Question

There is an example of how to modify SortableBindingList to use a stable sort. However, there is an updated version of SortableBindingList. What is the best way to modify this new version to use a stable sort? I think I would want a flag on the SortableBindingList to let the user of the SortableBindingList decide if they want to use (slower) stable sort or (faster) default sort.

Thanks

Answer 1

You can solve this problem by writing a stable sort extension method for List<T>:

public static class ListExtensions
{
    public static void StableSort<T>(this List<T> list, IComparer<T> comparer)
    {
        var pairs = list.Select((value, index) => Tuple.Create(value, index)).ToList();
        pairs.Sort((x, y) =>
            {
                int result = comparer.Compare(x.Item1, y.Item1);
                return result != 0 ? result : x.Item2 - y.Item2;
            });
        list.Clear();
        list.AddRange(pairs.Select(key => key.Item1));
    }
}

and then in the new version of SortableBindingList change this line:

itemsList.Sort(comparer);

to:

itemsList.StableSort(comparer);

This works by using the unstable sort supplemented with a secondary key on the item index within the list. Since this version doesn’t use the pathologically slow insertion sort to achieve a stable sort, it should be fast enough for general use.