How to subtract one huge list from another efficiently in C#

Question

I have a very long list of Ids (integers) that represents all the items that are currently in my database:

var idList = GetAllIds();

I also hav

Answer 1

Transform temporarily idList to an HashSet and use the same method i.e.:

items.RemoveAll(e => idListHash.Contains(e.Id));

it should be much faster