using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var ps = Program.GetDatas();

        var startDate = new DateTime(2023, 01, 07);

        var endDate = new DateTime(2023, 01, 14);

        ps.ApplyFilter(startDate, endDate).ToList()

            .ForEach(r => Console.WriteLine($"[{r.Id}] {r.StartDate:dd/MM/yyyy} - {r.EndDate:dd/MM/yyyy}"));

    }

    public static List<Period> GetDatas() {

        var list = new List<Period>();

        // - CORRECT PERIOD

        var id = 1;

        list.AddRange(new List<Period>()

        {

            new Period(id++, new DateTime(2023, 01, 07), new DateTime(2023, 01, 14)),   

            new Period(id++, new DateTime(2023, 01, 06), new DateTime(2023, 01, 14)),   

            new Period(id++, new DateTime(2023, 01, 08), new DateTime(2023, 01, 13)),

            new Period(id++, new DateTime(2023, 01, 01), null),                         

            new Period(id++, new DateTime(2020, 01, 01), null),                         

            new Period(id++, null, new DateTime(2024, 01, 31)),                     

            new Period(id++, null, null),

        });

        // - INCORRECT PERIOD

        id += 100;

        list.AddRange(new List<Period>()

        {

            new Period(id++, new DateTime(2023, 01, 01), new DateTime(2023, 01, 05)), 

            new Period(id++, new DateTime(2023, 01, 15), new DateTime(2023, 01, 20)),   

            new Period(id++, new DateTime(2023, 01, 21), new DateTime(2023, 01, 24)),

            new Period(id++, new DateTime(2023, 01, 25), new DateTime(2023, 01, 30)),   

            new Period(id++, new DateTime(2023, 01, 31), null), 

            new Period(id++, null, new DateTime(2022, 01, 31)),

        });

        return list;

    }

}

​

public static class EnumerableExtension {

    public static IEnumerable<Period> ApplyFilter(this IEnumerable<Period> periods, DateTime startDate, DateTime endDate) {

        //return periods.Where(o => !(startDate > o.EndDate || endDate < o.StartDate));

        // FOR EF

        return periods.Where(

                o =>

                    !(startDate > o.EndDate || endDate < o.StartDate) ||

                    (o.EndDate == null && o.StartDate <= endDate) ||

                    (o.StartDate == null && o.EndDate >= startDate)

            );

    }

}

​

​

public class Period {

    public Period(int id, DateTime? startDate, DateTime? endDate) {

        this.Id = id;

        this.StartDate = startDate;

        this.EndDate = endDate;

    }

    public int Id { get; set; }

    public DateTime? StartDate { get; set; }    

    public DateTime? EndDate { get; set; }  

}

​

​