using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        List<Order> orders = new List<Order>();

        List<ProductSold> products1 = new List<ProductSold>();

        products1.Add(new ProductSold() { OrderId = 1, ProductId = 2, Nome = "Produto2" } );

        List<ProductSold> products2 = new List<ProductSold>();

        products2.Add(new ProductSold() { OrderId = 2, ProductId = 3, Nome = "Produto3" } );

        orders.Add(new Order(){

            Id = 1,

            Products = products1

        });

        orders.Add(new Order(){

            Id = 2,

            Products = products2

        });

        var result = orders.SelectMany(x => x.Products).ToList();

        result.ForEach(x => Console.WriteLine(x.Nome));

    }

}

​

​

public class Order 

{

    public int Id {get;set;}

    public List<ProductSold> Products {get;set;}

​

}

​

public class ProductSold

{

    public int OrderId {get;set;}

    public int ProductId {get;set;}

    public string Nome {get;set;}

}