Returns the gap in list of type int

GapList.cs

public static List<int> GetGaps(List<int> originalList)
{
    // Check if the list is empty
    if (originalList == null)
        return new List<int>();

    // Order the list
    originalList = originalList.OrderBy(o => o).ToList();

    // Get the first element
    var first = originalList.First();
    // Get the last element
    var last = originalList.Last();
    // Get the element count
    var count = last - first + 1;

    // Create a new range
    var rangeList = Enumerable.Range(first, count);

    // Get all entries which are in the range list but not in the original list
    var gapList = rangeList.Except(originalList).ToList();

    // Return the result
    return gapList;

    // Short variant
    // return Enumerable.Range(originalList.First(), originalList.Last() - originalList.First() + 1).Except(originalList).ToList();
}