提升数据访问能力：C#与Entity Framework快速入门指南

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"Code-First Development with Entity Framework: A Practical Guide for .NET Developers" 这本书《Code-First Development with Entity Framework》是一本专为有一定微软.NET框架经验的软件开发人员设计的实用指南，旨在帮助读者提升在关系型数据库中使用Entity Framework的数据访问技能。作者Sergey Barskiy以其深入浅出的方式，通过C#和VB.NET的实例，讲解了Entity Framework的核心概念，特别是针对代码首次生成（Code-First）方法论。在本书中，读者将从数据库配置入门，学习如何编写定义数据库结构的类。重点在于理解如何利用LINQ与Entity Framework配合，以便于访问和操作存储的数据。随后，章节会深入探讨如何利用Entity Framework持久化信息到关系型数据库管理系统（RDBMS），以及ORM（对象关系映射）技术在.NET编程中的优势。书中的主要内容包括五个核心章节： 1. 介绍Entity Framework：简要介绍Entity Framework的背景和其在.NET开发者需求中的角色。 2. 第一个Entity Framework应用：通过实际项目展示如何构建一个基础的Entity Framework应用程序。 3. 定义数据库结构：详细说明如何通过代码自动生成数据库模型。 4. 数据查询、插入、更新与删除：涵盖基本的CRUD操作和LINQ查询的使用。 5. 高级建模和查询技术：涉及视图、存储过程、异步API和并发处理等进阶主题。此外，书中还讨论了数据库迁移和额外功能，以及如何使用迁移解决数据库部署问题。附录提供了自我测试问题的答案，供读者检验学习成果。《Code-First Development with Entity Framework》不仅适合想要学习Entity Framework的开发者，也适合希望升级现有数据访问技能的专业人士。它强调实践，通过丰富的实例确保读者能够快速上手，并掌握这一强大的.NET ORM工具。同时，由于版权原因，所有复制或传播必须得到出版商的书面许可。出版日期为2015年3月，反映了当时的技术环境，但依然具有参考价值，尤其是对于那些寻求最新Entity Framework版本知识的开发者来说，可以结合后续版本进行补充学习。

Preface

[ ix ]

Downloading the example code

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Introducing Entity Framework

[ 2 ]

In the following screenshot, you can see a table that allows you to store a person's

information, specically their rst and last names, along with a unique identier for

each person. This type of storage, where similar data items are grouped together into

tabular structures is typical:

Each column can also be constrained in some ways. For example, PersonId is an

integer column. LastName is nvarchar(50) column, which means you can store

Unicode data of variable size in it, up to 50 characters. You will see in subsequent

chapters how we describe this information using Entity Framework.

The data stored in each column and row combination is scalar data, such as number

or string. When software needs to persist or retrieve data, it must describe its intent,

such as insert or select query, using the database-specic language called Structured

Query Language (SQL). SQL is a common standard for all relational database systems,

as issued by the American National Standards Institute (ANSI). However, some

database systems have their own dialect on top of the common standard. In this book,

we are not going to dive into the depths of SQL, but some concepts are important

to understand. There are some basic commands that we need to look at. These are

typically described as CRUD. CRUD stands for Create, Retrieve, Update, and Delete.

For example, if you want to retrieve or query the data from the preceding example,

you would type the following:

SELECT PersonId, FirstName, LastName

FROM Person

Historically, before tools such as Entity Framework, developers embedded SQL

language statements inside the software code using .NET languages, such as C# or

VB.NET or other programming languages, such as C++ or Java. The reason for this is

that these languages do not natively speak or understand SQL. For example, to retrieve

the data from the database and manipulate it as objects, you would write a fair amount

of code using ADO.NET, .NET Framework's data access built-in framework. You

would need to dene a class to hold a person's data. Then, you would need to open

a connection to the database, create a command that uses the preceding query as its

text, execute the command's reader, and iterate through the reader results, populating

an instance of our Person class with the data from the reader. As you can see, there

would be a lot of steps involved. More importantly, the code we'd write would be

quite fragile.

Chapter 1

[ 3 ]

For example, if we change the column name in our database from FirstName to

First_Name, our code would still compile just ne, but would throw an exception

when we try to run it. Moreover, the data in the database was stored as scalar values

organized in columns and rows in a table, but our destination was an object or object

graph. As you can see, this way of accessing the data has a number of issues.

First of all, there is a type mismatch between RDBMS column types and .NET

types. Second, there is a mismatch between storage, which is a collection of scalar

values, and destination, which is an object with properties. To further complicate

the situation, our person object could also have a complex property that contains

a list of phone numbers, which would be represented by a completely different

table. These problems are collectively referred to as impedance mismatch between

object-oriented programming and relational databases.

The set of tools called ORM came about to solve this mismatch problem. An ORM

tool represents data stored in database tables as objects, native to a programming

language, such as .NET languages, C#, and VB.NET. ORM tools have many

advantages over the traditional code, such as ADO.NET code that we mentioned.

They expose the data using native .NET types. They expose related data using simple

.NET properties. They provide compile time checking. They solve the problem with

typos. Developers do not have to use SQL, a different language. Instead in the .NET

world, developers use Language INtegrated Query (LINQ) to query the data. LINQ

is simply part of C# and VB.NET languages. We will cover the basics of LINQ in

subsequent chapters. By the same token, programmers use an ORM tool's API to

persist data to the database. Finally, as we will see later, you will write less code.

Less code means fewer bugs, right?

A brief history of Entity Framework

Over the years, there have been many ORM tools entering the market; some

commercial, others open source. Microsoft developed its own tools. First one was

LINQ to SQL, which was built on .NET 3.5. This ORM only worked with SQL Server

and SQL Server Compact. Entity Framework, which rst shipped in 2008, was the

second attempt. It had a number of advantages over LINQ to SQL. First of all, it had

provider architecture, thus was open to working with all relational database engines,

not just SQL Server, given that a provider was written for the engine in question.

All major RDBMSes have Entity Framework providers at this point in time.

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