Интерпретатор (interpreter)

using System;
using System.Collections.Generic;
using System.Text;
using System.Threading;
using System.Linq;
using System.Threading.Tasks;

class Program
{
   static void Main(string[] args)
   {
    Context context = new Context();
    int x = 5,
    y = 8,
    z = 2;

    context.SetVariable("x", 5);
    context.SetVariable("y", 8);
    context.SetVariable("z", 2);

    IExpression expression = new SubstractExpression(
        new AddExpression(
            new NumberExpression("x"), new NumberExpression("y")
        ),
        new NumberExpression("z")
    );
    int result = expression.Interpret(context);
    Console.WriteLine("результат {0}", result);

    Console.ReadLine();
   }
}

class Context
{
    Dictionary<string, int> variables;

    public Context()
    {
        variables = new Dictionary<string, int>();
    }

    public int GetVariables(string name)
    {
        return variables[name];
    }

    public void SetVariable(string name, int value)
    {
        if(variables.ContainsKey(name) == true)
            variables[name] = value;
        else variables.Add(name, value);
    }
}

interface IExpression
{
    int Interpret(Context context);
}

class NumberExpression : IExpression
{
    string name;

    public NumberExpression(string variableName)
    {
        name = variableName;
    }

    public int Interpret(Context context)
    {
        return context.GetVariables(name);
    }
}

class AddExpression : IExpression
{
    IExpression leftExpression;
    IExpression rightExpression;

    public AddExpression(IExpression left, IExpression right)
    {
        leftExpression = left;
        rightExpression = right;
    }

    public int Interpret(Context context)
    {
        return leftExpression.Interpret(context) + rightExpression.Interpret(context);
    }
}

class SubstractExpression : IExpression
{
    IExpression leftExpression;
    IExpression rightExpression;

    public SubstractExpression(IExpression left, IExpression right)
    {
        leftExpression = left;
        rightExpression = right;
    }

    public int Interpret(Context context)
    {
        return leftExpression.Interpret(context) - rightExpression.Interpret(context);
    }
}