Tài liệu Changing the Database for an Open Connection pdf

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Recipe 1.18 Changing the Database for an Open Connection
Problem
You want to change the database that a connection uses without recreating the
connection.
Solution
Use the ChangeDatabase( ) method to change the database for a connection.
The sample code creates a Connection to the Northwind database using the SQL Server
.NET data provider. The connection is changed to use the pubs database. Finally the
connection is closed. The Database property of the SqlConnection object is displayed
throughout the sample for the different connection states.
The C# code is shown in Example 1-12
.
Example 1-12. File: ChangeDatabaseForm.cs
// Namespaces, variables, and constants
using System;
using System.Configuration;
using System.Text;
using System.Data;
using System.Data.SqlClient;

// . . .

StringBuilder result = new StringBuilder( );

// Create the connection accessing Northwind database.
SqlConnection conn = new SqlConnection(
ConfigurationSettings.AppSettings["Sql_ConnectString"]);
result.Append("Connection String:" + Environment.NewLine);

result.Append(conn.ConnectionString + Environment.NewLine +
Environment.NewLine);

// Open the connection.
conn.Open( );
result.Append("Connection.State: " + conn.State + Environment.NewLine);
result.Append("Database: " + conn.Database + Environment.NewLine);

// Change the database to pubs.
conn.ChangeDatabase("pubs");
result.Append("Database: " + conn.Database + Environment.NewLine);

// Close the connection.
conn.Close( );
result.Append("Connection.State: " + conn.State + Environment.NewLine);
result.Append("Database: " + conn.Database);

resultTextBox.Text = result.ToString( );
Discussion
The ChangeDatabase( ) method is defined in the IDbConnection interface that represents
a connection to a data source and is implemented by .NET data providers for relational
databases including those for SQL Server, Oracle, and OLE DB. The ChangeDatabase( )
method is used to change the current database for an open connection. It takes a single
parameter that specifies the name of the database to use in place of the current database.
The name of the database must be valid or an ArgumentException will be raised. If the
connection is not open when the method is called, an InvalidOperationException is
raised. A provider-specific exception is raised if the database cannot be changed for any
reason.
The Database property of the Connection object is updated dynamically and returns the
current database for an open connection or the name of a database that will be used by a

closed connection when it is opened.
When the Connection is closed after ChangeDatabase( ) is called, the database is reset to
that specified in the original connection string.

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