terça-feira, 27 de maio de 2008

Linq to Objects - Part 2

Clique aqui para ler este post em Português

Hello everyone. I'll continue to talk about Linq to Objects in this post and show some other features present in this technology I have found very nice. I really hope you all like it as I continue to show the things I have seen and learned. Let's get to it.

In the previous posts I've shown the Linq query syntax, but there's another way to query data with Linq. All of the statements (in fact even more than that) we see in the query syntax can be represented by Collections' special method calls. They're called query expressions. Let's take a look at the previous example, this time using the method calls:

var Meses = new List<string> {

var MesesComJ =
.Where(mes => mes.StartsWith("J"))
.OrderByDescending(mes => mes)
.Select(mes => mes);

foreach (var m in MesesComJ) {

This form, imho, is not as cool as the another one, but it's also nice and shows us clearly what the code is going to get too.

Working with simple lists the select call is optional, but only in query expressions - in query syntax is still mandatory.

Hey, what is that weird operator over there? What, the =>? This is the operator we use for Lambda Expressions. Oh, ok... But, what are Lambda Expressions?

Some object methods take a function definition as an argument instead of a fixed value. Let's see the Where line again:

.Where(mes => mes.StartsWith("J"))

Let's note that there's only one argument passed to the method. When the Where method do the full iteration over the list it'll use the funcion defined in this argument to decide if the item at the given position will return in the result list or not.

For that we define the function right there, on the fly, creating the return condition for the items. But to create a dynamic filter we need a context. And here comes the "goes to" operator. It's used to point out what is the context in the function we're defining. This condition is going to be tested for each item in the main list, where mes is the current item. Use of this kind of expression is common in functional languages. The variable name before the operator is defined by us, and the code on the right side have to use the same name.

There are many query expressions in Linq; and even if I knew all of them this post would take ages! Let's see here other two interesting ones: Skip and Take.

There's no representation for Skip and Take in the query syntax in C#, only via method calls. Well, the Skip "Skips" a given number of items and the Take "Takes" a given number of items of the result.

var Meses456 =

This'll produce the result:


The order of the methods call already gives us a tip of what's goind to happen: "Skip 3 and take the 3 next".

We can combine the query syntax with query expressions too:

var MesesCom5Letras = (
from mes in Meses
where mes.Length == 5
orderby mes descending
select mes)



As I'm lerning new stuff I post here for you guys. Until there practice a bit and post your experiences here too. Thank you for the support and take care.

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