PalindromeNumber.cs

static void Main()
{
	var values = new[] { -12321, -123, -121, -12, -1, 0, 1, 12, 121, 123, 12321 };
	
	var query = values.Select(value => new { Value = value, IsPalindrome = IsPalindrome(value) });
	
	foreach(var item in query)
	{
		Console.WriteLine("{0}: {1}", item.Value, item.IsPalindrome);
	}
}

static bool IsPalindrome(int value)
{
	if (value < 0) value = -value;
	if (0 <= value && value < 10) return true;

	var array = ToSequence(value).ToArray();
	var length = array.Length;

	for(var i = 0; i < length / 2; i++)
	{
		if (array[i] != array[length - i - 1]) return false;
	}
	
	return true;
}

static IEnumerable<byte> ToSequence(int value)
{
	if (value < 0) throw new ArgumentOutOfRangeException("value");

	if (0 <= value && value < 10)
	{
		yield return (byte)value;
		yield break;
	}

	while (value > 0)
	{
		yield return (byte)(value % 10);
		value = value / 10;
	}
}