NodeBinary.cs

// 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;
    }

    Node _lhs;                              // Left hand side of the operation
    Node _rhs;                              // Right hand side of the operation
    Func<double, double, double> _op;       // The callback operator

    public override double Eval()
    {
        // Evaluate both sides
        var lhsVal = _lhs.Eval();
        var rhsVal = _rhs.Eval();

        // Evaluate and return
        var result = _op(lhsVal, rhsVal);
        return result;
    }
}