LINQ - Concatenated seq from subcollection under parent collection

I suspect this is super simple, but I'm just getting started with linq.

I have a collection, say Orders. Under each Order I have a collection of
Lines.

I want to get a distinct list of ItemNumbers for all Orders.

Does anyone know how to do this? Is it a SelectMany()?

Thank you.

Jon
Tue Mar 04 17:29:39 CST 2008

Todd Beaulieu <ToddBeaulieu@discussions.microsoft.com> wrote:
> I suspect this is super simple, but I'm just getting started with linq.

I almost envy you, having the wonderful journey of discovery still
almost entirely ahead of you. (That's not to say I've stopped learning
about LINQ - far from it.)

> I have a collection, say Orders. Under each Order I have a collection of
> Lines.
>
> I want to get a distinct list of ItemNumbers for all Orders.
>
> Does anyone know how to do this? Is it a SelectMany()?

I'm assuming we have a situation like this:

public class Order
{
public IEnumerable<Line> Lines { get; set; }
}

public class Line
{
public int ItemNumber;
public int Quantity { get; set; }
}

and you have an IEnumerable<Order> to examine.

You want to end up with an IEnumerable<int> which consists of all the
ItemNumbers of all the orders, but distinct. In other words, "all the
items we've sold *any* of". Is that about right?

The simplest way to do this is to use SelectMany(), as you've
suggested. That's usually more easily expressed in a query expression.
So, we'd have:

var allItems = from order in orders
from line in order.Lines
select line.ItemNumber;

var distinctItems = allItems.Distinct();


So, allItems effectively flattens the sequence of item numbers, and
then the second statement finds the distinct items. The query
expression boils down to this:

var allItems = orders.SelectMany (order => order.Lines,
(order, line) => line.ItemNumber);

Does that help? Let me know if I've missed the boat, and I'll have
another go :)

--
Jon Skeet - <skeet@pobox.com>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
World class .NET training in the UK: http://iterativetraining.co.uk

ToddBeaulieu
Tue Mar 04 17:46:00 CST 2008

Jon, you ROCK!

Thanks, bud. That was exactly what I needed.

It's funny about the "envying" bit. I *am* quite excited lately about all
these new wonderful technologies I'm learning.

After getting a simple, but nonetheless cool LINQ query going in my app, and
thus avoiding the usual enumeration mess, I actually looked for someone to
share the moment with. Sadly, nobody there would have cared. :(

I will certainly need to study your message some more so I can understand it.

Thanks again!

"Jon Skeet [C# MVP]" wrote:

> Todd Beaulieu <ToddBeaulieu@discussions.microsoft.com> wrote:
> > I suspect this is super simple, but I'm just getting started with linq.
>
> I almost envy you, having the wonderful journey of discovery still
> almost entirely ahead of you. (That's not to say I've stopped learning
> about LINQ - far from it.)
>
> > I have a collection, say Orders. Under each Order I have a collection of
> > Lines.
> >
> > I want to get a distinct list of ItemNumbers for all Orders.
> >
> > Does anyone know how to do this? Is it a SelectMany()?
>
> I'm assuming we have a situation like this:
>
> public class Order
> {
> public IEnumerable<Line> Lines { get; set; }
> }
>
> public class Line
> {
> public int ItemNumber;
> public int Quantity { get; set; }
> }
>
> and you have an IEnumerable<Order> to examine.
>
> You want to end up with an IEnumerable<int> which consists of all the
> ItemNumbers of all the orders, but distinct. In other words, "all the
> items we've sold *any* of". Is that about right?
>
> The simplest way to do this is to use SelectMany(), as you've
> suggested. That's usually more easily expressed in a query expression.
> So, we'd have:
>
> var allItems = from order in orders
> from line in order.Lines
> select line.ItemNumber;
>
> var distinctItems = allItems.Distinct();
>
>
> So, allItems effectively flattens the sequence of item numbers, and
> then the second statement finds the distinct items. The query
> expression boils down to this:
>
> var allItems = orders.SelectMany (order => order.Lines,
> (order, line) => line.ItemNumber);
>
> Does that help? Let me know if I've missed the boat, and I'll have
> another go :)
>
> --
> Jon Skeet - <skeet@pobox.com>
> http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
> World class .NET training in the UK: http://iterativetraining.co.uk
>

Jon
Tue Mar 04 18:09:17 CST 2008

Todd Beaulieu <ToddBeaulieu@discussions.microsoft.com> wrote:
> Jon, you ROCK!
>
> Thanks, bud. That was exactly what I needed.
>
> It's funny about the "envying" bit. I *am* quite excited lately about all
> these new wonderful technologies I'm learning.

I can't remember the last time I found a technology suite this
intoxicating. It's clearly useful, *and* fun to play with. It's easy to
come up with examples to write about, and they can be thoroughly
bizarre (check my blog for specimens).

> After getting a simple, but nonetheless cool LINQ query going in my app, and
> thus avoiding the usual enumeration mess, I actually looked for someone to
> share the moment with. Sadly, nobody there would have cared. :(

I remember the first time I used an iterator block - I showed a few
people, but I don't think any of them "got" quite how cool it was.

> I will certainly need to study your message some more so I can understand it.

Just let me know if I can help explain it further. This isn't a
selfless act - I want to learn how to express LINQ ideas as easily as
possible, hence Human LINQ and Visual LINQ. I think I've expressed the
C# side of LINQ reasonably clearly in my book, but that's no good for
newsgroup posts. (I refuse to turn *every* LINQ thread into a plug for
the book.)

Basically, if you find one particular part of a post difficult, it
could be for any or all of three reasons:

1) I've fouled up the explanation
2) It's a fundamentally tricky concept
3) You're not really reading what I've written

1 and 2 are more likely than 3, and 1 is often fixable.

--
Jon Skeet - <skeet@pobox.com>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
World class .NET training in the UK: http://iterativetraining.co.uk