Tuesday, March 25, 2008

Object Initializers and LINQ

I first saw the C#3.0's "object initializers" in March 2007, at the Microsoft MVP Summit in Redmond, WA. The feature didn't strike me as a big deal. Ok, you can save lots of lines of code
and make your code more readable, but I thought, that's about it.
After some time learning about LINQ, lambda expressions, expression trees, etc, I now realize that object initializers are a fundamental and important part of C# 3.0.
They aren't just syntactical sugar. Take a look at an example:

Class Person is a plain old class:

public class Person

public string FName { get; set; }
public string Country { get; set; }

Before C# 3.0, in order to create a list of Person and populate with 5 new Person with the properties initialized, I would have do the following:

List people = new List();
Person p1 = new Person();
p1.FName = "Mary";
p1.Country = "United States";
Person p2 = new Person();
p2.FName = "Raul";
p2.Country = "Argetina";
Person p3 = new Person();
p3.FName = "Sergio";
p3.Country = "Brazil";
Person p4 = new Person();
p4.FName = "Giuseppe";
p4.Country = "Italy";
Person p5 = new Person();
p5.FName = "Jean";
p5.Country = "France";

With C# 3.0 we can initialize a collection of Person like so:

var people = new List {
new Person { FName="Mary", Country="United States" },
new Person { FName="Raul", Country="Argentina" },
new Person { FName="Sergio", Country="Brazil" },
new Person { FName="Giuseppe", Country="Italy" },
new Person { FName="Jean", Country="France" } };

As you can see, using object (and collection) initializers results in a much more compact and readable code. Notice that the code in between the {} is actually an expression. Therefore, we can say that object initializers give us the "ability to initialize an object in an expression context".

Now, let's see how this feature relates to LINQ. I am assuming you know what LINQ is and have at least seen LINQ queries. The result of a LINQ query is a "projection", or in other words, it is a brand new object created on the fly. The structure of the object is really unknown, and that's the reason the "var" keyword is so important in LINQ (see Anonymous Types).

You can project the entire object as is:

var FrenchPeople = from p in people
where p.Country == "France"
select p;

Or project a completely different object. In the case below, I am projecting a string (Person's first name) preceded by "Mr. ":

var FrenchPeople = from p in people
where p.Country == "France"
select "Mr. " + p.FName;

When you do these projections, all you're really doing is using an "expression that creates a new object out of existing objects".

The point I am trying to make is that, when we use expressions like these in LINQ, we are inherently using object initializers. Without them, projections in LINQ the way we know them would be impossible. The whole LINQ feature would be a lot clunkier and the code would look a lot messier.

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