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// No parens
Assert.AreEqual(Parser.Parse("10 + 20 * 30").Eval(), 610);

// Parens
Assert.AreEqual(Parser.Parse("(10 + 20) * 30").Eval(), 900);

// Parens and negative
Assert.AreEqual(Parser.Parse("-(10 + 20) * 30").Eval(), -900);

// Nested

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// Parenthesis?
if (_tokenizer.Token == Token.OpenParens)
{
    // Skip '('
    _tokenizer.NextToken();

    // Parse a top-level expression
    var node = ParseAddSubtract();

    // Check and skip ')'

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// Negative
Assert.AreEqual(Parser.Parse("-10").Eval(), -10);

// Positive
Assert.AreEqual(Parser.Parse("+10").Eval(), 10);

// Negative of a negative
Assert.AreEqual(Parser.Parse("--10").Eval(), 10);

// Woah!

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// Parse a unary operator (eg: negative/positive)
Node ParseUnary()
{
    // Positive operator is a no-op so just skip it
    if (_tokenizer.Token == Token.Add)
    {
        // Skip
        _tokenizer.NextToken();
        return ParseUnary();
    }

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[TestMethod]
public void AddSubtractTest()
{
    // Add 
    Assert.AreEqual(Parser.Parse("10 + 20").Eval(), 30);

    // Subtract 
    Assert.AreEqual(Parser.Parse("10 - 20").Eval(), -10);

    // Sequence

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public class Parser
{
    // Constructor - just store the tokenizer
    public Parser(Tokenizer tokenizer)
    {
        _tokenizer = tokenizer;
    }

    Tokenizer _tokenizer;

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// Construct an expression tree
var expr = new NodeBinary(
  new NodeNumber(10),
  new NodeNumber(20),
  (a,b) => a + b
);

// Evaluate it
var result = expr.Eval();

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// NodeBinary for binary operations such as Add, Subtract etc...
class NodeBinary : Node
{
    // Constructor accepts the two nodes to be operated on and function
    // that performs the actual operation
    public NodeBinary(Node lhs, Node rhs, Func<double, double, double> op)
    {
        _lhs = lhs;
        _rhs = rhs;
        _op = op;

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public abstract class Node
{
    public abstract double Eval();
}

class NodeNumber : Node
{
    public NodeNumber(double number)
    {
        _number = number;

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var testString = "10 + 20 - 30.123";
var t = new Tokenizer(new StringReader(testString));

// "10"
Assert.AreEqual(t.Token, Token.Number);
Assert.AreEqual(t.Number, 10);
t.NextToken();

// "+"
Assert.AreEqual(t.Token, Token.Add);