Interpreter

// Interpreter pattern -- Real World example

using System;
using System.Collections;

namespace DoFactory.GangOfFour.Interpreter.RealWorld
{

  // MainApp test application

  class MainApp
  {
    static void Main()
    {
      string roman = "MCMXXVIII";
      Context context = new Context(roman);

      // Build the 'parse tree'
      ArrayList tree = new ArrayList();
      tree.Add(new ThousandExpression());
      tree.Add(new HundredExpression());
      tree.Add(new TenExpression());
      tree.Add(new OneExpression());

      // Interpret
      foreach (Expression exp in tree)
      {
        exp.Interpret(context);
      }

      Console.WriteLine("{0} = {1}",
        roman, context.Output);

      // Wait for user
      Console.Read();
    }
  }

  // "Context"

  class Context
  {
    private string input;
    private int output;

    // Constructor
    public Context(string input)
    {
      this.input = input;
    }

    // Properties
    public string Input
    {
      get{ return input; }
      set{ input = value; }
    }

    public int Output
    {
      get{ return output; }
      set{ output = value; }
    }
  }

  // "AbstractExpression"

  abstract class Expression
  {
    public void Interpret(Context context)
    {
      if (context.Input.Length == 0)
        return;

      if (context.Input.StartsWith(Nine()))
      {
        context.Output += (9 * Multiplier());
        context.Input = context.Input.Substring(2);
      }
      else if (context.Input.StartsWith(Four()))
      {
        context.Output += (4 * Multiplier());
        context.Input = context.Input.Substring(2);
      }
      else if (context.Input.StartsWith(Five()))
      {
        context.Output += (5 * Multiplier());
        context.Input = context.Input.Substring(1);
      }

      while (context.Input.StartsWith(One()))
      {
        context.Output += (1 * Multiplier());
        context.Input = context.Input.Substring(1);
      }
    }

    public abstract string One();
    public abstract string Four();
    public abstract string Five();
    public abstract string Nine();
    public abstract int Multiplier();
  }

  // Thousand checks for the Roman Numeral M
  // "TerminalExpression"

  class ThousandExpression : Expression
  {
    public override string One() { return "M"; }
    public override string Four(){ return " "; }
    public override string Five(){ return " "; }
    public override string Nine(){ return " "; }
    public override int Multiplier() { return 1000; }
  }

  // Hundred checks C, CD, D or CM
  // "TerminalExpression"

  class HundredExpression : Expression
  {
    public override string One() { return "C"; }
    public override string Four(){ return "CD"; }
    public override string Five(){ return "D"; }
    public override string Nine(){ return "CM"; }
    public override int Multiplier() { return 100; }
  }

  // Ten checks for X, XL, L and XC
  // "TerminalExpression"

  class TenExpression : Expression
  {
    public override string One() { return "X"; }
    public override string Four(){ return "XL"; }
    public override string Five(){ return "L"; }
    public override string Nine(){ return "XC"; }
    public override int Multiplier() { return 10; }
  }

  // One checks for I, II, III, IV, V, VI, VI, VII, VIII, IX
  // "TerminalExpression"

  class OneExpression : Expression
  {
    public override string One() { return "I"; }
    public override string Four(){ return "IV"; }
    public override string Five(){ return "V"; }
    public override string Nine(){ return "IX"; }
    public override int Multiplier() { return 1; }
  }
}