using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var cart = new CartDto

        {

            CartItems = new List<CartItemDto>

            {

                new CartItemDto { CartItemDetails = new List<CartItemDetailDto> { new CartItemDetailDto(), new CartItemDetailDto() } },

                new CartItemDto { CartItemDetails = new List<CartItemDetailDto> { new CartItemDetailDto(), new CartItemDetailDto() } }

            }

        };

        Console.WriteLine(cart.CartItems.SelectMany(x => x.CartItemDetails).Count());

    }

}

​

public class CartDto

{

    public long CartId { get; set; }       

    public int FinalPrice { get; set; }

    public int CartItemDetailCount { get; set; }

    public List<CartItemDto> CartItems { get; set; }

}

​

  public class CartItemDto

{

    public long Id { get; set; }        

    public long PackageId { get; set; }

    public string PackageTitle { get; set; }  

      public List<CartItemDetailDto> CartItemDetails  { get; set; } 

}

​

public class CartItemDetailDto

{

}

​