public class Post
{
    public int Id { get; set; }
    public int Subject { get; set; }
    public int Text { get; set; }
    public DateTime Date { get; set; }
}

string[] columnsNeeded = {"Subject", "Text"};

var list = _service.GetAllPosts();

var filteredList = list.Select(..)

//list = [{Subject="1", Text="1"}, {Subject="2", Text="2"},{Subject="3", Text="3"}...]