What is the most efficient pattern/algorithm to compare two lists and find the delta between those two lists?

Question

We have two lists, let\'s say students and their scores. I want to compare these two lists and find the delta between the new list and the old list, then find the least intrusi

Answer 1

This should be able to do the trick if you don't have the same name twice in your list. In your example, you have 2x Steve, but you need a way to distinct between them.

public static List CompareLists(List existingList, List newList)
{
    List mergedList = new List();
    mergedList.AddRange(newList);
    mergedList.AddRange(existingList.Except(newList, new ListItemComparer()));
    return mergedList.OrderByDescending(x => x.Score).Take(10).ToList();
}

public class ListItemComparer : IEqualityComparer
{
    public bool Equals(ListItem x, ListItem y)
    {
        return x.Name == y.Name;
    }

    public int GetHashCode(ListItem obj)
    {
        return obj.Name.GetHashCode();
    }
}

You can call it like this:

newList = CompareLists(existingList, newList);