var expr =
  products
  .GroupJoin(
    customers.SelectMany(c => c.Orders),
    p => p.IdProduct,
    o => o.IdProduct,
    (p, orders) => new { p.IdProduct, Orders = orders });

foreach(var item in expr) {
  Console.WriteLine("Produkt: {0}", item.IdProduct);
  foreach (var order in item.Orders) {
    Console.WriteLine("\t{0}", order); }}
