How to Quickly Remove Items From a List

If you're happy creating a new list, you don't have to go through setting items to null. For example:

// This overload of Where provides the index as well as the value. Unless
// you need the index, use the simpler overload which just provides the value.
List<string> newList = oldList.Where((value, index) => index % 5 != 0)
                              .ToList();

However, you might want to look at alternative data structures, such as LinkedList<T> or HashSet<T>. It really depends on what features you need from your data structure.

If the order does not matter then there is a simple O(1) List.Remove method.

public static class ListExt
{
    // O(1) 
    public static void RemoveBySwap<T>(this List<T> list, int index)
    {
        list[index] = list[list.Count - 1];
        list.RemoveAt(list.Count - 1);
    }

    // O(n)
    public static void RemoveBySwap<T>(this List<T> list, T item)
    {
        int index = list.IndexOf(item);
        RemoveBySwap(list, index);
    }

    // O(n)
    public static void RemoveBySwap<T>(this List<T> list, Predicate<T> predicate)
    {
        int index = list.FindIndex(predicate);
        RemoveBySwap(list, index);
    }
}

This solution is friendly for memory traversal, so even if you need to find the index first it will be very fast.

Notes:

• Finding the index of an item must be O(n) since the list must be unsorted.
• Linked lists are slow on traversal, especially for large collections with long life spans.

List isn't an efficient data structure when it comes to removal. You would do better to use a double linked list (LinkedList) as removal simply requires reference updates in the adjacent entries.

I feel a HashSet, LinkedList or Dictionary will do you much better.