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FeatureAll Data, All the Time…An introduction to .NET data accessand SQLXML Web servicesSERVERBY BOB BEAUCHEMINHOMEDESKTOPEVERYWHEREin this “Data Access” column,I will discuss topicsrelating to the Microsoft.NET data access stack,known as ADO.NET. This installmentstarts with an overview of thedifferent pieces of the stack and finisheswith an interesting developmentin the world of SQL Server,.NET, and XML Web services.The ADO.NET data access APIsconsist of three types of classes:1. Classes for executing databasecommands in connected mode:This is accomplished by a seriesof data providers that run inmanaged code as much as possible.Each of these data providersimplements approximately thesame set of interfaces andexposes an analogous class hierarchy.2. Disconnected data access classes:A series of classes in theSystem.Data namespace thatconsists of a complex in-memorycollection class (called theDataSet), which approximatesrelational database semantics,and supporting classes such asthe DataTable, DataRow, andDataRelation.3. Classes for XML integration:DataSets, with help from a fewadditional classes in the System.Xmlnamespace (althoughthey “live” in the System.Data.dll),integrate the XML and relationaldata access worlds. It’s also possibleto allow for data interchangewith heterogeneous platforms,such as Unix systems and IBMmainframes, using XML Web services.An ADO.NET Crash CourseWe’ll start with a crash course inADO.NET consisting of some simpleprograms.Executing Database Commandsin Connected ModeData providers include a SQLServer–specific provider (theSqlClient data provider) and adata provider, known as the OleDbdata provider, which is a bridgefor OLE DB data sources. Justbefore .NET shipped in February,Microsoft released a bridgeprovider for ODBC data sourcesfor download from the Web. So, ifyou use SQL Server or have anOLE DB provider or ODBC driverfor your database, you can use the.NET data access classes directlywith your data.The simplest starter program forreading a resultset (the results of aSQL SELECT statement) containingtwo columns from a SQL Server table(in C#) would look like Listing 1.The program uses the SqlClientdata provider’s Connection class(which includes a constructor thattakes a database connection string)to open a connection to SQL Server.The Command class encapsulatesSQL statement functionality andincludes a constructor that takes aSQL command string. We use this toexecute the SQL; the results arereturned in a DataReader class. Thisclass has methods that allow us tomove forward through the rows inthe resultset, along with methodsthat allow us to access the data ineach row by using zero-based columnordinals.We’ll discuss using dataproviders with different datasources in more detail in futurecolumns. In addition to Microsoft’sdata providers, third parties canbuild data providers. For example,DataDirect Technologies hasshipped native providers for Oracleand Sybase. You can also build yourown. I built a simple data providerthat’s available for download athttp://staff.develop.com/bobb.Disconnected Dataand the DataSet ClassThe DataSet class is a specializedcollection class that includes a collectionthat follows the relationaldata model. If you’re used to dealingwith tables, columns, rows, andrelationships, you’ll be right athome with DataSet. DataSets can beused as an in-memory cache or adata source for ASP.NET Web Formscontrols, or they can be directlybound and manipulated byWindows Forms controls. DataSetsinteract with data providers througha data provider’s DataAdapter class.Listing 2 shows a simple programthat reads data into a DataSet,updates the DataSet, and flushesthe updates back to the database.Note that you don’t have to open theconnection directly when you’reusing a DataAdapter because theDataAdapter.Fill method opens theconnection, reads the rows, andthen closes the connection automatically.Although using a DataSet is similarin concept to using the disconnectedADO Recordset class, aDataSet can contain multiple tables16Preview Issue • dotnetdevelopersjournal.com