Question

I’m matching some in memory lists entities with a .contains (subselect) query to filter out old from new users.

Checking for performance problems i saw this:

The oldList mostly has around 1000 users in them, while the new list varies from 100 to 500. Is there a way to optimize this query?

Answer 1

Absolutely – build a set instead of checking a list each time:

// Change string to whatever the type of UserID is.
var oldUserSet = new HashSet<string>(oldList.Select(o => o.UserID));
var newUsers = NewList.Where(n => !oldUserSet.Contains(n.UserID))
                      .ToList();

The containment check on a HashSet should be O(1) assuming few hash collisions, instead of the O(N) of checking each against the whole sequence (for each new user).