This works for both positive and negative numbers:

private static bool isEven(int number)
{
	bool result = true;

	while (number < 0)
	{
		number = number - 1;
		if (result == true)
			result = false;
		else
			result = true;
	}
	while (number > 0)
	{
		number = number - 1;
		if (result == true)
			result = false;
		else
			result = true;
	}
	return result;
}

Output:

isEven(4) = True
isEven(5) = False
isEven(-4) = True
isEven(-5) = False