using System;

using System.Collections.Generic;

​

public class Program

{

​

    public static void Main(string[] args)

    {

        string[] values;

        string[] separators;

        values = new[] { "a", "b", "c" };

        separators = new[] { "1", "2" };

        Console.WriteLine(string.Join("", values.Interleave(separators)));

        values =  new[] { "a", "b" };

        separators = new[] { "1", "2", "3", "4", "5", "6" }; 

        Console.WriteLine(string.Join("", values.Interleave(separators)));

        values = new[] { "a", "b", "c", "d", "e", "f" };

        separators = new[] { "1", "2" };

        Console.WriteLine(string.Join("", values.Interleave(separators)));

    }

​

}

​

public static class EnumerableExtensions

{

​

    public static IEnumerable<TSource> Interleave<TSource>(this IEnumerable<TSource> source1, IEnumerable<TSource> source2)

    {

        if (source1 == null) { throw new ArgumentNullException(nameof(source1)); }

        if (source2 == null) { throw new ArgumentNullException(nameof(source2)); }

​

        using (var enumerator1 = source1.GetEnumerator())

        {

            using (var enumerator2 = source2.GetEnumerator())

            {

​

                var continue1 = true;

                var continue2 = true;

​

                do

                {

                    if (continue1 = enumerator1.MoveNext())

                    {

                        yield return enumerator1.Current;

                    }

​

                    if (continue2 = enumerator2.MoveNext())

                    {

                        yield return enumerator2.Current;

                    }

                }

                while (continue1 || continue2);

            }

        }

​

    }

​

}