Using a SqlConnection Object to Connect to a SQL Server Database phần 1

Using a SqlConnection Object to Connect to a SQL Server Database You create a SqlConnection object using the SqlConnection constructor.. Assuming you've imported the System.Data.SqlClien

Using a SqlConnection Object to Connect to a SQL Server Database

You create a SqlConnection object using the SqlConnection() constructor This

constructor is overloaded, meaning that there are multiple versions of the constructor that

you can call The SqlConnection() constructors are as follows:

SqlConnection()

SqlConnection(string connectionString)

where connectionString contains the details for the database connection You'll learn the details of the connectionString in this section

Assuming you've imported the System.Data.SqlClient namespace, you can create a new SqlConnection object using the following statement:

SqlConnection mySqlConnection = new SqlConnection();

You can then set the details for the database connection using the ConnectionString property of mySqlConnection For example:

mySqlConnection.ConnectionString =

"server=localhost;database=Northwind;uid=sa;pwd=sa";

where

server specifies the name of the computer on which SQL Server is running

database specifies the name of the database

uid specifies the name of the database user

pwd specifies the password for the user

Warning For security reasons, do not include the username password in your production

code Instead ask the user to enter their name and password-or use integrated security, which you'll learn about shortly

One thing you need to bear in mind is that you can set the ConnectionString property only when your Connection object is closed

You can also pass a connection string directly to the SqlConnection() constructor For example:

string connectionString =

"server=localhost;database=Northwind;uid=sa;pwd=sa";

SqlConnection mySqlConnection =

new SqlConnection(connectionString);

Or more simply:

SqlConnection mySqlConnection =

new SqlConnection(

"server=localhost;database=Northwind;uid=sa;pwd=sa"

);

You can set an optional connection timeout, which specifies the number of seconds that the Open() method will wait for a connection to the database You do this by specifying a connection timeout in your connection string For example, the following string specifies

a connection timeout of 10 seconds:

string connectionString =

"server=localhost;database=Northwind;uid=sa;pwd=sa;" +

"connection timeout=10";

Note The default connection timeout is 15 seconds A connection timeout of 0 seconds means the connection attempt will wait indefinitely Avoid setting your connection timeout to 0

Before starting a Windows session, you typically log in to Windows with a username and password If you're using Windows integrated security, you can pass your username and password to SQL Server and use those credentials to connect to the database This saves you from providing a separate username and password to SQL Server You can use integrated security in your program by specifying integrated security=SSPI in your

connection string For example:

string connectionString =

"server=localhost;database=Northwind;integrated security=SSPI";

Notice that you don't provide the username and password Instead, the username and password you used when logging into Windows is passed to SQL Server SQL Server will then check its list of users to see if you have permission to access the database (For further details on integrated security, consult the SQL Server Books Online

documentation.)

You've now seen how to create a Connection object using program statements You'll see how to create a Connection object visually using Visual Studio NET later in the

"Creating a Connection Object using Visual Studio NET" section Next, you'll see how

to open and close a connection

Opening and Closing a Database Connection

Once you've created your Connection object and set its ConnectionString property to the appropriate details for your database connection, you can open the connection to the database You do this by calling the Open() method of your Connection object The following example calls the Open() method of mySqlConnection:

mySqlConnection.Open();

Once you've finished with your database connection, you call the Close() method of your Connection object For example:

mySqlConnection.Close();

SqlConnection object and display some of the properties of that object

Listing 7.1: MYSQLCONNECTION.CS

/*

MySqlConnection.cs illustrates how to use a

SqlConnection object to connect to a SQL Server database

*/

using System;

using System.Data;

using System.Data.SqlClient;

class MySqlConnection

{

public static void Main()

{

// formulate a string containing the details of the

// database connection

string connectionString =

"server=localhost;database=Northwind;uid=sa;pwd=sa";

// create a SqlConnection object to connect to the

// database, passing the connection string to the constructor

SqlConnection mySqlConnection =

new SqlConnection(connectionString);

// open the database connection using the

// Open() method of the SqlConnection object

mySqlConnection.Open();

// display the properties of the SqlConnection object

Console.WriteLine("mySqlConnection.ConnectionString = "+

mySqlConnection.ConnectionString);

Console.WriteLine("mySqlConnection.ConnectionTimeout = "+

mySqlConnection.ConnectionTimeout);

Console.WriteLine("mySqlConnection.Database = "+

mySqlConnection.Database);

Console.WriteLine("mySqlConnection.DataSource = "+

mySqlConnection.DataSource);

Console.WriteLine("mySqlConnection.PacketSize = "+

mySqlConnection.PacketSize);

Console.WriteLine("mySqlConnection.ServerVersion = "+

mySqlConnection.ServerVersion);

Console.WriteLine("mySqlConnection.State = "+

mySqlConnection.State);

Console.WriteLine("mySqlConnection.WorkstationId = "+

mySqlConnection.WorkstationId);

// close the database connection using the Close() method

// of the SqlConnection object

mySqlConnection.Close();

}

}

The output from this program is as follows:

mySqlConnection.ConnectionString = server=localhost;database=Northwind;uid=sa; mySqlConnection.ConnectionTimeout = 15

mySqlConnection.Database = Northwind

mySqlConnection.DataSource = localhost

mySqlConnection.PacketSize = 8192

mySqlConnection.ServerVersion = 08.00.0194

mySqlConnection.State = Open

mySqlConnection.WorkstationId = JMPRICE-DT1

Note Your results will differ from those here For example, your connection string and

workstation ID will be different

Connection Pooling

Opening and closing a database connection is a relatively time-consuming process For

this reason, ADO.NET automatically stores database connections in a pool Connection pooling offers a great performance improvement because you don't have to wait for a

brand new connection to the database to be established when there's a suitable connection already available When you close a connection, that connection isn't actually closed; instead, your connection is marked as unused and stored in the pool, ready to be used again

If you then supply the same details in the connection string (same database, username, password, and so on), then the connection from the pool is retrieved and returned to you You then use that same connection to access the database

When using a SqlConnection object, you can indicate the maximum number of

connections allowed in the pool by specifying max pool size in your connection string (the default is 100) You can also indicate the minimum number of connections in the pool by specifying min pool size (the default is 0) For example, the following

SqlConnection specifies a max pool size of 10 and a min pool size of 5:

SqlConnection mySqlConnection =

new SqlConnection(

"server=localhost;database=Northwind;uid=sa;pwd=sa;" +

"max pool size=10;min pool size=5"

);

In this example, a pool with five initial SqlConnection objects is created A maximum of

10 SqlConnection objects can be stored in the pool If you attempt to open a new

SqlConnection object and the pool is already full with currently used objects, your

request waits until a SqlConnection object is closed, at which point that object is returned for you to use If your request waits longer than the number of seconds in the

ConnectionTimout property, then an exception is thrown

Listing 7.2: CONNECTIONPOOLING.CS

/*

ConnectionPooling.cs illustrates connection pooling

*/

using System;

using System.Data;

using System.Data.SqlClient;

class ConnectionPooling

{

public static void Main()

{

// create a SqlConnection object to connect to the database,

// setting max pool size to 10 and min pool size to 5

SqlConnection mySqlConnection =

new SqlConnection(

"server=localhost;database=Northwind;uid=sa;pwd=sa;" + "max pool size=10;min pool size=5"

);

// open the SqlConnection object 10 times

for (int count = 1; count <= 10; count++)

{

Console.WriteLine("count = "+ count);

// create a DateTime object and set it to the

// current date and time

DateTime start = DateTime.Now;

// open the database connection using the

// Open() method of the SqlConnection object

mySqlConnection.Open();

// subtract the current date and time from the start,

// storing the difference in a TimeSpan

TimeSpan timeTaken = DateTime.Now - start;

// display the number of milliseconds taken to open

// the connection

Console.WriteLine("Milliseconds = "+ timeTaken.Milliseconds);

// display the connection state

Console.WriteLine("mySqlConnection.State = "+

mySqlConnection.State);

// close the database connection using the Close() method

// of the SqlConnection object

mySqlConnection.Close(); }

}

}