cardmarketbot/ConsoleApp3/UsedRepository.cs

using ConsoleApp3.Contracts;

namespace ConsoleApp3
{
    internal class UsedRepository : IUsedRepository
    {
        public List<string> Query => File
            .ReadLines("usedList.csv")
            .Select(l => l.Split(','))
            .Select(p => new String(p[0]))
            .ToList();

        public void insert(string key)
        {
            key += Environment.NewLine;
            File.AppendAllText("usedList.csv", key);
        }

        public void Insert(List<Kunde> list)
        {
            foreach(var kunde in list)
            {
                insert(kunde.BestellungID);
            }
        }
    }
}