namespace Q72428
{
    public class Product
    {
        public List<string> Barcodes { get; set; }
    }

    class Program
    {
        static void Main(string[] args)
        {
            IList<Product> products = new List<Product>()
            {
                new Product{ Barcodes = new List<string> { "a", "b" }},
                new Product{ Barcodes = new List<string> { "c", "d" }}
            };

            var barcodes = products.SelectMany(x => x.Barcodes);

            Console.WriteLine(string.Join(",", barcodes));
            Console.ReadKey();
        }
    }
}

a,b,c,d