Separation of Concerns

Your UI layer shouldn’t contain business logic. Your business logic shouldn’t know how data is stored. Your data access layer shouldn’t format strings for display.

Compare:

public class TicTacToe
{
  private readonly char[,] _board = new char[3, 3];

  public void Play()
  {
    while (true)
    {
      // Display logic...
      for (var r = 0; r < 3; r++)
        Console.WriteLine($"{_board[r, 0]}{_board[r, 1]}{_board[r, 2]}");

      // Input handling...
      var row = int.Parse(Console.ReadLine() ?? "0");
      var col = int.Parse(Console.ReadLine() ?? "0");
      _board[row, col] = 'X';

      // Game rules...
      if (_board[0, 0] == _board[1, 1] && _board[1, 1] == _board[2, 2])
      {
        Console.WriteLine("Winner!");
        break;
      }
    }
  }
}

… to:

public class TicTacToe
{
  private readonly Board _board = new();
  private readonly Display _display = new();
  private readonly InputHandler _inputHandler = new();

  public void Play()
  {
    while (!_board.HasWinner())
    {
      display.Render(_board);
      var move = _inputHandler.GetNextMove();
      _board.MakeMove(move);
    }

    _display.ShowWinner(_board.GetWinner());
  }
}

… where the game rules, display, and input handling are separate, e.g., switching from a console input to a GUI only touches InputHandler.

Design Principles | Hello Interview Low Level Design. www.hellointerview.com . Accessed Sep 10, 2026.