Строковый калькулятор на С#

StringCalculator.cs

public class Calc
{
    // Универсальная операция
    abstract class Operation 
    {
        public abstract float Eval ();
    }

    // Просто число
    class Number : Operation 
    {    
        public Number (float f)         { value = f; }
        public override float Eval ()   { return value; }

        private float value;
    }

    // Один операнд
    abstract class Unary : Operation 
    {
        public Unary (Operation op) { one = op; }

        protected Operation one;
    }

    // Два операнда
    abstract class Binary : Operation 
    {
        public Binary (Operation l, Operation r) { left = l; right = r; }

        protected Operation left, right;
    }

    // Одинокий минус
    class Negation : Unary 
    {    
        public Negation (Operation n) : base (n) {}
        public override float Eval() { return - one.Eval(); }
    }

    // Сложение
    class Plus : Binary 
    {
        public Plus (Operation l, Operation r) :base (l, r) {}
        public override float Eval() { return left.Eval() + right.Eval(); }
    }

    // Вычитание
    class Minus : Binary 
    {
        public Minus (Operation l, Operation r) : base (l, r) {}
        public override float Eval() { return left.Eval() - right.Eval(); }
    }

    // Умножение
    class Multiply : Binary 
    {
        public Multiply (Operation l, Operation r) : base (l, r) {}
        public override float Eval() { return left.Eval() * right.Eval(); }        
    }

    // Деление
    class Divide : Binary 
    {
        public Divide (Operation l, Operation r) : base (l, r) {}
        public override float Eval()
        { 
            float right_eval = right.Eval ();
            if (right_eval == 0.0f)
                System.Console.WriteLine ("Devide by zero");
            return (right_eval != 0.0f) ? (left.Eval() / right_eval) : float.MaxValue;
        }    
    }

    class Expression
    {
        public Expression (string s) { source = s; }

        public float Calc ()
        {
            pos = 0;
            Operation root = Parse0 ();
            return (root != null) ? root.Eval () : 0.0f;
        }

        // Низкий приоритет: сложение, вычитание
        private Operation Parse0 ()
        {
            Operation result = Parse1 ();

            for (;;)
            {
                if      (Match ('+'))   result = new Plus  (result, Parse1 ());
                else if (Match ('-'))   result = new Minus (result, Parse1 ());
                else                    return result;
            }
        }

        // Средний приоритет: умножение, деление
        private Operation Parse1 ()
        {
            Operation result = Parse2 ();
            for (;;)
            {
                if      (Match ('*'))   result = new Multiply (result, Parse2 ());
                else if (Match ('/'))   result = new Divide   (result, Parse2 ());
                else                    return result;
            }
        }

        // Высокий приоритет: одинокий минус, скобки, число
        private Operation Parse2 ()
        {
            Operation result = null;

            if (Match ('-'))    
            {
                result = new Negation (Parse0 ());
            }
            else if (Match ('('))   
            {
                result = Parse0 ();
                if (! Match (')'))
                    System.Console.WriteLine ("Missing ')'");
            }
            else 
            {
                // распарсим число
                float val = 0.0f;
                int start = pos;
                while (pos < source.Length && (char.IsDigit (source[pos]) || source[pos] == '.' || source[pos] == 'e') ) ++pos;

                try { val = float.Parse (source.Substring (start, pos-start)); }
                catch { System.Console.WriteLine ("Can't parse '" + source.Substring (start) + "'"); }
                result = new Number (val);

            }
            return result;
        }

        // Поищем символ в строке
        private bool Match (char ch) 
        {
            if ( pos >= source.Length ) return false;
            while (source [pos] == ' ' ) ++pos;             // пропустим пробелы

            if (source [pos] == ch) { ++pos; return true; } // нашли что искали?
            else return false;
        }

        private string  source;     // исходная строчка
        private int     pos;        // текущая позиция
    }

    public static void Main ()
    {
        System.Console.WriteLine ("Enter expression:");
        string buf = System.Console.ReadLine ();
    
        Expression expr = new Expression (buf);
        System.Console.WriteLine ( expr.Calc().ToString ("g") );
    }
}