using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var InputData = new List<Invoice>()

        {

            new Invoice{ Vendor = "Microsoft", NumberInvoice= 10, Type= "PC", Month = "1" },

            new Invoice{ Vendor = "Microsoft", NumberInvoice= 10, Type= "PC", Month = "2" },

            new Invoice{ Vendor = "Microsoft", NumberInvoice= 10, Type= "Surface", Month = "1" },

            new Invoice{ Vendor = "Microsoft", NumberInvoice= 20, Type= "PC", Month = "1" },

            new Invoice{ Vendor = "Microsoft", NumberInvoice= 20, Type= "PC", Month = "2" },

            new Invoice{ Vendor = "Microsoft", NumberInvoice= 30, Type= "Surface", Month = "1" },

            new Invoice{ Vendor = "IBM", NumberInvoice= 50, Type= "Network", Month = "5" },

            new Invoice{ Vendor = "IBM", NumberInvoice= 60, Type= "Graphic Card", Month = "6" }

        };

​

        var result = (InputData.AsEnumerable() ?? throw new InvalidOperationException())

            .GroupBy(

                y => new

                {

                    y.Vendor,

                    y.Type

                }

            )

            .Select(

                a => new GroupedInvoice

                {

                    Vendor = a.Key.Vendor,

                    Type = a.Key.Type,

                    NumberInvoice = a.Sum(x => x.NumberInvoice),

                    Months = a.Select(x => x.Month).Distinct().ToArray(),

                }

            ).ToList();

​

        foreach (var item in result)

        {

            Console.WriteLine(item.Vendor+" "+ item.NumberInvoice + " " + String.Join(",", item.Months) + " " + item.Type );

        }

    }

}

​

public class Invoice

{

    public string Vendor { get; set; }

    public decimal NumberInvoice { get; set; }

    public string Type { get; set; }

    public string Month { get; set; }

}

​

public class GroupedInvoice

{

    public string Vendor { get; set; }

    public decimal NumberInvoice { get; set; }

    public string Type { get; set; }

    public string[] Months { get; set; }

}