Mod10 algo in C#

mod10.cs

void Main()
{
	var mod = new Mod10("600649179422170350");
	var digit = mod.CheckDigit; //2
	var activationNumber = mod.ActivationCardNumber; //6006491794221703502
}

public class Mod10
{
	private string _serialNumber;

	public Mod10(string serialNumber)
	{
		_serialNumber = serialNumber;
	}
	
	public string ActivationCardNumber
	{
		get
		{
			return string.Format("{0}{1}", _serialNumber, CheckDigit);
		}
	}
	
	public int CheckDigit
	{
		get
		{
			List<int> serialNumberArray = new List<int>();
			var modCheck = _serialNumber.Length % 2;
			var loopCounter = 0;
			while (loopCounter < _serialNumber.Length)
			{
				var digit = Convert.ToInt32(_serialNumber.Substring(loopCounter, 1));
				serialNumberArray.Add(digit);
				loopCounter++;
			}
			
			for (var i = 0; i < serialNumberArray.Count; i++)
			{
				var check = i + 1;
				if ((check % 2) == modCheck)
				{
					serialNumberArray[i] = serialNumberArray[i] * 2;
				}
			}
			
			loopCounter = 0;
			var additiveResult = 0;
			var arrayItemValue = 0;
			var checkSum = 0;
			
			while (loopCounter < _serialNumber.Length)
			{
				arrayItemValue = serialNumberArray[loopCounter];
				if (arrayItemValue > 9)
				{
					additiveResult = 1 + (arrayItemValue % 10);
				}
				else
				{
					additiveResult = arrayItemValue;
				}
				checkSum = checkSum + additiveResult;
				loopCounter++;
			}
			
			var checkDigit = 0;
			if ((checkSum % 10) > 0)
			{
				checkDigit = 10 - (checkSum % 10);
			}
			
			return checkDigit;
		}
	}
}