.NET Framework - Linq. Select

Asked By shapper

07-Feb-09 11:39 PM

Hello,

I have the following code:

article.TagsCsv = String.Join(", ", article.Tags.Select(t =>
t.Name).ToArray());

If picks all the tags on an article, joins them, and place the CSV
result in the property TagsCsv.

I am trying to do the same but for many articles:

articles.Select(a => a.TagsCsv = String.Join(", ", a.Tags.Select
(t => t.Name).ToArray()));

This is not working.

What am I doing wrong?

Thanks,
Miguel

07-Feb-09 11:39 PM

=3D>
.Select

What exactly are you trying to do, and what exactly do you mean by
foreach would do:

foreach (var a in articles) { a.TagsCsv =3D ... }

Select() is _not_ a general purpose replacement for foreach. It's
supposed to be used for projection operations, not to perform some
action for all elements of the sequence.

Or are you trying to build a _single_ CSV string from tags of several
acticles? In this case, use SelectMany:

String.Join(", ", articles.SelectMany(a =3D> a.Tags.Select(t =3D>
t.Name)).ToArray())

07-Feb-09 11:39 PM

=3D>
gs.Select

No,

I am only trying to fill the property TagsCsv in each Article from its
Tags property.
I now I could use a for loop but I would also like to know how to make
this using Linq. Isn't it possible?

When I say that is not working is that after I apply my code line
TagsCsv keeps being null.

Thanks,
Miguel

07-Feb-09 11:39 PM

It is possible, but highly not desirable. LINQ and expressions with
side effects (this includes all kinds of statements) do not mix well.
To reiterate: if you need to _perform some action_ on each element of
a collection, you should use foreach - that's what it is designed for
(and what LINQ is _not_ designed for). If you need to project, filter,
order, or otherwise process a sequence, getting a _new_ sequence as a
result of the operation, only then you should use LINQ.


From MSDN documentation for Enumerable.Select (and most other methods
of class Enumerable):

return value is an object that stores all the information that is
required to perform the action. The query represented by this method
is not executed until the object is enumerated either by calling its
GetEnumerator method directly or by using foreach in Visual C# or For
Each in Visual Basic."

What happens here is that when you call select, it just remembers the
collection on which Select was called, and the lambda that you passed.
It does not do anything until you start enumerating the result;l and
if you never enumerate it - such as your case here, where you just
throw it away - it's not going to do anything. Even if you start
enumerating it, it will try to be as lazy as possible (i.e. it will
only call the lambda for those elements of the collection that you
enumerate, and not for all of it).

If you call ToArray() on the return value of Select(), then you'll see
your lambda actully execute for all elements.

And this is precisely what I mean when I say that LINQ is not designed
for such tasks (side-effects). Always use side-effect free lambdas
with LINQ, always. If you do, you won't fall into traps like this one,
and you'll be able to switch to PLINQ (or some other optimized LINQ
provider) easily if needed.

06-Feb-09 07:47 PM

A reminder: LINQ is not there so that you can solve _every_ programming
problem with it.


As Pavel said, the Enumerable.Select() method isn't intended to be an
alternative to "foreach".  In this particular case, what's going on is
that you never attempt to enumerate the collection, so the new enumeration
projected by Select() never even gets realized, so the lambda you provided
to Select() never gets executed.

Thus, nothing happens.

You could fix it by using something like ToArray() or ToList() on the
result of the Select() to force it to evaluate the entire collection.  But
really, that's the wrong thing to do, and is just a hack.  The right thing
to do is to not abuse the Select() method this way, and instead just use a
regular "foreach" loop.

Pete