using System;

using System.Linq;

using System.Data;

using System.Collections.Generic;

​

public class Program

{

    public class ConsecutiveDates

    {

        public DateTime StartDate;

        public DateTime EndDate;

        public int userId;

    }

​

    public static void Main()

    {

        // original code.

        List<ConsecutiveDates> datesList = new List<ConsecutiveDates>();

        datesList.Add(new ConsecutiveDates{userId = 0, StartDate = new DateTime(2022, 8, 20), EndDate = new DateTime(2022, 8, 23)});

        datesList.Add(new ConsecutiveDates{userId = 1, StartDate = new DateTime(2022, 8, 20), EndDate = new DateTime(2022, 8, 22)}); 

        datesList.Add(new ConsecutiveDates{userId = 2, StartDate = new DateTime(2022, 8, 20), EndDate = new DateTime(2022, 8, 26)}); 

        datesList.Add(new ConsecutiveDates{userId = 3, StartDate = new DateTime(2022, 8, 20), EndDate = new DateTime(2022, 8, 23)});

        // extrapolate the actual arrays of days that belong to each "Dates" object

        var compareRanges = datesList.Select(d => GetDateRange(d)).ToList();

        // do a bit of LINQ here...

        var result = CompareDates(compareRanges, 1);

        // output the results.

        Console.WriteLine("Overlaps");

        foreach(var dt in result)

        {

            Console.WriteLine(dt.ToString("M/d/yyyy"));

        }

        var maxOverlap = GetLongestOverlap(compareRanges);

​

        Console.WriteLine();

        Console.WriteLine("Most overlaps: " + maxOverlap.Item1.Count());

        foreach(var dt in maxOverlap.Item1)

        {

            Console.WriteLine(dt.ToString("M/d/yyyy"));

        }

    }

​

    // extrapolate a range of days based on the Dates object (evaluate start and end)

    private static DateTime[] GetDateRange(ConsecutiveDates dt)

    {

        // probably should put in some guard code here to validate the start/end dates!

        var start = dt.StartDate.Date;

        var end = dt.EndDate.Date;

        // we dont care about whether the date time is 11pm or other; we simply want to know if the day portions overlap

        // so let's build up a range of dates that let us do this.

        return Enumerable.Range(0, Convert.ToInt32((end.Date - start.Date).TotalDays) + 1)

            .Select(e => start.Date.AddDays(e)).ToArray();

    }

    // set the threshold to filter the overlapped dates...

    private static List<DateTime> CompareDates(List<DateTime[]> compareRanges, int overlapThreshold = 1)

    {

        // merge all the days into a grouped listing

        var grouped = compareRanges.SelectMany(r => r).GroupBy(d => d.Date);

        // filter the grouped list based on the overlap threshold, and pick out the appropriate dates

        var thresholdMatch = grouped.Where(g => g.Count() >= overlapThreshold)

            .Select(g => g.Key)

            .OrderBy(d => d)

            .ToList();

        return thresholdMatch;

    }   

    // which date in the ranges list has the most overlaps

    private static Tuple<List<DateTime>, int> GetLongestOverlap(List<DateTime[]> compareRanges)

    {

        var grouped = compareRanges.SelectMany(r => r).GroupBy(d => d.Date);

        var max = grouped.Max(g => g.Count());

        var maxMatch = grouped.Where(g => g.Count() == max)

            .Select(g => g.Key)

            .OrderBy(d => d)

            .ToList();

        return new Tuple<List<DateTime>, int>(maxMatch, max);

    }

}