Getting index of an item in an ObservableCollection inside the item

Question

I'd like to be able to display an index value from within a DataTemplate, but I don't want the data to be persisted or backed by the model or viewmodel. In other words, if the order of the items in the OC changes, I don't want to have to recalculate the indexes. The value should be intrinsically tied to the underlying index in the OC. It is okay if the index is 0-based (in fact, I'd expect it).

One method that others have used is the AlternationIndex AP, but this has its own pitfalls for certain situations.

One last thought: I can't help but think that a converter is going to be helpful in a final solution.

Answer 1

I would use a converter to do this.

The trick is giving it the source collection, either on the ConverterParameter or a Dependency Property. At that point, conversion is as simple as using IndexOf.

Here's a sample converter that does this:

public class ItemToIndexConverter : IValueConverter
{
    public object Convert(...)
    {
        CollectionViewSource itemSource = parameter as CollectionViewSource;
        IEnumerable<object> items = itemSource.Source as IEnumerable<object>;

        return items.IndexOf(value as object);
    }

    public object ConvertBack(...)
    {
        return Binding.DoNothing;
    }
}

You can make the implementation strongly typed, return a formatted string as a number, etc. The basic pattern will be as above though.

This implementation uses the parameter approach, as making a DP is more messy in my view. Because you can't bind ConverterParameter, I have it set to a static resource that is bound to the collection:

<CollectionViewSource x:Key="collectionSource" Source="{Binding Path=MyCollection}" />

...

<TextBlock Text="{Binding Converter={StaticResource ResourceKey=ItemToIndexConverter}, 
               ConverterParameter={StaticResource ResourceKey=collectionSource}}"/>