ADO.NET The SqlConnection Object

ADO.NET is the .NET technology for interacting with data sources.  You have several Data Providers, which allow communication with different data sources, depending on the protocols they use or what the data base is.  Regardless, of which Data Provider used, you'll use a similar set of objects to interact with a data source.  The SqlConnection object lets you manage a connection to a data source.  SqlCommand objects allow you to talk to a data source and send commands to it.  To have fast forward-only read access to data, use the SqlDataReader.  If you want to work with disconnected data, use a DataSet and implement reading and writing to/from the data source with a SqlDataAdapter.

This is just the beginning - the first of several lessons in the ADO.NET Tutorial.  The next one in this series is Lesson 02:  The SqlConnection Object.

The SqlConnection Object:

This lesson describes the SqlConnection object and how to connect to a data base.  Here are the objectives of this lesson:

• Know what connection objects are used for.
• Learn how to instantiate a SqlConnection object.
• Understand how the SqlConnection object is used in applications.
• Comprehend the importance of effective connection lifetime management.

Introduction

The first thing you will need to do when interacting with a data base is to create a connection.  The connection tells the rest of the ADO.NET code which data base it is talking to.  It manages all of the low level logic associated with the specific data base protocols.  This makes it easy for you because the most work you will have to do in code is instantiate the connection object, open the connection, and then close the connection when you are done.  Because of the way that otherclasses in ADO.NET are built, sometimes you don't even have to do that much work.
Although working with connections is very easy in ADO.NET, you need to understand connections in order to make the right decisions when coding your data access routines.  Understand that a connection is a valuable resource.  Sure, if you have a stand-alone client application that works on a single data base one one machine, you probably don't care about this.  However, think about an enterprise application where hundreds of users throughout a company are accessing the same data base.  Each connection represents overhead and there can only be a finite amount of them.  To look at a more extreme case, consider a Web site that is being hit with hundreds of thousands of hits a day.  Applications that grab connections and don't let them go can have seriously negative impacts on performance and scalability. 

ADO.NET Objects

ADO.NET includes many objects you can use to work with data.  This section introduces some of the primary objects you will use.  Over the course of this tutorial, you'll be exposed to many more ADO.NET objects from the perspective of how they are used in a particular lesson.  The objects below are the ones you must know.  Learning about them will give you an idea of the types of things you can do with data when using ADO.NET.

 The SqlConnection Object

To interact with a data base, you must have a connection to it.  The connection helps identify the data base server, the data base name, user name, password, and other parameters that are required for connecting to the data base.  A connection object is used by command objects so they will know which data base to execute the command on.

The SqlCommand Object

The process of interacting with a data base means that you must specify the actions you want to occur.  This is done with a command object.  You use a command object to send SQL statements to the data base.  A command object uses a connection object to figure out which data base to communicate with.  You can use a command object alone, to execute a command directly, or assign a reference to a command object to an SqlDataAdapter, which holds a set of commands that work on a group of data as described below.

The SqlDataReader Object

Many data operations require that you only get a stream of data for reading.  The data reader object allows you to obtain the results of a SELECT statement from a command object.  For performance reasons, the data returned from a data reader is a fast forward-only stream of data.  This means that you can only pull the data from the stream in a sequential manner.  This is good for speed, but if you need to manipulate data, then a DataSet is a better object to work with.

The DataSet Object

DataSet objects are in-memory representations of data.  They contain multiple DataTable objects, which contain columns and rows, just like normal data base tables.  You can even define relations between tables to create parent-child relationships.  The DataSet is specifically designed to help manage data in memory and to support disconnected operations on data, when such a scenario make sense.  The DataSet is an object that is used by all of the Data Providers, which is why it does not have a Data Provider specific prefix.

The SqlDataAdapter Object

Sometimes the data you work with is primarily read-only and you rarely need to make changes to the underlying data source.  Some situations also call for caching data in memory to minimize the number of data base calls for data that does not change.  The data adapter makes it easy for you to accomplish these things by helping to manage data in a disconnected mode.  The data adapter fills a DataSet object when reading the data and writes in a single batch when persisting changes back to the data base.  A data adapter contains 

ADO.net Data Providers

ADO.NET is an object-oriented set of libraries that allows you to interact with data sources.  Commonly, the data source is a data base, but it could also be a text file, an Excel spread sheet, or an XML file.  For the purposes of this tutorial, we will look at ADO.NET as a way to interact with a data base.
As you are probably aware, there are many different types of data bases available.  For example, there is Microsoft SQL Server, Microsoft Access, Oracle, Borland Interbase, and IBM DB2, just to name a few.  To further refine the scope of this tutorial, all of the examples will use SQL Server. 
The examples will use the Northwind database.  This tutorial is specifically for ADO.NET.

Data Providers

We know that ADO.NET allows us to interact with different types of data sources and different types of data bases.  However, there isn't a single set of classes that allow you to accomplish this universally.  Since different data sources expose different protocols, we need a way to communicate with the right data source using the right protocol.  Some older data sources use the ODBC protocol, many newer data sources use the OleDb protocol, and there are more data sources every day that allow you to communicate with them directly through .NET ADO.NET class libraries. 

ADO.NET provides a relatively common way to interact with data sources, but comes in different sets of libraries for each way you can talk to a data source.  These libraries are called Data Providers and are usually named for the protocol or data source type they allow you to interact with.  Table 1 lists some well known data providers, the API prefix they use, and the type of data source they allow you to interact with.

Table 1.  ADO.NET Data Providers are class libraries that allow a common way to interact with specific data sources or protocols.  The library APIs have prefixes that indicate which provider they support.

Provider Name	API Prefix	Data Source Description
ODBC Data Provider	Odbc	Data Sources with an ODBC interface.  Normally older data bases.
OleDb Data Provider	OleDb	Data Sources that expose an OleDb interface, i.e. Access or Excel.
Oracle Data Provider	Oracle	For Oracle Data Bases.
SQL Data Provider	Sql	For interacting with Microsoft SQL Server.
Borland Data Provider	Bdp	Generic access to many data bases such as Interbase, SQL Server, IBM DB2, and Oracle.

An example may help you to understand the meaning of the API prefix.  One of the first ADO.NET objects you'll learn about is the connection object, which allows you to establish a connection to a data source.  If we were using the OleDb Data Provider to connect to a data source that exposes an OleDb interface, we would use a connection object named OleDbConnection.  Similarly, the connection object name would be prefixed with Odbc or Sql for an OdbcConnection object on an Odbc data source or a SqlConnection object on a SQL Server data base, respectively.