using System;

using System.Diagnostics;

using System.Threading.Tasks;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static async Task Main()

    {

        var stopwatch = Stopwatch.StartNew();

        await foreach (var pair in Zip(First(), Second()))

            Console.WriteLine(pair);

        Console.WriteLine($"Duration: {stopwatch.ElapsedMilliseconds:#,0} msec");

​

        static async IAsyncEnumerable<int> First()

        {

            for (int i = 1; i <= 5; i++) { await Task.Delay(100); yield return i; }

        }

        static async IAsyncEnumerable<int> Second()

        {

            for (int i = 1; i <= 5; i++) { await Task.Delay(100); yield return i; }

        }

    }

    public static async IAsyncEnumerable<(TFirst, TSecond)> Zip<TFirst, TSecond>(IAsyncEnumerable<TFirst> first, IAsyncEnumerable<TSecond> second)

    {

        await using var e1 = first.GetAsyncEnumerator();    

        await using var e2 = second.GetAsyncEnumerator();

        while (true)

        {

            var t1 = e1.MoveNextAsync().AsTask();

            var t2 = e2.MoveNextAsync().AsTask();

            await Task.WhenAll(t1, t2);

            if (!t1.Result || !t2.Result)

                yield break;

            yield return (e1.Current, e2.Current);

        }

    }

}