C# 4 and .NET 4

272 ❘ ChaPTer 11 lAnGuAGe inteGrAted Query
It is important to note that the variable query just has the LINQ query assigned to
it. The query is not performed by this assignment but rather as soon as the query is
accessed using the foreach loop. This is discussed in more detail later in the section
“ Deferred Query Execution.”
extension methods
The compiler modifi es the LINQ query to invoke methods instead. LINQ offers various extension methods
for the IEnumerable < T > interface so you can use the LINQ query across any collection that implements
this interface.
Extension methods make it possible to write a method to a class that doesn ’ t offer the method at fi rst. You
can also add a method to any class that implements a specifi c interface, so multiple classes can make use of
the same implementation.
For example, wouldn ’ t you like to have a Foo() method with the String class The String class is
sealed, so it is not possible to inherit from this class. You can do an extension method, as shown in the
following code:
public static class StringExtension
{
public static void Foo(this string s)
{
Console.WriteLine("Foo invoked for {0}", s);
}
}
An extension method is defi ned as a static method where the fi rst parameter defi nes the type it extends
and it is declared in a static class. The Foo() method extends the string class, as is defi ned with the fi rst
parameter. For differentiating extension methods from normal static methods, the extension method also
requires the this keyword with the fi rst parameter.
Indeed, it is now possible to use the Foo() method with the string type:
string s = "Hello";
s.Foo();
The result shows Foo invoked for Hello in the console, because Hello is the string passed to the
Foo() method.
This might appear to be breaking object - oriented rules because a new method is defi ned for a type
without changing the type or deriving from it. However, this is not the case. The extension method
cannot access private members of the type it extends. Calling an extension method is just a new syntax
of invoking a static method. With the string you can get the same result by calling the method Foo()
this way:
string s = "Hello";
StringExtension.Foo(s);
To invoke the static method, write the class name followed by the method name. Extension methods are
a different way to invoke static methods. You don ’ t have to supply the name of the class where the static
method is defi ned. Instead, the static method is taken because of the parameter type. You just have to
import the namespace that contains the class to get the Foo() extension method in the scope of the
String class.
One of the classes that defi ne LINQ extension methods is Enumerable in the namespace System.Linq .
You just have to import the namespace to open the scope of the extension methods of this class. A sample
implementation of the Where() extension method is shown in the following code. The fi rst parameter of
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