using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var t1 = new [] {"A", "B", "C"};

        var t2 = new [] {"1", "2", "3"};        

        IEnumerable<string> lst = null;

        lst = t1.SelectMany(x => t2, (x, y) => String.Concat(x, y));    // A1 A2 A3 B1 B2 B3 C1 C2 C3

        //lst = t.Zip(t, (x, y) => String.Concat(x, y));                // A1 B2 C3

        foreach(var s in lst)

            Console.Write(String.Format("{0} ", s));

    }

}