Style.Triggers>
<Trigger Property="ItemsControl.AlternationIndex" Value="0">
<Setter Property="Background" Value="Blue"/>
</Trigger>
<Trigger Property="ItemsControl.AlternationIndex" Value="1">
<Setter Property="Background" Value="Red"/>
</Trigger>
</Style.Triggers>
</Style>
</ItemsControl.Resources>
<ItemsControl.ItemTemplate>
<DataTemplate>
<Grid Margin="0,0,0,10" Style="{StaticResource FooBar}">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="25" />
<ColumnDefinition Width="100" />
<ColumnDefinition Width="100" />
<ColumnDefinition Width="100" />
<ColumnDefinition Width="100" />
<ColumnDefinition Width="100" />
<ColumnDefinition Width="100" />
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions >
<RowDefinition Height="Auto" />
<!--<RowDefinition Height="Auto" />-->
</Grid.RowDefinitions>
<CheckBox IsChecked="{Binding Checked, UpdateSourceTrigger=PropertyChanged}" VerticalAlignment="Center" />
<Label Grid.Column="1" Content="{Binding CompanyName}" />
<Label Grid.Column="2" Content="{Binding TradeKey}" />
<Label Grid.Column="3" Content="{Binding TradeDate}" ContentStringFormat="d" />
<Label Grid.Column="4" Content="{Binding Cusip}" />
<Label Grid.Column="5" Content="{Binding IssueName}" />
<Label Grid.Column="6" Content="{Binding TotalUnits}" ContentStringFormat="N0" />
<!--<Expander Grid.Row="0" Grid.Column="7" Grid.ColumnSpan="7" IsExpanded="True">
<Expander.Header>
<StackPanel Orientation="Horizontal">
<Label Content="Allocations"/>
<Button Content="Edit" Name="cmdEdit" Click="cmdEdit_Click"  />
</StackPanel>
</Expander.Header>-->
<DataGrid Grid.Column="7" IsReadOnly="True" ItemsSource="{Binding Territories}" AutoGenerateColumns="False" CanUserAddRows="False" CanUserDeleteRows="False">
<DataGrid.Columns>
<DataGridTextColumn Header="Rep on Ticket" Binding="{Binding TradeCustomer.RepNameNotes}" />
<DataGridTextColumn Header="Rep # on Ticket" Binding="{Binding TradeCustomer.RepNumberNotes}" />
<DataGridTextColumn Header="State" Binding="{Binding TradeCustomer.AccountStateKey}" />
<DataGridTextColumn Header="Qty. on Ticket" Binding="{Binding TradeCustomer.Quantity, StringFormat=N0}" />
<DataGridTextColumn Header="Zip Code" Binding="{Binding ZipCode}" />
<DataGridTextColumn Header="State" Binding="{Binding State}" />
<DataGridTextColumn Header="Territory" Binding="{Binding Territory}" />
</DataGrid.Columns>
</DataGrid>
<!--</Expander>-->
</Grid>
</DataTemplate>
</ItemsControl.ItemTemplate>
<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<VirtualizingStackPanel />
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>
<ItemsControl.Template>
<ControlTemplate>
<Border BorderThickness="{TemplateBinding Border.BorderThickness}" Padding="{TemplateBinding Control.Padding}" BorderBrush="{TemplateBinding Border.BorderBrush}" Background="{TemplateBinding Panel.Background}" SnapsToDevicePixels="True">
<ScrollViewer Padding="{TemplateBinding Control.Padding}" Focusable="False">
<ItemsPresenter SnapsToDevicePixels="{TemplateBinding UIElement.SnapsToDevicePixels}" />
</ScrollViewer>
</Border>
</ControlTemplate>
</ItemsControl.Template>
</ItemsControl>