using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var t = new List<string>(){"a1", "a2", "a3"};

        var t1 = Foo(t);

        Console.WriteLine(t1); // a1, a2, a3

    }

    // like join

    public static string Foo(List<string> input)

    {

    }

}