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CHAPTER 10 MANAGED PROVIDERS OF DATA ACCESS
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Version 2.0 of the Framework introduces a factory model for creating instances of data access objects. Using this factory enables you to write code that s generic across different database vendors. This feature is available through ADO, but to write vendor-neutral code in .NET requires using an interface-based late bound programming model or one of the backwards compatibility Managed Providers, such as OLEDB or ODBC. The interface-based approach involves programming against the interfaces common to all Managed Providers, and then using Reflection or a custom-written factory to actually load instances of types at runtime. The backwards compatibility layer introduces a serious performance hit to your managed applications. This is particularly painful if the databases you re supporting do provide Managed Providers coded specifically for those vendors platforms. You don t ever want to, for example, use the OLEDB Managed Provider to talk to SQL Server 2000. So in 2.0, there s a Provider Factory model. This factory enables you to let configuration entries drive what database you re to use, but get instances of types from specific Managed Providers at runtime. So if you re using SQL Server, you get a SqlConnection object to use from your code. If you re using Oracle, you get an OracleConnection. The services of the Provider Factory are exposed through a couple of types that Microsoft added to the System.Data.Common namespace. DbProviderFactories exposes a couple of simple shared methods (see Table 10-6). Table 10-6. The Shared Methods of the DbProviderFactories type
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This method returns a DataTable with metadata about all of the installed providers on the system. This method returns an instance of a DbProviderFactory type. It accepts an argument that describes the factory either a DataRow from the DataTable returned by GetFactoryClasses, or an invariant name of the provider, which is a column returned in the DataTable.
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All of the configured Managed Providers are available through the services of the Provider Factory. To get a list of installed providers on a system, you can use the GetFactoryClasses method on the DbProviderFactory type (see Default.aspx in Web10). protected void Page_Load(object sender, EventArgs e) { GridView gv = new GridView(); gv.DataSource = DbProviderFactories.GetFactoryClasses(); gv.DataBind(); form1.Controls.Add(gv); } By binding this DataTable to a grid, you can see the information it returns, and a list of the installed providers on the machine, shown here in Figure 10-4.
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CHAPTER 10 MANAGED PROVIDERS OF DATA ACCESS
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Figure 10-4. The data returned by GetProviderFactories, listing the installed Managed Providers on the system In Figure 10-4, you can see the data contained in the DataTable returned by GetObjectFactories, including all of the invariant names of the providers, which you can use to create instances of the factory for a specific provider. All of the installed providers are registered in the machine.config under a new element named DbProviderFactories. <system.data> <DbProviderFactories> <add name="Odbc Data Provider" invariant="System.Data.Odbc" description=".Net Framework Data Provider for Odbc" type="System.Data.Odbc.OdbcFactory, System.Data, Version=..." /> <add name="OleDb Data Provider" invariant="System.Data.OleDb" description=".Net Framework Data Provider for OleDb" type="System.Data.OleDb.OleDbFactory, System.Data, Version=..." /> <add name="OracleClient Data Provider" invariant="System.Data.OracleClient" description=".Net Framework Data Provider for Oracle" type="System.Data.OracleClient.OracleClientFactory, System.Data..." /> <add name="SqlClient Data Provider" invariant="System.Data.SqlClient" description=".Net Framework Data Provider for SqlServer" type="System.Data.SqlClient.SqlClientFactory, System.Data, Version=..." /> <add name="SQL Server CE Data Provider" invariant="Microsoft.SqlServerCe.Client" support="3F7"
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CHAPTER 10 MANAGED PROVIDERS OF DATA ACCESS
description=".NET Framework Data Provider for Microsoft SQL Server ..." type="Microsoft.SqlServerCe.Client.SqlCeClientFactory, Microsoft...." /> </DbProviderFactories> </system.data> <system.web> If you want to use a new or third-party provider with the Provider Factory, you must register it in configuration using the DbProviderFactory element. You can make this entry in the machine.config, or if you only plan to use the provider from a specific application, you can enter into the web.config. The other method on DbProviderFactories returns an instance of DbProviderFactory. This type exposes eight factory methods for creating instance of connections, commands, parameters, and whatever other objects you need for interacting with your database. Here, you create an instance of the SQL Server Managed Provider factory and use it to execute a parameterize query against the database using the generic types served by the DbProviderFactory. (You can find this code in UseFactory.aspx of Web10.) protected void Page_Load(object sender, EventArgs e) { DbProviderFactory factory = DbProviderFactories.GetFactory("System.Data.SqlClient"); DbConnection cn = factory.CreateConnection(); cn.ConnectionString = ConfigurationManager.ConnectionStrings ["localPubs"].ToString(); DbCommand cm = factory.CreateCommand(); cm.Connection = cn; cm.CommandText = "select * from authors where [state] = @state"; DbParameter pm = factory.CreateParameter(); pm.ParameterName = "@state"; pm.Value = "CA"; cm.Parameters.Add(pm); GridView gv = new GridView(); cn.Open(); gv.DataSource = cm.ExecuteReader(CommandBehavior.CloseConnection); gv.DataBind(); form1.Controls.Add(gv); } The code listed previously generates the Web Form displayed in Figure 10-5. Notice the types you re using aren t SQL Server-specific. You re not using a SqlConnection object; you re using a DbConnection object. There is a whole set of these new types in the System.Data.Common namespace. They re created by the factory to represent a specific provider, but expose a generic type and programming model to the developer using the types (see Table 10-7).
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