using System;

using System.Linq;

using System.Collections;

using System.Collections.Generic;

public class Program

{

    class InvoiceProducts 

    {

        public int InvoiceID { get; set; }

        public int ProductID { get; set; }

    }

​

    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 lookup = list.Select(x => new { x.InvoiceID, x.ProductID })

                 .Distinct()

                 .ToLookup(x => x.InvoiceID, x => x.ProductID);

         foreach(var x in lookup)

            {

              Console.WriteLine(x.Key);

            }

    }

}