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CHAPTER 4 s INTRODUCING ADO.NET
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in the Finally block to close the connection. Note that creating a connection doesn t actually connect to the database. You need to call the Open method on the connection. To execute the query, you first created a command object, passing its constructor the SQL to run and the connection on which to run it. Next, you created a data reader by calling ExecuteReader() on the command object. This not only executed the query but also set up the data reader. Note that unlike most objects, you have no way to create a data reader with a new expression.
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' Execute query Dim cmd As SqlCommand = New SqlCommand(sql, conn) rdr = cmd.ExecuteReader()
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You then produced a header for your output:
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' Display output header Console.WriteLine( _ "This program demonstrates the use of " _ & "the SQL Server Data Provider." _ & ControlChars.NewLine _ ) Console.WriteLine("First Name Last Name")
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You retrieved all the rows in the result set by calling the data reader s Read method, which returns true if there are more rows and false otherwise. Note that the data reader is positioned immediately before the first row prior to the first call to Read.
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' Process result set While rdr.Read Console.WriteLine( _ "{0} | {1}" _ , rdr("firstname").ToString().PadLeft(10) _ , rdr(1).ToString().PadLeft(10) _ ) End While
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You accessed each row s columns with the data reader s indexer (here, the SqlDataReader.Item property), which is overloaded to accept either a column name or a zero-based integer index. You used both to demonstrate the indexer s use, but using column numbers is more efficient than using column names. Next you handled any exceptions, quite simplistically, but at least you re developing a good habit. We ll cover exception handling much more thoroughly in 13.
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Catch e As Exception Console.WriteLine("Error: " & e.ToString)
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Finally, in a Finally block, you closed the data reader and the connection by calling their Close methods. As a general rule, you should close things in a Finally block to be sure they get closed no matter what happens within the Try block.
Finally ' Close connection rdr.Close() conn.Close()
Technically, closing the connection also closes the data reader, but closing both (in the previous order) is another good habit. A connection with an open data reader can t be used for any other purpose until the data reader has been closed.
Using the OLE DB Data Provider
Outside .NET, OLE DB is still Microsoft s high-performance data access technology. You can use it to access data stored in any format, so even in ADO.NET it plays an important role in accessing data sources that don t have their own ADO.NET data providers. The .NET Framework data provider for OLE DB is in the namespace System.Data.OleDb. Table 4-3 describes some important classes in the OleDb namespace.
Table 4-3. Commonly Used OleDb Classes Classes
OleDbCommand OleDbConnection OleDbDataAdapter OleDbDataReader OleDbError OleDbParameter OleDbTransaction
Description
Executes SQL queries, statements, or stored procedures Represents a connection to an OLE DB data source Represents a bridge between a dataset and a data source Provides a forward-only, read-only data stream of rows from a data source Holds information on errors and warnings returned by the data source Represents a command parameter Represents a SQL transaction
Notice the similarity between the two data providers, SqlClient and OleDb. The differences in their implementations are transparent, and the user interface is fundamentally the same. The ADO.NET OLE DB data provider requires that an OLE DB provider be specified in the connection string. Table 4-4 describes some OLE DB providers.
CHAPTER 4 s INTRODUCING ADO.NET
Table 4-4. Some OLE DB Providers Provider
DB2OLEDB SQLOLEDB Microsoft.Jet.OLEDB.4.0 MSDAORA MSDASQL
Description
Microsoft OLE DB provider for DB2 Microsoft OLE DB provider for SQL Server Microsoft OLE DB provider for Access (which uses the Jet engine) Microsoft OLE DB provider for Oracle Microsoft OLE DB provider for ODBC
Let s use the OLE DB data provider (SQLOLEDB) to access the Northwind database, making a few straightforward changes to the code in Listing 4-1. (Of course, you d use the SQL Server data provider for real work since it s more efficient.)
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