using System;

using System.Collections.Generic;

using System.Linq;

​

​

public class Program

{

    public static void Main()

    {

        var Db = new List<Checkout>()

        {

            new Checkout { 

                IsCheckoutComplete = true, 

                Order = new Order

                {

                    OrderDate = new DateTime(10, 10, 10), 

                    TotalPrice = 100m

                }

            },

            new Checkout { 

                IsCheckoutComplete = true, 

                Order = new Order

                {

                    OrderDate = new DateTime(11, 11, 11), 

                    TotalPrice = 111m

                }

            },

            new Checkout { 

                IsCheckoutComplete = true, 

                Order = new Order

                {

                    OrderDate = new DateTime(11, 11, 11), 

                    TotalPrice = 100m

                }

            },

            new Checkout { 

                IsCheckoutComplete = true, 

                Order = new Order

                {

                    OrderDate = new DateTime(10, 10, 10), 

                    TotalPrice = 122m

                }

            }

        };

        var list = Db.Where(x => x.IsCheckoutComplete)

            .GroupBy(x => new { OrderDate = x.Order.OrderDate.Date})

            .Select(a => new { OrderDate = a.Key.OrderDate, TotalPrice = a.Sum(b => b.Order.TotalPrice)})

            .OrderBy(c => c.OrderDate)

            .ToList();

        foreach(var item in list)

        {

            Console.WriteLine("{0} -- {1}", item.OrderDate, item.TotalPrice);

        }

    }

​

}

​

public class Checkout

{

    public bool IsCheckoutComplete {get;set;}

    public Order Order {get;set;}

}

​

public class Order

{

    public DateTime OrderDate {get;set;}

    public decimal TotalPrice {get;set;}

}