CollectionViewSource sort algorithm

Question

How to change the sort algorithm of CollectionViewSource? In fact, i found that the sort algorithm of CollectionViewSource is not stable. And i want to use a stable algorithm on the CollectionViewSource. How can i do that?

Answer 1

I've managed to get a stable sorting using a custom Comparer, but it kinda feels like a big hack...

Just like Benjamin suggested, I get the ListCollectionView from the collection and set its CustomSort property with my custom Comparer. The only difference is that I pass the collection to the Comparer when instantiating it.

private void Sorting(IEnumerable collection)
{
    var view = CollectionViewSource.GetDefaultView(collection) as ListCollectionView;

    if (view != null)
    {
        view.CustomSort = new StableComparer(collection);
    }
}

Then, in my custom Comparer, I use the collection in the Compare method just to fallback to the items indexes when the comparison returns a zero (they are the same or have the same value).

public class StableComparer : IComparer
{
    public IEnumerable Collection { get; set; }

    public StableComparer(IEnumerable collection)
    {
        Collection = collection;
    }

    public int Compare(object x, object y)
    {
        IComparable x_Comparable = x as IComparable;
        IComparable y_Comparable = y as IComparable;

        if (x_Comparable != null && y_Comparable != null)
        {
            var comparison = x_Comparable.CompareTo(y_Comparable);

            // A zero value means x and y are equivalent for sorting, and they could
            //  be rearranged by an unstable sorting algorithm
            if (comparison == 0 && Collection != null)
            {
                // IndexOf is an extension method for IEnumerable (not included)
                var x_Index = Collection.IndexOf(x);
                var y_Index = Collection.IndexOf(y);

                // By comparing their indexes in the original collection, we get to
                //  preserve their relative order
                if (x_Index != -1 && y_Index != -1)
                    comparison = x_Index.CompareTo(y_Index);
            }

            return comparison;
        }

        return 0;
    }
}

I'm still testing this, so I can't guarantee this would work all the time... One problem would be keeping the Collection property inside the Comparer updated, for instance. Or supporting two sort directions.

But I think the idea is clear, though hacky, as I said.