using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var items = new List<Item>

        {

                new Item {id = 1, StartDate = DateTime.Now, EndDate = DateTime.Now.AddDays(2)},

                new Item {id = 2, StartDate = DateTime.Now, EndDate = DateTime.Now.AddMinutes(120)}

        };

        var results = items.SelectMany(i => 

           Enumerable.Range(0, 1 + i.EndDate.Subtract(i.StartDate).Days)

          .Select(offset => new { id = i.id, date = i.StartDate.AddDays(offset).Date})

          .ToArray());

        foreach (var result in results)

        {

            Console.WriteLine(result);

        }

    }

}

​

public class Item

{

    public int id;

    public DateTime StartDate { get; set; }

    public DateTime EndDate { get; set; }

}