using System;

using System.Collections.Generic;

public static class Program

{

    public static void Main()

    {

    Run(new List<DateTime>(){

        new DateTime(2015, 1, 1), new DateTime(2016, 2, 1)

    }, new List<DateTime?>(){

        null, new DateTime(2016, 2, 13)

    });

    Run(new List<DateTime>(){

        new DateTime(2015, 1, 1), new DateTime(2016, 1, 1)

    }, new List<DateTime?>(){

        null, null

    });

    Run(new List<DateTime>(){

        new DateTime(2015, 1, 1), new DateTime(2016, 1, 1)

    }, new List<DateTime?>(){

        new DateTime(2015, 12, 31), null

    });

    Run(new List<DateTime>()

    {

        new DateTime(2015, 1, 1),

        new DateTime(2016, 1, 11)

    }, new List<DateTime?>()

    {

        new DateTime(2016, 1, 31),

        null

    });

    Run(new List<DateTime>()

    {

        new DateTime(2015, 1, 1),

        new DateTime(2016, 2, 1),

        new DateTime(2016, 3, 1),

        new DateTime(2016, 4, 1),

        new DateTime(2016, 1, 10),

        new DateTime(2016, 5, 1)

    }, new List<DateTime?>()

    {

        new DateTime(2016, 1, 31),

        new DateTime(2016, 2, 28),

        new DateTime(2016, 3, 31),

        new DateTime(2016, 4, 30),

        new DateTime(2016, 1, 15),

        null

    });

}

​

public static void Run(List<DateTime> effectiveStartDates, List<DateTime?> effectiveEndDates)

{

    var result = Validate(effectiveStartDates, effectiveEndDates);

    if(result == string.Empty) result = "No Error";

    Console.WriteLine(result);

}

​

public static string Validate(List<DateTime> effectiveStartDates, List<DateTime?> effectiveEndDates)

{

    var result = string.Empty;

    //I am not checking whether end Dates are greater than start date since it wasn't on the question.

    var infIndex = -1;

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

        if(infIndex == -1 && !effectiveEndDates[i].HasValue){

            infIndex = i;

        }

        else if(infIndex > -1 && !effectiveEndDates[i].HasValue){

            return "cannot have two infinite ranges";

        }

    }

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

        if(i!= infIndex && IsOverlap(effectiveStartDates[i], effectiveEndDates[i].Value, effectiveStartDates[infIndex], DateTime.MaxValue)){

            return "Overlap with infinite date range";  

        }

    }

    //Checking to make sure all effective dates have end date, except the last one.

    for(var i=1; i < effectiveEndDates.Count; i++){

        if(!effectiveEndDates[i - 1].HasValue){

            result = "cannont have a previous billing address without and end date.";

        }

    }

    //Ensure all the startDates Dates are on or after the next end Date

    //This solution will work if the effective dates are in order.

    for(var i=1; i < effectiveStartDates.Count; i++){

        if(effectiveEndDates[i - 1].HasValue && effectiveEndDates[i - 1].Value > effectiveStartDates[i]){

            result = string.Format("cannot have 2 active billing address. start:{0}, end:{1}", effectiveStartDates[i], effectiveEndDates[i-1].Value);

        }

    }

    //otherwise we have to check each effective date with all others

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

        if(effectiveEndDates.Count > i && effectiveEndDates[i].HasValue)

        {

            for(var j=0; j < effectiveStartDates.Count && i !=j; j++){

                if(effectiveEndDates.Count > j &&   effectiveEndDates[j].HasValue)

                {

                    if(IsOverlap(effectiveStartDates[i], effectiveEndDates[i].Value,

                        effectiveStartDates[j], effectiveEndDates[j].Value)){

                        return string.Format("Following ranges overlap {0} - {1} && {2} - {3}",effectiveStartDates[i], effectiveEndDates[i].Value,effectiveStartDates[j], effectiveEndDates[j].Value);

                    }

                }

            }

        }

    }

    return result;

}

​

public static bool IsOverlap(DateTime startDate, DateTime endDate, DateTime otherStartDate, DateTime otherEndDate)

{

    return (startDate > otherStartDate && startDate < otherEndDate) || 

    (endDate > otherStartDate && endDate < otherEndDate);

}

​

// Define other methods and classes here

​

}