How to use AlternationIndex in ItemsControls?

The ItemContainerStyle is applied to the elements generated by the ItemsControl: ContentPresenter. The ContentPresenter will in turn contain whatever you put in your ItemTemplate. In the case of a ListBox, the ItemContainerStyle is applied to the generated ListBoxItem.

The AlternationCount is, based on what you posted, only available on these generated items. You cannot use the ItemContainerStyle to set the Grid's background, because the Grid is unknown to that Style.

The following would be ideal, but unfortunately ContentPresenter has no background property. It would work for a ListBox (with ListBoxItems) however.

<ItemsControl    ItemsSource="{Binding ObservableCollectionItems}"    AlternationCount="2">    <ItemsControl.ItemContainerStyle>        <Style TargetType="ContentPresenter">            <Style.Triggers>                <Trigger Property="ItemsControl.AlternationIndex" Value="0">                    <Setter Property="Background" Value="Red"></Setter>                </Trigger>                <Trigger Property="ItemsControl.AlternationIndex" Value="1">                    <Setter Property="Background" Value="Blue"></Setter>                </Trigger>            </Style.Triggers>        </Style>    </ItemsControl.ItemContainerStyle></ItemsControl>

So you end up writing a style for the grid which binds to the AlternationIndex of your parent ContentPresenter.

<DataTemplate DataType="{x:Type vm:ObservableCollectionItem}">    <Grid>        <Grid.Style>            <Style TargetType="Grid">                <Style.Triggers>                    <DataTrigger Binding="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ContentPresenter}}, Path=(ItemsControl.AlternationIndex)}" Value="0">                        <Setter Property="Background" Value="Red"/>                    </DataTrigger>                    <DataTrigger Binding="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ContentPresenter}}, Path=(ItemsControl.AlternationIndex)}" Value="1">                        <Setter Property="Background" Value="Blue"/>                    </DataTrigger>                </Style.Triggers>            </Style>        </Grid.Style>    </Grid></DataTemplate>

hm... After about 2 hours of playing around, I finally found the solution that simply works:

<ItemsControl ItemsSource="{Binding}" AlternationCount="2">    <ItemsControl.ItemTemplate>        <DataTemplate>            <Grid Background="Transparent" x:Name="__PART_GRID"></Grid>            <DataTemplate.Triggers>                <Trigger Property="ItemsControl.AlternationIndex" Value="0">                    <Setter TargetName="__PART_GRID" Property="Background" Value="Red"/>                </Trigger>                <Trigger Property="ItemsControl.AlternationIndex" Value="1">                    <Setter TargetName="__PART_GRID" Property="Background" Value="Blue"/>                </Trigger>            </DataTemplate.Triggers>        </DataTemplate>    </ItemsControl.ItemTemplate></ItemsControl>

I hope this answer helps others to save some time.

Or, as I found on another post, and it works great for me...You can simply use a binding...

{Binding    RelativeSource={RelativeSource Mode=TemplatedParent},     Path=(ItemsControl.AlternationIndex)}

NB: remember to add AlterationCount="100" on your ItemsControl