using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var projectList = new List<Project>

        {

            new Project{Name = "Object 1", BeginDate = new DateTime(2017,12,21), EndDate = new DateTime(2018,01,20)},

            new Project{Name = "Object 2", BeginDate = new DateTime(2017,12,01), EndDate = new DateTime(2017,12,21)},

            new Project{Name = "Object 3", BeginDate = new DateTime(2017,10,25), EndDate = new DateTime(2017,12,01)},

            new Project{Name = "Object 4", BeginDate = new DateTime(2017,09,17), EndDate = new DateTime(2017,10,25)},

            new Project{Name = "Object 5", BeginDate = new DateTime(2017,08,01), EndDate = new DateTime(2017,09,02)},

            new Project{Name = "Object 6", BeginDate = new DateTime(2017,06,25), EndDate = new DateTime(2017,07,26)},

            new Project{Name = "Object 7", BeginDate = new DateTime(2017,04,20), EndDate = new DateTime(2017,06,25)},

        };

        var newList = new List<Project>();

        while(projectList.Count > 0)

        {

            var item = projectList.ElementAt(0);

            projectList.Remove(item);

            newList.Add(item);

​

            //Console.WriteLine(item);

            var match = Match(item, projectList);

            while (match != null)

            {

                //Console.WriteLine("match: " + match.ToString());

                projectList.Remove(match);

                item.BeginDate = item.BeginDate < match.BeginDate ? item.BeginDate : match.BeginDate;

                item.EndDate = item.EndDate > match.EndDate ? item.EndDate : match.EndDate;

                item.IsLongTerm = true;

                //Console.WriteLine("adjusted: " + item.ToString());

                match = Match(item, projectList);

            }

        }

        foreach(var project in newList)

        {

            Console.WriteLine(project.ToString());

        }

    }

    private static Project Match(Project project, IEnumerable<Project> projects)

    {

        var result = projects.FirstOrDefault(p => 

            (project.BeginDate.AddDays(-1) < p.BeginDate && p.BeginDate < project.EndDate.AddDays(1) )

            || (project.BeginDate.AddDays(-1) < p.EndDate && p.EndDate < project.EndDate.AddDays(1)) );

        return result;

    }

}

​

public class Project

{

    public string Name { get; set; }

    public DateTime BeginDate { get; set; }

    public DateTime EndDate { get; set; }

    public bool IsLongTerm { get; set; }

    public override string ToString()

    {

        return Name + " " + BeginDate.ToString("yyyy-MM-dd") + " " + EndDate.ToString("yyyy-MM-dd");

    }

}

​

​

​