Using the DataReader Class

Using the DataReader Class

The DataSet class is a central figure in ADO.NET. However, you have other options for reading data. Suppose, for example, you only want to iterate through a set of data, perhaps browsing. You don't need to scroll back and forth, and you don't need to update the data. You have the option of using a DataReader object for read-only and forward-only data. The DataReader class yields better performance heuristics than the DataSet class for forward-only browsing. The technique for using a DataReader is similar to that for initializing a DataSet .

The DataReader is requested from a command object. We still need a connection, but we don't need an adapter or a DataSet . Listing 11.4 demonstrates how to initialize a command and DataReader object, using the reader to iterate over every row represented by the SQL SELECT statement.

Listing 11.4 Using a Connection and Command to Initialize a DataReader Object
 Private Sub InitializeDropDownList()   Dim Connection As OleDbConnection = _     New OleDbConnection(Database.ConnectionString)   Dim Command As OleDbCommand = _     New OleDbCommand( _     "SELECT CompanyName FROM CUSTOMERS", Connection)   Dim Reader As OleDbDataReader   Connection.Open()   Try     Reader = Command.ExecuteReader()     While (Reader.Read())       DropDownList1.Items.Add(Reader.GetString(0))     End While   Finally     Reader.Close     Connection.Close()   End Try End Sub 

There are a couple of variations on Listing 11.4. For example, you can pass a CommandBehavior argument to Command.ExecuteReader , like CommandBehavior.CloseConnection , which instructs the reader to close the connection after the reader is obtained. In our example we used a verbose version that requires that we close the reader and then the connection explicitly. Readers and connections represent resources; hence it is helpful to use a Try . . . Finally block (referred to as a resource protection block ) to ensure that the Close methods are called.

The basic behavior of the reader after it is obtained is to read all rows in a loop and process each row with a getter. To demonstrate , our example uses a While loop and the GetString method. The argument passed to GetString represents the ordinal column index. Because I requested only CompanyName , I knew that column would contain the company name .

Visual Basic. NET Power Coding
Visual Basic(R) .NET Power Coding
ISBN: 0672324075
EAN: 2147483647
Year: 2005
Pages: 215
Authors: Paul Kimmel © 2008-2017.
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