We can avoid expensive branches (gasp) by using some bitwise arithmetic to achieve the so-called "absolute value", an advanced hacker technique I learnt at Blizzard. Also unlike c, c# is not enlightened enough to understand that my code is perfect so it complains about "not all code paths returning a value".

private bool IsEven(int number)
{
    number *= 1 - 2*(int)(((uint)number & 2147483648) >> 31);
    if (number > 1) return IsEven(number - 2);
    if (number == 0) return true;
    if (number == 1) return false;
    throw new Exception();
}