using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var xs = new List<string>() {"A","B","C"};

        var firstPart = xs.SkipLast(1);

        Console.WriteLine(string.Join(",", firstPart)); // A,B

    }

}

​

​