using System;

using System.Data;

using System.Collections.Generic;

​

public class Program

{

    public class Dates

    {

        public DateTime ActiveDate;

        public DateTime ExpireDate;

        public string Name;

    }

​

    public static void Main()

    {

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

        datesList.Add(new Dates{Name = "A", ActiveDate = new DateTime(2017, 8, 20), ExpireDate = new DateTime(2017, 8, 23)}); //Aug 20 - Aug 23

        datesList.Add(new Dates{Name = "B", ActiveDate = new DateTime(2017, 8, 20), ExpireDate = new DateTime(2017, 8, 22)}); //Aug 20 - Aug 22

        datesList.Add(new Dates{Name = "C", ActiveDate = new DateTime(2017, 8, 22, 23, 0, 0), ExpireDate = new DateTime(2017, 8, 26)}); //Aug 22 11pm - Aug 26

​

        datesList.Add(new Dates{Name = "D", ActiveDate = new DateTime(2017, 8, 20), ExpireDate = new DateTime(2017, 8, 23)}); //Aug 20 - Aug23

        int numOverlapped = 0;

        for (var i = 0; i < datesList.Count; i++)

        {

            var item = datesList[i];

            for (var j = i + 1; j < datesList.Count; j++)

            {

                bool isOverlap = Compare(item, datesList[j]);

                if (isOverlap)

                {

                    numOverlapped++;

                }

            }

        }

    }

​

    private static DateTime? overlapStart

    {

        get;

        set;

    }

​

    private static DateTime? overlapEnd

    {

        get;

        set;

    }

​

    private static bool Compare(Dates a, Dates b)

    {

        DateTime? tStartA = a.ActiveDate;

        DateTime? tEndA = a.ExpireDate;

        DateTime? tStartB = b.ActiveDate;

        DateTime? tEndB = b.ExpireDate;

        bool overlap = (tStartA <= tEndB || tEndB == null) && (tStartB <= tEndA || tEndA == null);

        DateTime? overlapStart = null;

        DateTime? overlapEnd = null;

        if (overlap)

        {

            //Find maximum start date. 

            overlapStart = (tStartA >= tStartB) ? tStartA : tStartB;

            //Find Min End date between the two

            overlapEnd = (tEndA <= tEndB) ? tEndA : tEndB;

            if (overlapStart > overlapStart || overlapStart == null)

            {

                overlapStart = overlapStart;

            }

​

            if (overlapEnd < overlapEnd || overlapEnd == null)

            {

                overlapEnd = overlapEnd;

            }

        }

​

        string overlapStartShortDateString = overlapStart.ToString();

        string overlapEndShortDateString = overlapEnd.ToString();

        if (overlap)

            Console.WriteLine(a.Name + " and " + b.Name + " Overlap from " + overlapStartShortDateString + " TO:  " + overlapEndShortDateString);

        else

        {

            Console.WriteLine(a.Name + " and " + b.Name + " DO NOT Overlap");

        }

​

        return overlap;

    }

}