A helper to resolve the list of differences (delta) between two lists

DeltaHelper.cs

    public static class DeltaHelper
    {
        public static (IEnumerable<TItem> Left, IEnumerable<TItem> Both, IEnumerable<TItem> Right) Compute<TItem, TValue>(
            IEnumerable<TItem> list1,
            IEnumerable<TItem> list2,
            Func<TItem, TValue> getItemValue)
            where TValue : IEquatable<TValue>
        {
            var comparableList1 = list1.Select(x => new EqualityComparer<TItem, TValue>
            {
                GetItemValue = getItemValue,
                Item = x
            });

            var comparableList2 = list2.Select(x => new EqualityComparer<TItem, TValue>
            {
                GetItemValue = getItemValue,
                Item = x
            });

            return (
                Left: comparableList1.Intersect(comparableList2).Select(x => x.Item),
                Both: comparableList1.Except(comparableList2).Select(x => x.Item),
                Right: comparableList2.Except(comparableList1).Select(x => x.Item)
                );
        }

        public static (IEnumerable<TItem> Left, IEnumerable<TItem> Both, IEnumerable<TItem> Right) Delta<TItem, TValue>(
            this IEnumerable<TItem> list1,
            IEnumerable<TItem> list2,
            Func<TItem, TValue> getItemValue)
            where TValue : IEquatable<TValue>
        {
            return Compute(list1, list2, getItemValue);
        }

        class EqualityComparer<TItem, TValue> : IEqualityComparer<TItem>
        {
            public Func<TItem, TValue> GetItemValue { get; set; }
            public TItem Item { get; set; }

            public bool Equals(TItem x, TItem y)
            {
                return GetItemValue(x).Equals(GetItemValue(y));
            }

            public int GetHashCode(TItem obj)
            {
                return GetItemValue(obj).GetHashCode();
            }
        }
    }