using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<Employee> employees = new List<Employee>

            {

                new Employee { Name = "John", WorkDays = new List<WorkDay>

                {

                    new WorkDay { Date = new DateTime(2015,04,15), Hours = 7 },

                    new WorkDay { Date = new DateTime(2015,04,16), Hours = 8 }

                }},

                new Employee { Name = "Alex", WorkDays = new List<WorkDay>

                {

                    new WorkDay { Date = new DateTime(2015,04,12), Hours = 11 },

                    new WorkDay { Date = new DateTime(2015,04,15), Hours = 9 }

                }},

                new Employee { Name = "Mary", WorkDays = new List<WorkDay>

                {

                    new WorkDay { Date = new DateTime(2015,04,16), Hours = 9 },

                    new WorkDay { Date = new DateTime(2015,04,12), Hours = 10 }

                }}

            };

​

​

            var result = employees.SelectMany(x => x.WorkDays, (employeeObj, workDays) => new { employeeObj, workDays })

                                  .GroupBy(x => x.workDays.Date)

                                  .Select(x => new

                                      {

                                          Date = x.Key,

                                          NameAndHours = x.Select(z => new { Name = z.employeeObj.Name, Hours = z.workDays.Hours }).ToList()

                                      }).ToList();

​

            foreach (var item in result)

            {

                Console.WriteLine("Date: {0}", item.Date.ToShortDateString());

                foreach (var data in item.NameAndHours)

                {

                    Console.WriteLine("\tName: {0}, Hours: {1}", data.Name, data.Hours);

                }

                Console.WriteLine();

            }

    }

}

​

 public  class Employee

{

  public String Name { get; set; }

  public List<WorkDay> WorkDays { get; set; }

}

​

public class WorkDay

{

  public DateTime Date { get; set; }

  public int Hours { get; set; }

}