IFormatter.cs

  public interface IFormatter
  {
    string Format(Model model);
  }

Model.cs

  public class Model
  {
    readonly IEnumerable<string> _header;
    readonly IEnumerable<IEnumerable<string>> _rows;

    public Model(IEnumerable<string> header, IEnumerable<IEnumerable<string>> rows)
    {
      _header = header;
      _rows = rows;
    }

    public virtual IEnumerable<string> Header
    {
      get
      {
        return _header;
      }
    }

    public virtual IEnumerable<IEnumerable<string>> Rows
    {
      get
      {
        return _rows;
      }
    }
  }