using System;

using System.Collections.Generic;

​

public static class Program

{

    public static void Main()

    {

        IList<int> items = new List<int>() { 1, 2, 3, 4, 5 };

        IEnumerable<int> itemsToDelete = new int[] { 2, 5 };

        Console.WriteLine($"Original: [{String.Join(", ", items)}]");

        Console.WriteLine($"ToDelete: [{String.Join(", ", itemsToDelete)}]");

        HashSet<int> itemsToDeleteSet = new(itemsToDelete);

        items.RemoveAll((x, _) => itemsToDeleteSet.Contains(x));

        Console.WriteLine($"After RemoveAll: [{String.Join(", ", items)}]");

    }

​

    /// <summary>

    /// Removes all the elements that match the conditions defined by the specified predicate.

    /// </summary>

    public static int RemoveAll<T>(this IList<T> list, Func<T, int, bool> predicate)

    {

        ArgumentNullException.ThrowIfNull(list);

        ArgumentNullException.ThrowIfNull(predicate);

​

        int i = 0, j = 0;

        try

        {

            for (; i < list.Count; i++)

            {

                if (predicate(list[i], i)) continue;

                if (j < i) list[j] = list[i];

                j++;

            }

        }

        finally

        {

            if (j < i)

            {

                for (; i < list.Count; i++, j++)

                    list[j] = list[i];

                while (list.Count > j)

                    list.RemoveAt(list.Count - 1);

            }

        }

        return i - j;

    }

}

​