using System;

using System.Collections.Generic;

using System.Linq;

​

public class Contract

{

    public DateTime? StartEffectiveDate

    {

        get;

        set;

    }

​

    public DateTime? EndEffectiveDate

    {

        get;

        set;

    }

​

    public DateTime? EndDate

    {

        get;

        set;

    }

}

​

public class Program

{

    public static void Main()

    {

        var contracts = new List<Contract>();

        var contractA = new Contract()

        {

            StartEffectiveDate = new DateTime(2021, 03, 20), 

            EndEffectiveDate = new DateTime(2021, 04, 8), 

        };

        contracts.Add(contractA);

        var contractB = new Contract()

        {

            StartEffectiveDate = new DateTime(2021, 03, 22), 

            EndEffectiveDate = new DateTime(2021, 04, 12), 

            EndDate = new DateTime(2021, 04, 10), 

        };

        contracts.Add(contractB);

        var contractC = new Contract()

        {

            StartEffectiveDate = new DateTime(2021, 03, 25), 

            EndEffectiveDate = new DateTime(2021, 04, 25), 

        };

        contracts.Add(contractC);

        var contractD = new Contract()

        {

            StartEffectiveDate = new DateTime(2021, 03, 10), 

        };

        contracts.Add(contractD);

        List<DateTime?> dates = new List<DateTime?>();

        if (contracts.Any())

        {

            dates.AddRange(contracts.Where(m => m.StartEffectiveDate.HasValue).Select(m => m.StartEffectiveDate));

            foreach (var m in contracts) 

            {

                DateTime? date = null;

                if (m.EndDate.HasValue && m.EndEffectiveDate.HasValue) date = m.EndDate > m.EndEffectiveDate ? m.EndEffectiveDate : m.EndDate;

                else if (m.EndDate.HasValue) date = m.EndDate;

                else date = m.EndEffectiveDate;

                dates.Add(date);

            }

        }

​

        Console.WriteLine("Dates: " + string.Join(" | ", dates.Select(m => m.ToString())));

        Console.WriteLine();

        DateTime startEffectiveDate = dates.Where(m => m.HasValue).Select(m => m.Value).Min();

        DateTime? endEffectiveDate = dates.Any(m => !m.HasValue) ? null : dates.Where(m => m.HasValue).Max();

        Console.WriteLine("Start Effective Date: " + startEffectiveDate);

        Console.WriteLine("End Effective Date: " + endEffectiveDate);

    }

}