c# - Subtract a generic list from another

Question

I am trying remove a list of firmIDs from one list from another. I don't really understand linq but I am pretty sure I need to use it.

List<Firm> firms = GetBusinessDevelopmentFirms(database);
List<Firm> trackedFirms = GetAllCLIFirmsBeingTrackedByUser();

var result = firms.Contains(i => trackedFirms.Contains(i.FirmID));

The last line doesn't work and the system says "unknown method Contains(?)" even though I have put "using System.Linq;" At the top of the class.

My idea was to remove a list of tracked firms from a list of all firms to find the untracked firms.

I hope this makes sense.

Answer 1

var result = firms.Except(trackedFirms); // returns all the firms except those in trackedFirms