using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var products = new List<Product>()

        {

            new Product() { ProductCode = 1, ClientId = 1 },

            new Product() { ProductCode = 2, ClientId = 1 },

            new Product() { ProductCode = 3, ClientId = 2 },

            new Product() { ProductCode = 4, ClientId = 3 } // this product has no corresponding client

        };

        var clients = new List<Client>()

        {

            new Client() { ClientId = 1, ClientName = "Bob" },

            new Client() { ClientId = 2, ClientName = "Laura" }

        };

        var productClientModels = products

            .GroupJoin(

                clients,

                p => p.ClientId,

                c => c.ClientId,

                (p, c) => new ProductClientModel

                { 

                    ProductCode = p.ProductCode, 

                    Clients = c.DefaultIfEmpty()

                }

            )

            .OrderByDescending(p => p.ProductCode)

            .ToList();

        foreach (var productClient in productClientModels)

            foreach (var client in productClient.Clients)

                Console.WriteLine("ProductCode = " + productClient.ProductCode + "; ClientId: " + client?.ClientId + "; Client Name: " + client?.ClientName);

    }

}

​

public class Product

{

    public long ProductCode { get; set; }

    public long ClientId { get; set; }

}

​

public class Client

{

    public long ClientId { get; set; }

    public string ClientName { get; set; }

}

​

public class ProductClientModel

{

    public long ProductCode { get; set; }

    public IEnumerable<Client> Clients { get; set; }

}