Trying to create an IsFirstItem and IsLastItem IValueConverter for object collection bindings

Question

The end goal to be able to set a specific ItemContainerStyle on the first and last element in my list box;

The converters thus far are:

public class IsFirstItemConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        bool result = false;
        result = ((IList<object>)parameter).First() == value;

        return result;
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}
public class IsLastItemConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        bool result = false;
        result = ((IList<object>)parameter).Last() == value;

        return result;
    }

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

And implementation:

<DataTrigger Value="True" Binding="{Binding Converter={StaticResource IsFirstItemConverter},ConverterParameter=Items,  ElementName=SubItems}">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type ListBoxItem}">
                <StackPanel Orientation="Horizontal">
                    <TextBlock Text="First"/>
                    <ContentPresenter/>
                </StackPanel>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</DataTrigger>

And the error is:

InvalidCastException: Unable to cast object of type 'System.String' to type 'System.Collections.Generic.IList`1[System.Object]'.

Im sure I screwed up in multiple spots, just not experienced enough with XAML and bindings to narrow down where.

Take a look at [this](http://stackoverflow.com/questions/13613053/how-can-i-know-if-a-listboxitem-is-the-last-item-inside-a-wpfs-listbox) — Ponas Justas, Mar 04 '16 at 15:17
@Sinatr Ohh my, that works even cleaner than the way I was trying to do it, please post as an answer (even though its just a reference to elsewhere) so I can accept it. This is definitely getting added to my library of common converters. — Wobbles, Mar 04 '16 at 15:26
@Wobbles, I rather expect from you to post an answer ;). After you adapt it to work for you case: to find first and last item. If you are not going to, then feel free to simply delete question. — Sinatr, Mar 04 '16 at 15:30
Anybody know why these data triggers wouldn't be firing when the collection changes? — Wobbles, Mar 05 '16 at 12:58

score 1 · Answer 1 · answered Mar 04 '16 at 15:51

The following adapted from the questions comments work perfectly:

public class IsLastItemConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        DependencyObject item = (DependencyObject)value;
        ItemsControl ic = ItemsControl.ItemsControlFromItemContainer(item);

        return ic.ItemContainerGenerator.IndexFromContainer(item) == ic.Items.Count - 1;
    }

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

public class IsFirstItemConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        DependencyObject item = (DependencyObject)value;
        ItemsControl ic = ItemsControl.ItemsControlFromItemContainer(item);

        return ic.ItemContainerGenerator.IndexFromContainer(item) == 0;
    }

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

public class IsOnlyItemConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        DependencyObject item = (DependencyObject)value;
        ItemsControl ic = ItemsControl.ItemsControlFromItemContainer(item);

        return (ic.ItemContainerGenerator.IndexFromContainer(item) == 0 && ic.ItemContainerGenerator.Items.Count == 1);
    }

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

Implementation:

<DataTrigger Binding="{Binding RelativeSource={RelativeSource Self}, Converter={StaticResource IsLastItemConverter}}" Value="True">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type ListBoxItem}">
                <StackPanel Orientation="Horizontal">
                    <TextBlock Text="└"/>
                    <ContentPresenter/>
                </StackPanel>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</DataTrigger>

The magic seems to be that ItemsControlFromItemContainer makes it so I do not have to pass both the item and its collection instance to the converter but just the item and the converter can infer the parent collection.

Trying to create an IsFirstItem and IsLastItem IValueConverter for object collection bindings

1 Answers1