.NET中的抽象工厂

  l         我们说过，抽象工厂针对系列产品的应变。在使用ADO.NET进行数据访问的时候，如果目标数据库是Access，我们会使用OleDbConnection、OleDbCommand以及OleDbDataAdapter等一系列ADO.NET对象。那么如果数据库是SQL Server，我们又会改用SqlConnection、SqlCommand以及SqlDataAdapter等一系列ADO.NET对象。如果只使用一套对象，没有什么大问题，如果我们的数据访问有系列变化的需求，比如可以针对Access和SQL Server，而且希望改换数据库尽量对客户端代码透明，那么就需要引入抽象工厂模式。

l         好在，ADO.NET 2.0中已经有了整套抽象工厂的类型。看下面的代码，你应该能辨别这些类型在抽象工厂中的角色：

    publicabstractclassDbProviderFactory

    {

        // Methods

        protected DbProviderFactory()

        {

        }

        publicvirtual DbCommand CreateCommand()

        {

            returnnull;

        }

        publicvirtual DbCommandBuilder CreateCommandBuilder()

        {

            returnnull;

        }

        publicvirtual DbConnection CreateConnection()

        {

            returnnull;

        }

        publicvirtual DbConnectionStringBuilder CreateConnectionStringBuilder()

        {

            returnnull;

        }

        publicvirtual DbDataAdapter CreateDataAdapter()

        {

            returnnull;

        }

        publicvirtual DbDataSourceEnumerator CreateDataSourceEnumerator()

        {

            returnnull;

        }

        publicvirtual DbParameter CreateParameter()

        {

            returnnull;

        }

        publicvirtual CodeAccessPermission CreatePermission(PermissionState state)

        {

            returnnull;

        }

        // Properties

        publicvirtualbool CanCreateDataSourceEnumerator

        {

            get

            {

                returnfalse;

            }

        }

    }

   publicsealedclassOleDbFactory : DbProviderFactory

    {

        // Fields

        publicstaticreadonlyOleDbFactory Instance = newOleDbFactory();

        // Methods

        private OleDbFactory()

        {

        }

        publicoverride DbCommand CreateCommand()

        {

            returnnewOleDbCommand();

        }

        publicoverride DbCommandBuilder CreateCommandBuilder()

        {

            returnnewOleDbCommandBuilder();

        }

        publicoverride DbConnection CreateConnection()

        {

            returnnewOleDbConnection();

        }

        publicoverride DbConnectionStringBuilder CreateConnectionStringBuilder()

        {

            returnnewOleDbConnectionStringBuilder();

        }

        publicoverride DbDataAdapter CreateDataAdapter()

        {

            returnnewOleDbDataAdapter();

        }

        publicoverride DbParameter CreateParameter()

        {

            returnnewOleDbParameter();

        }

        publicoverride CodeAccessPermission CreatePermission(PermissionState state)

        {

            returnnewOleDbPermission(state);

        }

    }

    publicsealedclassSqlClientFactory : DbProviderFactory, IServiceProvider

    {

        // Fields

        publicstaticreadonlySqlClientFactory Instance = newSqlClientFactory();

        // Methods

        private SqlClientFactory()

        {

        }

        publicoverride DbCommand CreateCommand()

        {

            returnnewSqlCommand();

        }

        publicoverride DbCommandBuilder CreateCommandBuilder()

        {

            returnnewSqlCommandBuilder();

        }

        publicoverride DbConnection CreateConnection()

        {

            returnnewSqlConnection();

        }

        publicoverride DbConnectionStringBuilder CreateConnectionStringBuilder()

        {

            returnnewSqlConnectionStringBuilder();

        }

        publicoverride DbDataAdapter CreateDataAdapter()

        {

            returnnewSqlDataAdapter();

        }

        publicoverride DbDataSourceEnumerator CreateDataSourceEnumerator()

        {

            return SqlDataSourceEnumerator.Instance;

        }

        publicoverride DbParameter CreateParameter()

        {

            returnnewSqlParameter();

        }

        publicoverride CodeAccessPermission CreatePermission(PermissionState state)

        {

            returnnewSqlClientPermission(state);

        }

        objectIServiceProvider.GetService(Type serviceType)

        {

            object obj2 = null;

            if (serviceType == GreenMethods.SystemDataCommonDbProviderServices_Type)

            {

                obj2 = GreenMethods.SystemDataSqlClientSqlProviderServices_Instance();

            }

            return obj2;

        }

        // Properties

        publicoverridebool CanCreateDataSourceEnumerator

        {

            get

            {

                returntrue;

            }

        }

}