Visual Basic .NET ADO.NET数据库编程入门指南

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本资源是《Mastering Visual Basic .NET Database Programming》一书的第6章，专注于ADO.NET的详细介绍。ADO.NET是Microsoft提供的一种用于.NET框架下数据库访问的技术，它在Visual Basic .NET中扮演着核心角色。以下是章节中涉及的关键知识点： 1. **ADO.NET工作原理**： ADO.NET（ActiveX Data Objects for .NET）是一种组件模型，它允许开发者以一种统一的方式处理多种类型的数据库。它基于接口和抽象类，提供了一种面向对象的方式来操作数据库，简化了数据库访问的复杂性。 2. **ADO.NET对象模型**： - **Connection对象**：这是与数据库建立连接的起点，负责打开、关闭和管理数据库连接。开发者可以通过SqlConnection类实例化。 - **Command对象**：代表SQL命令，可以执行查询、插入、更新或删除操作，如SqlCommand类。 - **DataAdapter对象**：负责填充DataSet，将数据库中的数据转换为可操作的数据结构。 - **DataReader对象**：逐行读取数据集的结果，通常在遍历后会被自动关闭。 - **DataSet对象**：是一个内存中的数据集合，可以存储来自多个数据源的结果，并支持数据的检索、修改和分页。 3. **Navigating through DataSets**：数据集提供了遍历数据的灵活性，通过DataTable对象可以获取列和行，同时支持分组、排序和过滤数据。 4. **Updating Your Database by using DataSets**：通过DataAdapter的Update方法，开发者可以将DataSet中的更改同步回数据库。这涉及到创建一个Command对象来执行UPDATE语句。 5. **Managing concurrency**： ADO.NET提供了事务管理和锁机制来处理并发操作，确保数据的一致性和完整性。开发者需要考虑如何正确地使用Transaction对象来管理数据库操作的原子性。本章内容涵盖了ADO.NET的基本用法和关键概念，对于想要深入学习Visual Basic .NET数据库编程的开发者来说，理解并掌握这些知识点至关重要。通过实践，开发者可以有效地在.NET应用程序中集成数据库操作，提高开发效率并确保数据安全。

Note If you have worked with collection objects, this experience will be a bonus to programming with ADO.NET.

ADO.NET contains a collection-centric object model, which makes programming easy if you already know how to work

with collections.

Four core objects belong to .NET data providers, within the ADO.NET managed provider archi-

tecture: the Connection, Command, DataReader, and DataAdapter objects. The Connection object is the

simplest one, because its role is to establish a connection to the database. The Command object exposes a

Parameters collection, which contains information about the parameters of the command to be exe-

cuted. If you’ve worked with ADO 2.x, the Connection and Command objects should seem familiar

to you. The DataReader object provides fast access to read-only, forward-only data, which is reminiscent

of a read-only, forward-only ADO RecordSet. The DataAdapter object contains Command objects that

enable you to map specific actions to your data source. The DataAdapter is a mechanism for bridging

the managed providers with the disconnected DataSets.

The DataSet object is not part of the ADO.NET managed provider architecture. The DataSet

exposes a collection of DataTables, which in turn contain both DataColumn and DataRow collec-

tions. The DataTables collection can be used in conjunction with the DataRelation collection to

create relational data structures.

First, you will learn about the connected layer by using the .NET data provider objects and

touching briefly on the DataSet object. Next, you will explore the disconnected layer and examine

the DataSet object in detail.

Note Although there are two different namespaces, one for OleDb and the other for the SqlClient, they are quite

similar in terms of their classes and syntax. As we explain the object model, we use generic terms, such as Connection, rather

than SqlConnection. Because this book focuses on SQL Server development, we gear our examples toward SQL Server data

access and manipulation.

In the following sections, you’ll look at the five major objects of ADO.NET in detail. You’ll

examine the basic properties and methods you’ll need to manipulate databases, and you’ll find

examples of how to use each object. ADO.NET objects also recognize events, which we discuss

in Chapter 12, “More ADO.NET Programming.”

The Connection Object

Both the SqlConnection and OleDbConnection namespaces inherit from the IDbConnection object.

The Connection object establishes a connection to a database, which is then used to execute commands

against the database or retrieve a DataReader. You use the

SqlConnection object when you are working

with SQL Server, and the

OleDbConnection for all other data sources. The ConnectionString property

is the most important property of the Connection object. This string uses name-value pairs to specify

the database you want to connect to. To establish a connection through a Connection object, call its

Open() method. When you no longer need the connection, call the Close() method to close it. To find

out whether a Connection object is open, use its

State property.

Chapter 6 A FIRST LOOK AT ADO.NET

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What Happened to Your ADO Cursors?

One big difference between classic ADO and ADO.NET is the way they handle cursors. In ADO 2.x, you have

the option to create client- or server-side cursors, which you can set by using the

CursorLocation property

of the Connection object. ADO.NET no longer explicitly assigns cursors. This is a good thing.

Under classic ADO, many times programmers accidentally specify expensive server-side cursors, when

they really mean to use the client-side cursors. These mistakes occur because the cursors, which sit in the

COM+ server, are also considered client-side cursors. Using server-side cursors is something you should

never do under the disconnected, n-tier design. You see, ADO 2.x wasn’t originally designed for disconnected

and remote data access. The

CursorLocation property is used to handle disconnected and connected access

within the same architecture. ADO.NET advances this concept by completely separating the connected and

disconnected mechanisms into managed providers and DataSets, respectively.

In classic ADO, after you specify your cursor location, you have several choices in the type of cursor to

create. You could create a static cursor, which is a disconnected, in-memory representation of your data-

base. In addition, you could extend this static cursor into a forward-only, read-only cursor for quick

database retrieval.

Under the ADO.NET architecture, there are no updateable server-side cursors. This prevents you from

maintaining state for too long on your database server. Even though the DataReader does maintain state on

the server, it retrieves the data rapidly as a stream. The ADO.NET DataReader works much like an ADO read-

only, server-side cursor. You can think of an ADO.NET DataSet as analogous to an ADO client-side, static

cursor. As you can see, you don’t lose any of the ADO disconnected cursor functionality with ADO.NET;

it’s just architected differently.

Connecting to a Database

The first step to using ADO.NET is to connect to a data source, such as a database. Using the Con-

nection object, you tell ADO.NET which database you want to contact, supply your username and

password (so that the DBMS can grant you access to the database and set the appropriate privileges),

and, possibly, set more options. The Connection object is your gateway to the database, and all the

operations you perform against the database must go through this gateway. The Connection object

encapsulates all the functionality of a data link and has the same properties. Unlike data links, how-

ever, Connection objects can be accessed from within your VB .NET code. They expose a number of

properties and methods that enable you to manipulate your connection from within your code.

Note You don’t have to type this code by hand. The code for all the examples in this chapter is located on the companion

CD that comes with this book. You can find many of this chapter’s code examples in the solution file

Working with

ADO.NET.sln

. Code related to the ADO.NET Connection object is listed behind the Connect To Northwind button on

the startup form.

Let’s experiment with creating a connection to the Northwind database. Create a new Win-

dows Application solution and place a command button on the Form; name it Connect to

Northwind. Add the

Imports statement for the System.Data.SqlClient name at the top of

the form module. Now you can declare a Connection object with the following statement:

Dim connNorthwind As New SqlClient.SqlConnection()

233

THE CONNECTION OBJECT

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