Writing Database-Independent Code

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 here.

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 provider-specific class of the provider-independent interface. Branching logic is then used execute code specific to that class.

Technorati : DB Independent code