using System;

using System.Collections.Generic;

using System.Linq;

using System.Threading.Tasks;

​

namespace ConsoleApp1

{

    class Program

    {

        static async Task Main(string[] args)

        {

            var numbers = new int[] { 1, 2, 3 };

            var numberStrings = GetNumberStringsAsync(numbers);

​

            await foreach (var numberString in numberStrings)

            {

                Console.WriteLine($"{numberString.Item1} = {numberString.Item2}");

            }

        }

​

        static async IAsyncEnumerable<(int, string)> GetNumberStringsAsync(int[] numbers)

        {

            var tasks = numbers.Select(GetNumberStringAsync).ToList();

​

            while (tasks.Any())

            {

                var completedTask = await Task.WhenAny(tasks);

                tasks.Remove(completedTask);

​

                yield return completedTask.Result;

            }

        }

​

        private static async Task<(int, string)> GetNumberStringAsync(int number)

        {

            var result = await NumberToStringAsync(number);

            return (number, result);

        }

​

        private static async Task<string> NumberToStringAsync(int number)

        {

            await Task.Delay(100);

            return number.ToString();

        }

    }

}

​