How can I use an IComparer<T> to sort by arbitrary values?

Question

I receive a sorted list of integers which I would like to use to sort a ListCollectionView via ListCollectionView.CustomSort. This property accepts an IComparer and I've created one but, as expected, I can only compare values on generic objects x and y.

public List<int> SortedIds = new List<int> { 13, 7, 5, 3, 1 };
public class Item {
  int Id { get; set; }    
}

public class IdComparer : IComparer<Item> {
  public int Compare(Item x, Item y) {
    // Comparisons 
    // ???
    // return 0;
  }
}

Answer 1

If you want to compare by their index in your SortedIds list, you can do:

public class IdComparer : IComparer<Item> {
    private readonly Dictionary<int, int> idIndexes;
    public IdComparer(IEnumerable<int> sortedIds)
    {
        idIndexes = sortedIds
            .Select((id, idx) => new { Id = id, Index = idx })
            .ToDictionary(p => p.Id, p.Index);
    }

    public int Compare(Item x, Item y) {
        xIndex = idIndexes[x.Id];
        yIndex = idIndexes[y.Id]
        return xIndex.CompareTo(yIndex);
    }
}

However you could use linq:

IEnumerable<Item> sorted = SortedIds
    .Select((id, idx) => new { Id = id, Index = idx })
    .Join(items, p => i.Id, item => item.Id, (p, item) => new { Item = item, Index = idx })
    .OrderBy(p => p.Index)
    .Select(p => p.Item);

Answer 2

Maybe you want to return SortedIds.IndexOf(x.Id).CompareTo(SortedIds.IndexOf(y.Id))?

Answer 3

You can use OrderBy to order by any field you want. Like this:

List<Item> sortedById = items.OrderBy(x=>x.Id).ToList();

and

List<Item> sortedByEgo = items.OrderBy(x=>x.Ego).ToList();

(for types that don't compare directly you can also use the variant of OrderBy that takes an IComparer argument, but that's not needed here if Id and Ego are ints).