using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        A();

        B();

    }

    public static void A()

    {

        var myList = new List<string>{ "AB", "CD", "EF", "GK" };

        var result = from a in myList from b in myList where a != b select a+b;

        var resultStr = string.Join(",", result);

        Console.WriteLine(resultStr);

    }

    public static void B()

    {

        var myList = new List<string> { "AB", "CD", "EF", "GK" };

        var result = myList.SelectMany(a => myList.Where(b => b != a).Select(b => a + b));

        var resultStr = string.Join(",", result);

        Console.WriteLine(resultStr);

    }

}