How to Quickly Remove Items From a List

Question

I am looking for a way to quickly remove items from a C# List. The documentation states that the List.Remove() and List.RemoveAt()<

Answer 1

If you still want to use a List as an underlying structure, you can use the following extension method, which does the heavy lifting for you.

using System.Collections.Generic;
using System.Linq;

namespace Library.Extensions
{
    public static class ListExtensions
    {
        public static IEnumerable RemoveRange(this List list, IEnumerable range)
        {
            var removed = list.Intersect(range).ToArray();
            if (!removed.Any())
            {
                return Enumerable.Empty();
            }

            var remaining = list.Except(removed).ToArray();
            list.Clear();
            list.AddRange(remaining);

            return removed;
        }
    }
}

A simple stopwatch test gives results in about 200ms for removal. Keep in mind this is not a real benchmark usage.

public class Program
    {
        static void Main(string[] args)
        {
            var list = Enumerable
                .Range(0, 500_000)
                .Select(x => x.ToString())
                .ToList();

            var allFifthItems = list.Where((_, index) => index % 5 == 0).ToArray();

            var sw = Stopwatch.StartNew();
            list.RemoveRange(allFifthItems);
            sw.Stop();

            var message = $"{allFifthItems.Length} elements removed in {sw.Elapsed}";
            Console.WriteLine(message);
        }
    }

Output:

100000 elements removed in 00:00:00.2291337