How to get unique entries of products in a List<Order> with LINQ

Question

I have a classic Order -> Order Row scenario something like this:

public class Order 
{
    public int Id { get; set; }
    public string Name { get; set; }
    public List<OrderRow> OrderRows { get; set; }
}

public class OrderRow
{
    public int Id { get; set; }
    public int ProductId { get; set; }
    public int Amount { get; set; }
}

and a List<Order>. From that list of orders I want to get a list of all the unique ProductIds.

I solved it like this:

var orders = new List<Order>();
// TODO get the orders from DB
var products = new Dictionary<int,int>();
orders.ForEach(order => order.OrderRows.ForEach(row => products[row.ProductId]=row.ProductId));

but I got questions about my use of Dictionary and why I didn't use LINQs GroupBy and also comments that it wasn't very clear how it worked. I think the solution is fine but the comments made me want to try to solve it in LINQ but for some reason I hit a brick wall trying.

How do I get a list of unique products ids from a list of orders with LINQ here?

Answer 1

Untested, but how about:

var products = orders.SelectMany(o => o.OrderRows)
                     .Select(r => r.ProductId)
                     .Distinct();

Answer 2

This should get the unique product ids from the list of orders.

var orders = new List<Order>();

// Get orders from database.

IEnumerable<int> uniqueIds = orders.SelectMany(order => order.OrderRows)
    .Select(row => row.ProductId).Distinct();

Answer 3

If you don't want to remember SelectMany, this query comprehension syntax will still get you where you want to go:

var productIDs =
(
  from order in orders
  from orderrow in order.OrderRows
  select orderrow.ProductId
).Distinct();