Tài liệu Changing the Database for an Open Connection pdf

[ Team LiB ]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 th

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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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