using System;

using System.Threading.Tasks;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var source = new List<int>{ 1, 2, 3, 4, 5 };

        var source2 = new List<int>();

        var tasks = source.Select(x => Task.Run(() => Console.WriteLine(x)));

        var tasks2 = source2.Select(x => Task.Run(() => Console.WriteLine(x)));

        var res = Extensions.Append<Task>(tasks, tasks2);

        Console.WriteLine("Total tasks:" + res.Count());

        Task.WaitAll(res.ToArray());

        Console.WriteLine("Done");

    }

    public static class Extensions

    {

        public static IEnumerable<T> Append<T>(IEnumerable<T> source, IEnumerable<T> items)

        {           

            source.ToList().AddRange(items);

            return source;

        }

    }

}