Question

I am removing the users from a user collection using linq as below. Is there a better of doing it? Can I merge the first and second query?

List<int> userIDs = ConfigurationManager.AppSettings["users"].Split(',').Select(userId =>            Convert.ToInt32(userid)).ToList();

foreach (int userId in userIDs)
{
   userInfoList.RemoveAll(user => (user.UserId.Equals(userId)));
}

UserInfoList is a collection of users.

Thanks in advance

Answer 1

This would be more efficient:

HashSet<int> userIDs = new HashSet<int>(ConfigurationManager.AppSettings["users"].Split(',').Select(userId => Convert.ToInt32(userId)));

userInfoList.RemoveAll(user => userIDs.Contains(user.UserId));

If you have n elements in userIDs and m elements in userInfoList, then your solution is O(n * m). Using HashSet, the complexity becomes O(n + m) – much better. (Assuming the hash table operations are constant time).