using System;

using System.Collections.Generic;

using System.Linq;

using System.Threading.Tasks;

​

public class Program

{

    public static async Task Main()

    {

        await foreach (var i in FibonacciAsync(4).Take(20))

        {

            Console.WriteLine(i);

        }

    }

​

    private static IAsyncEnumerable<int> FibonacciAsync(int batchSize = 100)

    {

        return FibonacciAsyncBatches(batchSize).SelectMany(batch => batch.ToAsyncEnumerable());

    }

    private static async IAsyncEnumerable<IEnumerable<int>> FibonacciAsyncBatches(int batchSize = 100)

    {

        var fib = Fibonacci();

        int skip = 0;

        // Generate in batches of 100 

        while (true) 

        {

            Console.Out.WriteLine("Fetching batch {0} to {1}", skip, skip + batchSize);

            await Task.Delay(100);

            yield return fib.Skip(skip).Take(batchSize);

            skip = (skip + batchSize);

        }

    }

    private static IEnumerable<int> Fibonacci()

    {

        int current = 1, next = 1;

​

        while (true) 

        {

            yield return current;

            next = current + (current = next);

        }

    }

}

​