using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var managers = new []{

            new{

                creationDate = new DateTime(2020, 1, 1),

                sponsor = "Kowalski",

                pm = "Jankowski"

            },

            new{

                creationDate = new DateTime(2020, 2, 1),

                sponsor = "Kowalski",

                pm = "Felek"

            },          

            new{

                creationDate = new DateTime(2020, 3, 1),

                sponsor = "Kowalski",

                pm = ""

            },

            new{

                creationDate = new DateTime(2020, 4, 1),

                sponsor = "Piki",

                pm = "Karo"

            }

        };

        // Step 1: fill dateTo

        var last = managers.Last();

        var managersWithDateTo = managers.Skip(2).Aggregate(

            new {

                list = new [] { new { sponsor = managers[0].sponsor, pm = managers[0].pm, dateFrom = managers[0].creationDate, dateTo =  managers[1].creationDate }}.ToList(),

                prev =  managers[1]          

            },

            (acc, curr) => {

                acc.list.Add(new {sponsor = acc.prev.sponsor, pm = acc.prev.pm, dateFrom = acc.prev.creationDate, dateTo = curr.creationDate });

                return new {

                    list = acc.list,

                    prev = curr 

                };

            }                   

        ).list.Concat(new [] { new {

            sponsor = last.sponsor,

            pm = last.pm, 

            dateFrom = last.creationDate,

            dateTo = DateTime.MinValue

        }});                

        // Step 2: split into roles

        var roles = managersWithDateTo.Select(x => new [] { 

            new {roleName = "sponsor", resourceName = x.sponsor, dateFrom = x.dateFrom, dateTo = x.dateTo}, 

            new {roleName = "pm", resourceName = x.pm, dateFrom = x.dateFrom, dateTo = x.dateTo }

        }).SelectMany( x => x ).Where(x => x.resourceName != String.Empty).OrderBy(x => x.roleName);

        // Step 3: merge where there was no change

        var rolesMerged = roles.Skip(1).Aggregate(          

            new {

                list = roles.Take(1).ToList(),

                prev = roles.First()        

            },

            (acc, curr) => {

                if (acc.prev.roleName ==  curr.roleName && acc.prev.resourceName == curr.resourceName) {

                    var x = acc.list.Last();

                    acc.list.RemoveAt(acc.list.Count() - 1);

                    acc.list.Add(new {

                        roleName = x.roleName,

                        resourceName = x.resourceName,

                        dateFrom = x.dateFrom,

                        dateTo = curr.dateTo

                    }); 

                } else {

                    acc.list.Add(curr); 

                }               

                return new {

                    list = acc.list,

                    prev = curr

                };              

            }

        ).list;

        foreach (var r in rolesMerged){         

            Console.WriteLine(r);

        }       

    }

}