Merge.cs

namespace Shared.Helpers;

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 = (TCurrent?)currentEntity,
                toBeEntity = toBeEntity.DefaultIfEmpty().SingleOrDefault()
            });
        
        var rightOuterJoin = toBeList.GroupJoin(currentList, toBeIdSelector, currentIdSelector,
            (toBeEntity, currentEntity) => new
            {
                currentEntity = currentEntity.DefaultIfEmpty().SingleOrDefault(),
                toBeEntity = (TToBe?)toBeEntity
            });
        
        var fullOuterJoin = leftOuterJoin.Union(rightOuterJoin).ToArray();
        
        foreach (var entity in fullOuterJoin)
        {
            if (entity.toBeEntity is null && entity.currentEntity is not null)
            {
                onRemove?.Invoke(entity.currentEntity);
            }
            else if (entity.currentEntity is null && entity.toBeEntity is not null)
            {
                onAdd?.Invoke(entity.toBeEntity);
            }
            else if (entity.currentEntity is not null && entity.toBeEntity is not null)
            {
                onUpdate?.Invoke(entity.currentEntity, entity.toBeEntity);
            }
        }
    }
}