LignAlgorithmArithmeticApproach.cs

public long CalculateChecksum(long value)
{
	long sum = 0;
	for (int i = 0; value != 0; i++)
	{
		for (var j = (value % 10) * (2 - i % 2); j != 0; j /= 10)
		{
			sum += j % 10;
		}
		value /= 10;
	}
 
	return (10 - sum % 10) % 10;
}

LuhnAlgorithmLinqFlavour.cs

public long CalculateChecksum(long value)
{
	var sum = value.ToString()
		.Reverse()
		.Select((c, i) => int.Parse(c.ToString()) * (2 - i % 2))
		.Select(i => i.ToString().Sum(c => c - '0'))
		.Sum();
 
	return (10 - sum % 10) % 10;
}