Understanding LINQ (C#) - SPIRO THE TECH GURU

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This article is about LINQ which I think is one of the most exciting features in Orcas. LINQ makes the concept of querying a first-class programming concept in .NET. The data to be queried can take the form of XML (LINQ to XML), databases (LINQ-enabled ADO.NET: LINQ to SQL, LINQ to Dataset and LINQ to Entities) and objects (LINQ to Objects). LINQ is also highly extensible and allows you to build custom LINQ enabled data providers (e.g.: LINQ to Amazon, LINQ to NHibernate, LINQ to LDAP).

I will discuss some of the new language features and improvements that are introduced in C# 3.0, and it is those features that enable the full power of LINQ which makes it possible to write something like this:

var result = from c in Customers
             where c.City == Boston"
             orderby c.LastName descending
             select new { c.FirstName, c.LastName, c.Address };

Remember that if you want to play around with LINQ or try the examples yourself, you will need to download Visual Studio Orcas Beta 1.
In case you don't want to download Visual Studio, you can check the LINQ Preview (May 2006 CTP) which runs on top of Visual Studio 2005 (there are a few changes in Beta 1 from the way LINQ worked in the May CTP).


New Language Features


I. Automatic Properties


public class Point {
    private int _x, _y;
    public int X {
        get { return _x; }
        set { _x = value; }
    }
    public int Y {
        get { return _y; }
        set { _y = value; }
    }
}

The above code simply defining a class with basic properties. Now with 
the new C# compiler in Orcas, we can write a shorter cleaner version 
using Automatic Properties which automatically generates the private fields with get/set operations :

public class Point {
    public int X { get; set; }
    public int Y { get; set; }
}

The code above is even more readable and less verbose.
Note that this
 feature has nothing to do with LINQ. I just thought it would be 
appropriate to list it with the other new language features.


II. Local Variable Type Inference




With this feature, the type of the local variable being declared is 
inferred from the expression used to initialize the variable. This is 
achieved using the var keyword (familiar to those who work 
with scripting languages, but actually it is quite different). It allows
 us to write the following code:




var num = 50;
var str = "simple string";
var obj = new myType();
var numbers = new int[] {1,2,3};
var dic = new Dictionary<int,myType>();

The compiler would generate the same IL as if we compiled:


int num = 50;
string str = "simple string";
myType obj = new myType();
int[] numbers = new int[] {1,2,3};
Dictionary<int,myType> dic = new Dictionary<int,myType>();

Note that there is no un-typed variable reference nor late-binding 
happening, instead the compiler is inferring and declaring the type of 
the variable from the right-hand side of the assignment. As a result, 
the var keyword is generating a strongly typed variable reference.


III. Object Initializers & Collection Initializers




Let's use the same Point class defined earlier, and suppose
 we want to define an instance of this class. We will have to create the
 object and start setting its properties, the code would look like this:




Point p = new Point();
p.X = 0;
p.Y = 0;

This could be rewritten using Objects Initializers and combined into:

Point p = new Point() { X = 0, Y = 0 };

This feature can also be used with collection. Take a look at this example:

List<Point> points = new List<Point> {
    new Point { X = 2,  Y = 5 },
    new Point { X = 1, Y = -10 },
    new Point { X = 3, Y = 0 }
};

Note that the compiler will generate a long hand code equivalent to the above one. It makescalls to the Add() method to add elements to the collection one at a time.


IV. Anonymous Types

This language feature enable us to define inline types without having to
 explicitly define a class declaration for this type. In other words, 
imagine we want to use a Point object without defining the class Point (it would be anonymous). We will use the same object initializer syntax discussed earlier, but without the type name:




var p = new {X = 0, Y = 2};

Inside Orcas, you will have full intellisense support. So when you use the variable p you will get a list of properties that this anonymous type has.


V. Lambda Expressions




C# 2.0 introduced anonymous methods, which allow code blocks to be 
written "in-line" where delegate values are expected. While anonymous 
methods provide the power of functional programming languages, the 
syntax is rather verbose. Lambda expressions provide a more concise, 
functional syntax for writing anonymous methods. A lambda expression is 
written as a parameter list (can be implicitly typed), followed by the => token, followed by an expression or a statement block.



As an example, let's define a delegate type MyDeleg as:



delegate R MyDeleg<A,R>(A arg);

We can then write using anonymous methods:
MyDeleg<int,bool> IsPositive = delegate(int num) {
                                   return num > 0;
                               };

Or we can use the new lambda expressions to write:

MyDeleg<int,bool> IsPositive = num => num > 0;



VI. Extension Methods

Extension methods make it possible to extend existing types and 
constructed types with additional methods, without having to derive from
 them or recompile the original type. So instead of writing helper 
methods for objects, they become part of that object itself.



As an example, suppose we want to check a string to see if it is a valid email address. We would do this by writing a function that takes a string as an argument and returns a true/false. With Extension Methods, we can do the following:



public static class MyExtensions {
    public static bool IsValidEmailAddress(this string s) {
        Regex regex = new Regex( @"^[w-.]+@([w-]+.)+[w-]{2,4}$" );
        return regex.IsMatch(s);
    }
}


We defined a static class with a static method containing the Extension Method. Note how the static method above has a this keyword before the first parameter argument of type string. This tells the compiler that this particular Extension Method should be added to objects of type string. And then we can call it from the string as a member function:


using MyExtensions;

string email = Request.QueryString["email"];
if ( email.IsValidEmailAddress() ) {
    // ...
}

It is worth mentioning that LINQ syntax makes use of built-in Extension Methods (e.g. where(), orderby(), select(), sum(), average() and many more) that reside in the new System.Linq namespace
 in Orcas and define standard query operators that can be used against 
relational databases, XML and any .NET objects that implement IEnumerable<T>.


VII. Query Syntax

Query expressions provide a language integrated syntax for queries 
that is similar to relational and hierarchical query languages such as 
SQL and XQuery. It is a shorthand for writing queries using the LINQ 
query operators (i.e. from...where...select). Visual Studio provides full intellisense and compile-time checking support for query syntax.
When
 the C# compiler encounters a query syntax expression, it actually 
transforms it into explicit method invocation code that uses Extension 
Methods and Lambda Expressions.



To explain this, let me give an example:



var result = from c in Customers
             where c.City.StartsWith("B")
             orderby c.LastName
             select new { c.FirstName, c.LastName, c.Address };

The above code is equivalent to the following:











var result = Customers.Where( c => c.City.StartsWith("B") )
                      .OrderBy( c => c.LastName  )
                      .Select( c => new { c.FirstName, c.LastName, c.Address } );

The advantage of using Query Syntax is that the code is easier and more readable.
Also note that a query expression begins with a from clause and ends with either a select or group clause.