using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var ords = new List<order>();

        ords.Add(new order{guid = "aaa",parent="",ProductID="100",Description="Burger",Quantity=1,Total=2.00});

        ords.Add(new order{guid = "bbb",parent="aaa",ProductID="800",Description="+ Bacon",Quantity=1,Total=0.70});

        ords.Add(new order{guid = "ccc",parent="",ProductID="101",Description="Fries",Quantity=1,Total=1.80});

        ords.Add(new order{guid = "ddd",parent="",ProductID="101",Description="Fries",Quantity=1,Total=1.80});

        ords.Add(new order{guid = "eee",parent="",ProductID="101",Description="Fries",Quantity=1,Total=1.80});

        ords.Add(new order{guid = "fff",parent="",ProductID="102",Description=" Large Fries",Quantity=1,Total=2.80});

        ords.Add(new order{guid = "ggg",parent="",ProductID="100",Description="Burger",Quantity=1,Total=2.00});

        ords.Add(new order{guid = "hhh",parent="ggg",ProductID="800",Description="+ Bacon",Quantity=1,Total=0.70});     

        ords.Add(new order{guid = "iii",parent="",ProductID="102",Description=" Large Fries",Quantity=1,Total=2.80});

        var lst = ords.GroupBy(x => new

{

    x.ProductID,

    GuidIfHaveChildren = (ords.Any(x2 => x2.parent == x.guid) ? x.guid : ""),

    x.parent

}).Select(x => new

 {

     guid = x.First().guid,

     parent = x.First().parent,

     ProductID = x.Key.ProductID,

     Desc = x.First().Description,

     Quantity = x.Sum(p => p.Quantity),

     Total = x.Sum(p => p.Total)

 });

        foreach(var ord in lst)

        { 

            Console.WriteLine(ord.ProductID+"     "+ord.Quantity+"        "+ord.Total);

        }

    }

}

​

public  class order

{

    public  string guid {get;set;}

    public  string parent {get;set;}

    public  string ProductID{get;set;}

    public  string Description{get;set;}

    public  double Quantity{get;set;}

    public  double Total{get;set;}

    public  order(){}

}