Question

I need to style the first and last items of a list view differently. To achieve that, I started working on a solution based on that answer: Use different template for last item in a WPF itemscontrol

Basically, I have a custom ItemsTemplateSelector that decide on the template to apply based on the item’s index in the list view items (code below).

It works properly except that when the list gets updated (an item is added or removed), the templates do not get selected again (for instance, initially, the SingleItemTemplate gets selected because there is a single item. When I add an item to the list, that first item’s template does not get switched to FirstItemTemplate). How to force template selection for all items?

public class FirstLastTemplateSelector : DataTemplateSelector 
{
    public DataTemplate DefaultTemplate { get; set; }
    public DataTemplate FirstItemTemplate { get; set; }
    public DataTemplate LastItemTemplate { get; set; }
    public DataTemplate SingleItemTemplate { get; set; }

    public override DataTemplate SelectTemplate(object item, DependencyObject container)
    {
        ListView lv = VisualTreeHelperEx.FindParentOfType<ListView>(container);
        if (lv != null)
        {
            if (lv.Items.Count == 1)
            {
                return SingleItemTemplate;
            }

            int i = lv.Items.IndexOf(item);
            if (i == 0)
            {
                return FirstItemTemplate;
            }
            else if (i == lv.Items.Count - 1)
            {
                return LastItemTemplate;
            }
        }
        return DefaultTemplate;
    }
}

Answer 1

As an alternative approach, I would suggest binding the AlternationCount of your ItemsControl to the number of items in your collection (e.g. the Count property). This will then assign to each container in your ItemsControl a unique AlternationIndex (0, 1, 2, … Count-1). See here for more information:

http://msdn.microsoft.com/en-us/library/system.windows.controls.itemscontrol.alternationcount.aspx

Once each container has a unique AlternationIndex you can use a DataTrigger in your container Style to set the ItemTemplate based off of the index. This could be done using a MultiBinding with a converter that returns True if the index is equal the count, False otherwise. Of course you could also build a selector around this approach as well. With the exception of the converter, this approach is nice since it is a XAML only solution.

An example using a ListBox:

<Window x:Class="WpfApplication4.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:Collections="clr-namespace:System.Collections;assembly=mscorlib"
        xmlns:System="clr-namespace:System;assembly=mscorlib"
        xmlns:l="clr-namespace:WpfApplication4"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Grid.Resources>
            <Collections:ArrayList x:Key="MyCollection">
                <System:String>Item One</System:String>
                <System:String>Item Two</System:String>
                <System:String>Item Three</System:String>
            </Collections:ArrayList>

            <l:MyAlternationEqualityConverter x:Key="MyAlternationEqualityConverter" />

            <Style x:Key="MyListBoxItemStyle" TargetType="{x:Type ListBoxItem}">
                <Style.Triggers>
                    <DataTrigger Value="True">
                        <DataTrigger.Binding>
                            <MultiBinding Converter="{StaticResource MyAlternationEqualityConverter}">
                                <Binding RelativeSource="{RelativeSource FindAncestor, AncestorType={x:Type ListBox}}" Path="Items.Count" />
                                <Binding RelativeSource="{RelativeSource Self}" Path="(ItemsControl.AlternationIndex)" />
                            </MultiBinding>
                        </DataTrigger.Binding>
                        <!-- Could set the ItemTemplate instead -->
                        <Setter Property="Background" Value="Red"/>
                    </DataTrigger>
                </Style.Triggers>
            </Style>
        </Grid.Resources>

        <ListBox ItemsSource="{Binding Source={StaticResource MyCollection}}"
                 AlternationCount="{Binding RelativeSource={RelativeSource Self}, Path=Items.Count}"
                 ItemContainerStyle="{StaticResource MyListBoxItemStyle}" />
    </Grid>

Where the converter might look something like:

class MyAlternationEqualityConverter : IMultiValueConverter
{
    #region Implementation of IMultiValueConverter

    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        if (values != null && values.Length == 2 &&
            values[0] is int && values[1] is int)
        {
            return Equals((int) values[0], (int) values[1] + 1);
        }

        return DependencyProperty.UnsetValue;
    }

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

    #endregion
}