Find DateTime Gaps

toolkit

using System;
using System.Collections.Generic;
using System.Linq;
 
class Toolkit
{
    public class DateTimeInterval
    {
        public DateTime BeginDate { get; set; }
        public DateTime EndDate { get; set; }
    }
 
    public static List<DateTimeInterval> FindGaps<T>(DateTimeInterval baseInterval, IEnumerable<T> list,
        Func<T, DateTime> beginDateKeySelector, Func<T, DateTime> endDateKeySelector)
    {
        List<DateTimeInterval> result = new List<DateTimeInterval>();
        List<T> tempList = list.ToList();
        T currentItem, nextItem;
 
        DateTime dateMin = baseInterval.BeginDate;
        DateTime dateMax = baseInterval.EndDate;
 
        if (dateMin >= dateMax)
            return result;
 
        if (tempList == null || tempList.Count() == 0)
        {
            result.Add(new DateTimeInterval() { BeginDate = dateMin, EndDate = dateMax });
            return result;
        }
 
        currentItem = tempList.OrderBy(o => beginDateKeySelector(o)).FirstOrDefault();
 
        tempList.RemoveAll(o => endDateKeySelector(o) <= endDateKeySelector(currentItem));
 
        if (beginDateKeySelector(currentItem) > dateMin)
        {
            result.Add(new DateTimeInterval()
                {
                    BeginDate = dateMin,
                    EndDate = beginDateKeySelector(currentItem)
                }
            );
        }
 
        while (true)
        {
            nextItem = tempList.
                Where(o => beginDateKeySelector(o) <= endDateKeySelector(currentItem)).
                OrderByDescending(o => endDateKeySelector(o)).FirstOrDefault();
            if (nextItem == null)
            {
                nextItem = tempList.OrderBy(o => beginDateKeySelector(o)).FirstOrDefault();
                if (nextItem == null)
                {
                    if (endDateKeySelector(currentItem) < dateMax)
                    {
                        result.Add(new DateTimeInterval()
                            {
                                BeginDate = endDateKeySelector(currentItem),
                                EndDate = dateMax
                            }
                        );
                    }
                    break;
                }
                else
                {
                    tempList.RemoveAll(o => endDateKeySelector(o) <= endDateKeySelector(nextItem));
                    result.Add(new DateTimeInterval()
                        {
                            BeginDate = endDateKeySelector(currentItem),
                            EndDate = beginDateKeySelector(nextItem)
                        }
                    );
                    currentItem = nextItem;
                }
            }
            else
            {
                tempList.RemoveAll(o => endDateKeySelector(o) <= endDateKeySelector(nextItem));
                currentItem = nextItem;
            }
        }
        return result;
    }
}

usage

using System;
using System.Collections.Generic;
 
class MyIntervalClass
{
    public int Id { get; set; }
    public string Name { get; set; }
    public DateTime Start { get; set; }
    public DateTime Finish { get; set; }
}
 
class IntervalRepo
{
    public static List<MyIntervalClass> GetIntervals()
    {
        List<MyIntervalClass> result = new List<MyIntervalClass>();
        // Add some MyIntervalClass instances to the list
        return result;
    }
}
 
class Program
{
    static void Main(string[] args)
    {
        var list = IntervalRepo.GetIntervals();
        var baseInterval = new Toolkit.DateTimeInterval() { BeginDate = DateTime.Today, EndDate = DateTime.Now };
 
        var gapList = Toolkit.FindGaps<MyIntervalClass>(baseInterval, list, o => o.Start, o => o.Finish);
    }
}