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.
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}}"/>