EF是啥？【What is Entity Framework?】（EF基础系列2）

EF产生的背景：

编写ADO.NET访问数据的代码，是沉闷而枯燥的，所以微软提供了一个对象关系映射框架（我们称之为EF），通过EF可以自动帮助我们的程序自动生成相关数据库。

Writing and managing ADO.Net code for data access is a tedious and monotonous job. Microsoft has provided an O/RM framework called "Entity Framework" to automate database related activities for your application.

EF的定义：

EF是一个对象关系映射【ORM】的框架,它保证程序员把相关的数据作为特别的领域对象，减少了程序员编写大量的访问数据库的代码。开发人员，使用LINQ来查询，检索，操作数据为强类型对象来处理。EF框架提供了更改跟踪，身份识别，懒加载，和查询转换，所以开发人员可以专注于他们的程序中的逻辑代码，而不是数据访问基础。

The Microsoft ADO.NET Entity Framework is an Object/Relational Mapping (ORM) framework that enables developers to work with relational data as domain-specific objects, eliminating the need for most of the data access plumbing code that developers usually need to write. Using the Entity Framework, developers issue queries using LINQ, then retrieve and manipulate data as strongly typed objects. The Entity Framework's ORM implementation provides services like change tracking, identity resolution, lazy loading, and query translation so that developers can focus on their application-specific business logic rather than the data access fundamentals.

EF的三个应用场景：

Entity framework is an Object/Relational Mapping (O/RM) framework. It is an enhancement to ADO.NET that gives developers an automated mechanism for accessing & storing the data in the database.

Entity framework is useful in three scenarios. First, if you already have existing database or you want to design your database ahead of other parts of the application. Second, you want to focus on your domain classes and then create the database from your domain classes. Third, you want to design your database schema on the visual designer and then create the database and classes.

The following figure illustrates the above scenarios.

1.根据已经存在的数据库，生成相关类；

2.根据相关领域类，生成数据库；

3.在可视化的设计器中，来创建类和数据库；

As per the above figure, EF creates data access classes for your existing database, so that you can use these classes to interact with the database instead of ADO.Net directly.

EF can also create the database from your domain classes, thus you can focus on your domain-driven design.

EF provides you a model designer where you can design your DB model and then EF creates database and classes based on your DB model.

什么是O/RM？

ORM is a tool for storing data from domain objects to relational database like MS SQL Server, in an automated way, without much programming. O/RM includes three main parts: Domain class objects, Relational database objects and Mapping information on how domain objects map to relational database objects (tables, views & storedprocedures). ORM allows us to keep our database design separate from our domain class design. This makes the application maintainable and extendable. It also automates standard CRUD operation (Create, Read, Update & Delete) so that the developer doesn't need to write it manually.

A typical ORM tool generates classes for the database interaction for your application as shown below.

ORM是一个工具，它可以自动的存储数据库（比如MS SQL Server）中领域对象中的数据，而不需要很多编程。

O/RM包含3个主要的部分：

1.领域类对象

　　　 2.关系数据库对象

3.映射信息（领域对象，怎样映射到数据库对象【表，视图，存储过程】？）

ORM允许我们把数据库设计和领域类设计分开来，这样保证了程序的可维护性和可扩展性，ORM同样自动帮助我们生成了增删查改（CURD）操作，所以我们不必手动去写了。

Tips：要了解更多的ORM技术，可以访问 这个链接： Object-relational Mapping

Visit wikipedia for more information on Object-relational Mapping

There are many ORM frameworks for .net in the market such as DataObjects.Net, NHibernate, OpenAccess, SubSonic etc. Entity Framework is an open source ORM framework from Microsoft.

Please note that Entity Framework is an open source framework by Microsoft. You can contribute to the Entity Framework project oncodeplex.

ORM框架分类：

1. DataObjects.Net

2. NHibernate

3. OpenAccess

4. SubSonic

5. 请注意：EF是微软的开源ORM框架，所谓开源就是你可以参与开发，有兴趣的可以去这个网站看看：codeplex.

好了，EF的介绍到此结束了。后面继续更新，觉得不错的，园友可以点赞，你的支持是我前进的动力！