using System;

using System.Collections.Generic;

// see: https://stackoverflow.com/a/65127942/1016343

public class Program

{

    public static void Main()

    {

        var lst1 = new List<int>() { 1, 2, 1, 3, 4, 1 };

        Console.WriteLine("Match 1");

        lst1.FindEveryIndex(x => x == 1).Dump();

        Console.WriteLine("Match 2");

        lst1.FindEveryIndex(x => x == 2).Dump();

        Console.WriteLine("Match 9");

        lst1.FindEveryIndex(x => x == 9).Dump();

        var lst2 = new List<string>() { "A", "B", "A", "C", "D", "A"};

        Console.WriteLine("Match A");

        lst2.FindEveryIndex(x => x=="A").Dump();

        Console.WriteLine("Match B");

        lst2.FindEveryIndex(x => x=="B").Dump();

        Console.WriteLine("Match X");

        lst2.FindEveryIndex(x => x=="X").Dump();

    }

}

​

    public static class Exttension

    {

        public static IEnumerable<int> FindEveryIndex<T>(this IEnumerable<T> items, Predicate<T> predicate)

        {

            int index = 0; bool found = false;

            foreach (var item in items)

            {

                if (predicate(item))

                {

                    found = true; yield return index;

                };

                index++;

            }

            if (!found) yield return -1;

        }

    }