using System;

using System.Threading;

using System.Threading.Tasks;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var filters = new List<string>

        {

            "A", "B", "C"

        }

​

        ;

        var api = new API();

        var TaskList = new List<Task>();

        foreach (var f in filters)

        {

            var LastTask = new Task<List<String>>(() =>

            {

                return api.GetArray(f);

            }

​

            );

            LastTask.Start();

            TaskList.Add(LastTask);

        }

​

        Task.WaitAll(TaskList.ToArray());

        foreach (var t in TaskList)

        {

            // I need to get the List<string> returned in each GetArray and merge them

        }

    }

}

​

public class API

{

    public List<string> GetArray(string param)

    {

        Thread.Sleep(1000);

        return new List<String>{

            param + "1",

            param + "2",

            param + "3",

        };

    }

}