Linq - SelectMany Confusion

Question

From what I understand from the documentation of SelectMany, one could use it to produce a (flattened) sequence of a 1-many relationship.

I have following classes

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Answer 1

Here is your query using SelectMany, modeled exactly after your example. Same output!

        var customerOrders2 = customers.SelectMany(
            c => orders.Where(o => o.CustomerId == c.Id),
            (c, o) => new { CustomerId = c.Id, OrderDescription = o.Description });

The first argument maps each customer to a collection of orders (completely analagous to the 'where' clause you already have).

The second argument transforms each matched pair {(c1, o1), (c1, o2) .. (c3, o9)} into a new type, which I've made the same as your example.

So:

• arg1 maps each element in the base collection to another collection.
• arg2 (optional) transforms each pair into a new type

The resulting collection is flat like you'd expect in your original example.

If you were to omit the second argument, you would end up with a collection of all orders the match up to a customer. It'd be just that, a flat collection of Order objects.

Using it takes a lot of getting used to, I still have trouble wrapping my head around it sometimes. :(