using System;

using System.Collections.Generic;

using System.Threading;

using System.Threading.Tasks;

using System.Runtime.CompilerServices;

public static class Program

{

    // Was having issues with the NuGet package so wrote a custom Range function instead, at the bottom

    public static async Task Main()

    {

        var sequence_A = Range("A{0}", 1, 5);

        var sequence_B = Range("B{0}", 1, 3);

        var sequence_C = Range("C{0}", 1, 7);

        var merged = Interleave(sequence_A, sequence_B, sequence_C);

        await foreach (var item in merged) Console.WriteLine(item);

    }

    public static IAsyncEnumerable<T> Interleave<T>(params IAsyncEnumerable<T>[] sources) =>

        Interleave(default, sources);

    public static async IAsyncEnumerable<T> Interleave<T>([EnumeratorCancellation] CancellationToken token, IAsyncEnumerable<T>[] sources)

    {

        if(sources.Length == 0)

            yield break;

        var enumerators = new List<IAsyncEnumerator<T>>(sources.Length);

        try

        {

            for(var i = 0; i < sources.Length; i++)

                enumerators.Add(sources[i].GetAsyncEnumerator(token));

​

            var j = 0;

            do

            {

                if(await enumerators[j].MoveNextAsync())

                {

                    yield return enumerators[j].Current;

                    j++;

                    if(j >= enumerators.Count)

                        j = 0;

                }

                else

                {

                    try

                    {

                        await enumerators[j].DisposeAsync();

                    }

                    catch

                    { //

                    }

                    enumerators.RemoveAt(j);

                }

            } while (enumerators.Count > 0);

        }

        finally

        {

            for(var i = 0; i < enumerators.Count; i++)

            {

                try

                {

                    await enumerators[i].DisposeAsync();

                }

                catch

                { //

                }

            }

        }

    }

    static async IAsyncEnumerable<string> Range(string text, int start, int count) {

        for (int i = 0; i < count; i++)

            yield return string.Format(text, start + i);

    }

}