C# 4 and .NET 4

294 ❘ ChaPTer 11 lAnGuAGe inteGrAted Query
The Where() method of LINQ to objects is defined with the Enumerable class:
public static IEnumerable Where(this IEnumerable source,
Func predicate);
Inside the System.Linq namespace there’s another class that implements the operator Where. This
implementation is used by LINQ to SQL. You can find the implementation in the class Queryable:
public static IQueryable Where(this IQueryable source,
Expression predicate);
Both of these classes are implemented in the System.Core assembly in the System.Linq namespace. How
is it defined and what method is used The Lambda expression is the same no matter whether it is passed
with a Func parameter or with an Expression parameter.
Just the compiler behaves differently. The selection is done based on the source parameter. The method
that matches best based on its parameters is chosen by the compiler. The CreateQuery() method of the
ObjectContext class that is defined by ADO.NET Entity Framework returns an ObjectQuery object
that implements IQueryable, and thus the Entity Framework uses the Where() method of the
Queryable class.
summary
In this chapter, you’ve seen the LINQ query and the language constructs that the query is based on, such
as extension methods and Lambda expressions. You’ve seen the various LINQ query operators not just
for filtering and ordering of data sources, but also for partitioning, grouping, doing conversions, joins,
and so on.
With Parallel LINQ you’ve seen how longer queries can easily be parallelized.
Another important concept is the expression tree. Expression trees allow building the query to the data
source at runtime because the tree is stored in the assembly. You can read about the great advantages of it
in Chapter 31. LINQ is a very in-depth topic, and you should see Chapters 31 and 33 for more information.
Other third-party providers are available for download; for example, LINQ to MySQL, LINQ to
Amazon, LINQ to Flickr, LINQ to LDAP, and LINQ to SharePoint. No matter what data source you
have, with LINQ you can use the same query syntax.
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