Interpreter Design Pattern

Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language..

using System;
using System.Collections.Generic;

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

             // Build the 'parse tree'
           List<Expression> tree = new List<Expression>();

             tree.Add(new ThousandExpression());
             tree.Add(new HundredExpression());
             tree.Add(new TenExpression());
             tree.Add(new OneExpression());

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

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

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

    /// <summary>
    /// The 'Context' class
    /// </summary>
    class Context
    {
        // Gets or sets input
        public string Input { get; set; }

        // Gets or sets output
        public int Output { get; set; }
    }

    /// <summary>
    /// The 'AbstractExpression' abstract class
    /// </summary>
    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();
    }

    /// <summary>
    /// A 'TerminalExpression' class
    /// <remarks>
    /// Thousand checks for the Roman Numeral M
    /// </remarks>
    /// </summary>
    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; }
    }

    /// <summary>
    /// A 'TerminalExpression' class
    /// <remarks>
    /// Hundred checks C, CD, D or CM
    /// </remarks>
    /// </summary>
    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; }
    }

    /// <summary>
    /// A 'TerminalExpression' class
    /// <remarks>
    /// Ten checks for X, XL, L and XC
    /// </remarks>
    /// </summary>
    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; }
    }

    /// <summary>
    /// A 'TerminalExpression' class
    /// <remarks>
    /// One checks for I, II, III, IV, V, VI, VI, VII, VIII, IX
    /// </remarks>
    /// </summary>
    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; }
    }
}