public class InvertibleList<T> : List<T>
  {
    public new T this[int index]
    {
      get
      {
        if (index >= 0) return base[index];
        if (Count + index < 0)
          throw new IndexOutOfRangeException();
        return this[Count + index];
      }
      set
      {
        if (index >= 0)
          base[index] = value;
        else
        {
          if (Count + index < 0) 
            throw new IndexOutOfRangeException();
          this[Count + index] = value;
        }
      }
    }
    
  }

InvertibleList<string> list=new InvertibleList<string>
      {
        "1",
        "2",
        "3",
        "4",
        "5",
      };

      list[-2] = "asd";
      list.ForEach(Console.WriteLine);