18.cs

using System;
using System.Collections;

public abstract class Expression {
  public abstract double Evaluate(Hashtable vars); 
}

public class Constant: Expression {
  double value;
  public Constant(double value) { 
    this.value = value;
  }
  public override double Evaluate(Hashtable vars) { 
    return value;
  } 
}

public class VariableReference: Expression{
  string name;
  public VariableReference(string name) {
    this.name = name; 
  }
  public override double Evaluate(Hashtable vars) { 
    object value = vars[name];
    if (value == null) {
      throw new Exception("Unknown variable: " + name); }
    return Convert.ToDouble(value); 
  }
}

public class Operation: Expression {
  Expression left; 
  char op; 
  Expression right;

  public Operation(Expression left, char op, Expression right) { 
    this.left = left;
    this.op = op;
    this.right = right;
  }

  public override double Evaluate(Hashtable vars) { 
    double x = left.Evaluate(vars);
    double y = right.Evaluate(vars);
    switch (op) {
      case '+': return x + y; 
      case '-': return x - y; 
      case '*': return x * y; 
      case '/': return x / y;
    }
    throw new Exception("Unknown operator"); 
  }
}

class Test {
  static void Main() {
    Expression e = new Operation( 
        new VariableReference("x"), 
        '*', 
        new Operation(       
          new VariableReference("y"), 
          '+',
          new Constant(2)
        ) 
    );
    Hashtable vars = new Hashtable();
    vars["x"] = 3;
    vars["y"] = 5; 
    Console.WriteLine(e.Evaluate(vars));
    vars["x"] = 1.5;
    vars["y"] = 9; 
    Console.WriteLine(e.Evaluate(vars));
  }
}