Tài liệu Writing Database-Independent Code doc

[ Team LiB ]Recipe 1.11 Writing Database-Independent Code Problem You need to develop an application that can be used with different data providers, but does not lose functionality tha

[ Team LiB ]

Recipe 1.11 Writing Database-Independent Code

Problem

You need to develop an application that can be used with different data providers, but does not lose functionality that is specific to the different providers You want to create provider-independent code and use it with the provider-specific code that you might need

Solution

The solution shows how to use interfaces that are inherited by NET connected classes (such as Connection and DataReader) to create provider-independent code that can be used with provider-specific code to access unique functionality

The sample code contains a method and two event handlers:

GetData( )

This method is a NET data provider-independent method that accepts NET data provider-specific Connection and DataAdapter arguments as generic

IDbConnection and IDbDataAdapter interface types The interfaces are used to fill

a DataSet from the Customers table in Northwind The default view of the

Customers DataTable is bound to the data grid on the form

Finally, the provider-specific Connection for the IDbConnection is identified and provider-specific logic executed

SQL Button.Click

This event handler is provider-specific code that creates a SqlConnection and a SqlDataAdapter object and passes them as arguments into the

provider-independent GetData( ) method

OLE DB Button.Click

This event handler is provider-specific code that creates an OleDbConnection and

an OleDbDataAdapter object and passes them as arguments into the provider-independent GetData( ) method

The C# code is shown in Example 1-9

Example 1-9 File: DatabaseIndependentCodeForm.cs

// Namespaces, variables, and constants

using System;

using System.Configuration;

using System.Windows.Forms;

using System.Data;

using System.Data.Common;

using System.Data.SqlClient;

using System.Data.OleDb;

//

private void GetData(IDbConnection conn, IDbDataAdapter da)

{

// Create the command and assign it to the IDbDataAdapter interface IDbCommand cmd = conn.CreateCommand( );

cmd.CommandText = "SELECT * FROM Customers";

da.SelectCommand = cmd;

// Add a table mapping

da.TableMappings.Add("Table", "Customers");

dataGrid.DataSource = null;

// Fill the DataSet

DataSet ds = new DataSet( );

da.Fill(ds);

// Bind the default view for the Customer table to the grid

dataGrid.DataSource = ds.Tables["Customers"].DefaultView;

// Identify provider-specific connection type and process appropriately

if (conn is SqlConnection)

{

MessageBox.Show("Specific processing for SQL data provider."); }

else if(conn is OleDbConnection)

{

MessageBox.Show("Specific processing for OLE DB data provider."); }

}

private void sqlButton_Click(object sender, System.EventArgs e)

{

// Create a SQL Connection and DataAdapter

SqlConnection conn = new SqlConnection(

ConfigurationSettings.AppSettings["Sql_ConnectString"]);

SqlDataAdapter da = new SqlDataAdapter( );

dataGrid.CaptionText = "SQL NET Provider";

// Call provider-independent function to retrieve data

GetData(conn, da);

}

private void oleDbButton_Click(object sender, System.EventArgs e)

{

// Create a OLE DB Connection and DataAdapter

OleDbConnection conn = new OleDbConnection(

ConfigurationSettings.AppSettings["OleDb_ConnectString"]);

OleDbDataAdapter da = new OleDbDataAdapter( );

dataGrid.CaptionText = "OLE DB NET Provider";

// Call provider-independent function to retrieve data

GetData(conn, da);

}

Discussion

The IDbConnection, IDbCommand, IDataAdapter, and IDataReader interfaces are

implemented by Connection, Command, DataAdapter, and DataReader classes in NET data providers You can pass these provider-independent base classes as interface

arguments instead of the provider-specific inherited classes This allows applications that support multiple data providers to reuse common provider-independent code

The provider-specific functionality of the classes is not available when the base interfaces are used The is operator is used to identify the specific class of the provider-independent interface Branching logic is then used execute code specific to that class

[ Team LiB ]