Add.cs

// Adds the integers in a string like "1,2,3" = 6, 
// or "//;\n1;2;3" = 6
public int Add(string value)
{
  if (string.IsNullOrEmpty(value))
    return 0;
  
  var delimters = new char[] { ',', '\n' };
  if (value.StartsWith("//"))
  {
    delimters = value.Substring(2, 1).ToCharArray();
    value = value.Substring(4, value.Length - 4);
  }

  var values = value.Split(delimters);
  
  var sum = 0;
  foreach (var item in values)
  {
    sum += int.Parse(item);
  }
  
  return sum;
}