Guide to Entity Framework with

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The Object Model layer contains .NET objects and a collection of .NET objects, types, 
properties, and methods. You can use the Object Model or the O-Space Model to 
query your business objects, or the collections of your business objects, using LINQ 
to Entities or Entity SQL. The C-Space and O-Space models are actually mapped by 
the O-C Mapping Layer using code attributes applied to the O-Space Model.
LINQ to Entities
LINQ is a query translation pipeline that has been introduced as part of the C# 
3.0 library. It comprises a set of query operators for different data sources (LDAP, 
objects, CSV, XML, entities, SQL, and so on). It is an extension of the C# language 
(it's actually a series of extension methods) and provides a simplified framework to 
access relational data in a strongly typed, object-oriented manner.
LINQ to Entities is a Microsoft technology that enables you to query your business 
objects from within the language in a strongly typed manner. You can use LINQ to 
Entities, a superset of LINQ to SQL, to query data against a conceptual data model, 
namely, the 
Entity Data Model. You will learn 
more about LINQ and LINQ to 
Entities in 
Chapter 6
, 
Working with LINQ to Entities
.
Here is an example of a typical LINQ to Entities query:
AdventureWorksEntities ctx = new AdventureWorksEntities();
var query = from e in ctx.Employees
select e;
foreach (var employee in query)
Console.WriteLine (employee.EmployeeID);
LINQ to Entities rests on top of Entity Framework's Object 
Services Layer and the LINQ to Entities queries are internally 
translated to canonical query trees. This, in turn, gets converted 
internally to corresponding SQL queries in a form expected by 
your underlying database.
Entity Client
The Entity Framework allows you to write programs against the EDM and also add 
a level of abstraction on top of the relational model. This isolation of the logical view 
of data from the Object Model is accomplished by expressing queries in terms of 
abstractions using an enhanced query language called Entity SQL.

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Introducing the ADO.NET Entity Framework