IEnumerableExtensions.cs

public class EditedPair<T>
{
    public EditedPair(T old, T @new)
    {
        Old = old;
        New = @new;
    }

    public T Old { get; private set; }
    public T New { get; private set; }
}

public class CollectionDif<T>
{
    public CollectionDif(IEnumerable<T> added, IEnumerable<T> deleted, IEnumerable<EditedPair<T>> edited)
    {
        Added = added;
        Deleted = deleted;
        Edited = edited;
    }

    public IEnumerable<T> Added { get; private set; }
    public IEnumerable<T> Deleted { get; private set; }
    public IEnumerable<EditedPair<T>> Edited { get; private set; }
}

public static class Extensions
{
    public static IEnumerable<T> FindAdded<T, TKey>(this IEnumerable<T> newList, IEnumerable<T> oldList, Func<T, TKey> keySelector)
    {
        return newList.Where(n => !oldList.Any(o => keySelector(n).Equals(keySelector(o))));
    }

    public static IEnumerable<T> FindDeleted<T,TKey>(this IEnumerable<T> newList, IEnumerable<T> oldList, Func<T, TKey> keySelector)
    {
        return oldList.FindAdded(newList, keySelector);
    }

    public static IEnumerable<EditedPair<T>> FindEdited<T, TKey, TValue>(this IEnumerable<T> newList, IEnumerable<T> oldList, Func<T, TKey> keySelector, Func<T, TValue> valueSelector)
    {
        var commonElements = newList.Where(n => oldList.Any(o => keySelector(n).Equals(keySelector(o))));

        return commonElements
            .Select(e => new { New = e, Old = oldList.Single(x => keySelector(e).Equals(keySelector(x))) })
            .Where(t => !valueSelector(t.Old).Equals(valueSelector(t.New)))
            .Select(t => new EditedPair<T>(old: t.Old, @new: t.New));
    }

    public static CollectionDif<T> FindDiff<T, TKey, TValue>(this IEnumerable<T> newList, IEnumerable<T> oldList, Func<T, TKey> keySelector, Func<T, TValue> valueSelector)
    {
        return new CollectionDif<T>(
            added: newList.FindAdded(oldList, keySelector),
            deleted: newList.FindDeleted(oldList, keySelector),
            edited: newList.FindEdited(oldList, keySelector, valueSelector));
    }
}