CC C# calculator

This is my basic C# calculator; I'm sure there is much that can be improved. This design is based on Bjarne Stroustrup's C++ calculator, a purposely be-bugged version of which can be found at his website.

enum Operators
{
    Addition, Subtraction, Multiplication, Division, Modulo
}

static class ExtensionMethods
{
    public static string RemoveAll(this string str, char toRemove)
    {
        for (int i = 0; i < str.Length; i++)
        {
            if (str[i] == toRemove)
            {
                str = str.Remove(i, 1);
                i--;
            }
        }
        return str;
    }
}

class Calculator
{
    private static StreamReader dataStream;

    private static Operators getNextOperator()
    {
        const string OperatorSet = "+-*/%";

        char token = (char)dataStream.Read();

        if (OperatorSet.IndexOf(token) == -1)
        {
            throw new InvalidDataException("Invalid or missing operator.");
        }
        return (Operators)OperatorSet.IndexOf(token);
    }

    private static double getNextNum()
    {
        string value = string.Empty;

        while (true)
        {
            if ("0123456789.".IndexOf((char)dataStream.Peek()) == -1)
            {
                break;
            }

            value += (char)dataStream.Read();
        }

        double d;
        if (double.TryParse(value, out d))
        {
            return d;
        }
        Console.WriteLine(value);
        throw new InvalidDataException("Expected a number.");
    }

    private static double NumbersAndParens()
    {
        bool valueIsNegative = false;
        double val = -1;    // trick the compiler

        if ((char)dataStream.Peek() == '-')
        {
            valueIsNegative = true;
            dataStream.Read();
        }

        if ((char)dataStream.Peek() == '(')
        {
            dataStream.Read();
            val = AdditiveOperators();

            if ((char)dataStream.Peek() == ')')
            {
                dataStream.Read();
                return valueIsNegative ? -1 * val : val;
            }
            else
            {
                throw new InvalidDataException("Expected closing parenthesis.");
            }
        }

        val = getNextNum();

        return valueIsNegative ? -1 * val : val;
    }

    private static double MultiplicativeOperators()
    {
        double val = NumbersAndParens();

        while (true)
        {
            if ("*/%".IndexOf((char)dataStream.Peek()) == -1)
            {
                return val;
            }

            Operators op = getNextOperator();

            if (op == Operators.Multiplication)
            {
                val *= NumbersAndParens();
            }
            else if (op == Operators.Division)
            {
                double nextNum = NumbersAndParens();
                if (nextNum == 0)
                {
                    throw new DivideByZeroException("Divide by 0 error.");
                }
                val /= nextNum;
            }
            else
            {
                val %= NumbersAndParens();
            }
        }
    }

    private static double AdditiveOperators()
    {
        double val = MultiplicativeOperators();

        while (true)
        {
            if ("-+".IndexOf((char)dataStream.Peek()) == -1)
            {
                return val;
            }
            Operators op = getNextOperator();

            if (op == Operators.Addition)
            {
                val += MultiplicativeOperators();
            }
            else if (op == Operators.Subtraction)
            {
                val -= MultiplicativeOperators();
            }
        }
    }

    private static Stream StringToStream(string equation)
    {
        MemoryStream stream = new MemoryStream();
        StreamWriter writer = new StreamWriter(stream);

        writer.Write(equation.RemoveAll(' '));
        writer.Flush();

        stream.Position = 0;
        return stream;
    }

    /// <summary>
    /// 
    /// </summary>
    /// <param name="equation">A mathematical epxression needing evalation.</param>
    /// <returns>The evaluated expression.</returns>
    public static double EvaluateExpression(string equation)
    {
        dataStream = new StreamReader(StringToStream(equation));

        double evaluation = AdditiveOperators();

        if (!dataStream.EndOfStream)
        {
            throw new InvalidDataException("Invalid expression.");
        }

        return evaluation;
    }
}