C# - DTO to Entity Comparison that outputs items to add/delete/update to/from/in target collection

dto-to-entity-comparison.cs

// Let's say you have a list of DTOs (data tranfer objects) that someone
// posted on the web or anywhere.  Items have been removed and added to it.
// You need to compare the input with what exists in the database
// and find out 1.) new items to add to the database, 
// 2.) items to delete from the database, and
// 3.) items to update in the database.
// You don't want to continually rewrite that sort of thing over and over again, like I have.
// The problem with conventional methods is that the data types on either side of the
// equation are probably different types.
// The following will produce the items to add/delete/update by comparing them via 
// a user-defined comparison Func<T,U,bool>.
//
// This sample assumes a list of ThingDto objects that are posted from the web
// and using EntityFramework, we compare what is in the database versus what
// is coming in from the web.

DbContext context = new DbContext(); // pretend EntityFramework // context.Things is of type ThingEntity
List<ThingDto> listPostedFromWeb;// passed in parameter

Func<ThingDto, ThingEntity, bool> comparer = (compDto, compEntity) =>
{
	// what comparison between these different types ensures
	// that they are the same?
	return compDto.Id == compEntity.id;
};

var (insert, update, delete) = GetChanges(
	listPostedFromWeb ?? new List<ThingDto>(),      // list from the web or a new list
	context.Things.Where(x => x.CategoryId == XYZ), // EntityFramework query
	comparer); // your custom comparer.

public (IEnumerable<L> insert, IEnumerable<R> update, IEnumerable<R> delete) GetChanges<L,R>(IEnumerable<L> models, IEnumerable<R> dos, Func<L,R,bool> comparer)
{
	IEnumerable<L> itemsToInsert;
	if (models == null)
		itemsToInsert = new List<L>();
	else if (dos == null)
		itemsToInsert = models;
	else
		itemsToInsert = models.Where(x => !dos.Any(y => comparer(x, y))).ToList();

	IEnumerable<R> itemsToUpdate;
	if (models == null || dos == null)
		itemsToUpdate = new List<R>();
	else
		itemsToUpdate = dos.Where(x => models.Any(y => comparer(y, x))).ToList();

	IEnumerable<R> itemsToDelete;
	if (models == null)
		itemsToDelete = dos;
	else if (dos == null)
		itemsToDelete = new List<R>();
	else
		itemsToDelete = dos.Where(x => !models.Any(y => comparer(y, x))).ToList();

	return (itemsToInsert, itemsToUpdate, itemsToDelete);
}