using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<InvoiceProducts> list = new List<InvoiceProducts>();

    list.Add(new InvoiceProducts{ InvoiceID = 7000, ProductID=15});

    list.Add(new InvoiceProducts{ InvoiceID = 7000, ProductID=10});

    list.Add(new InvoiceProducts{ InvoiceID = 7000, ProductID=10});

    list.Add(new InvoiceProducts{ InvoiceID = 7000, ProductID=15});

​

    list.Add(new InvoiceProducts{ InvoiceID = 7010, ProductID=12});

    list.Add(new InvoiceProducts{ InvoiceID = 7010, ProductID=20});

    list.Add(new InvoiceProducts{ InvoiceID = 7010, ProductID=12});

​

    list.Add(new InvoiceProducts{ InvoiceID = 7021, ProductID=1});

    list.Add(new InvoiceProducts{ InvoiceID = 7021, ProductID=1});

         var result = list.GroupBy(x => x.InvoiceID)

                             .Select(x => x.Key + "," + String.Join(",", x.Select(z => z.ProductID).Distinct()));

​

            foreach (var item in result)

            {

                Console.WriteLine(item);

            }    

    }

}

​

class InvoiceProducts 

    {

        public int InvoiceID { get; set; }

        public int ProductID { get; set; }

    }