Merge.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace CollectionHelpers
{
public static class MergeHelper
    {
        public static void Merge<TCurrent, TToBe, TId>(
            this IEnumerable<TCurrent> current,
            IEnumerable<TToBe> toBe,
            Func<TCurrent, TId> currentIdSelector,
            Func<TToBe, TId> toBeIdSelector,
            Action<TToBe> onAdd = null,
            Action<TCurrent, TToBe> onUpdate = null,
            Action<TCurrent> onRemove = null)
        {
            var currentList = current.ToList();
            var toBeList = toBe.ToList();

            var leftOuterJoin = currentList.GroupJoin(toBeList, currentIdSelector, toBeIdSelector,
                (currentEntity, toBeEntity) => new
                {
                    currentEntity,
                    toBeEntity = toBeEntity.DefaultIfEmpty().SingleOrDefault()
                });
            var rightOuterJoin = toBeList.GroupJoin(currentList, toBeIdSelector, currentIdSelector,
                (toBeEntity, currentEntity) => new
                {
                    currentEntity = currentEntity.DefaultIfEmpty().SingleOrDefault(),
                    toBeEntity
                });
            var fullOuterJoin = leftOuterJoin.Union(rightOuterJoin).ToArray();
            foreach (var entity in fullOuterJoin)
                if (Equals(entity.toBeEntity, null))
                    onRemove?.Invoke(entity.currentEntity);
                else if (Equals(entity.currentEntity, null))
                    onAdd?.Invoke(entity.toBeEntity);
                else
                    onUpdate?.Invoke(entity.currentEntity, entity.toBeEntity);
        }
    }
}