Question

If I want to filter a list of objects against a specific id, I can do this:

list.Where(r => r.Id == idToCompare);   

What if, instead of a single idToCompare, I have a list of Ids to compare against?

What is the syntax for comparing against a predefined list? Something like:

int[] listofIds = GetListofIds();

list.Where(r => r.Id "in listofIds");   

Answer 1

If listOfIds is a list, this will work, but, List.Contains() is a linear search, so this isn’t terribly efficient.

You’re better off storing the ids you want to look up into a container that is suited for searching, like Set.

List<int> listOfIds = new List(GetListOfIds());
lists.Where(r=>listOfIds.Contains(r.Id));