C# 4 and .NET 4

fast Data access: The Data reader ❘ 833
}
}
OleDbCommand cmd = new OleDbCommand(select, conn);
OleDbDataReader aReader = cmd.ExecuteReader();
while(aReader.Read())
Console.WriteLine("'{0}' from {1}",
aReader.GetString(0), aReader.GetString(1));
aReader.Close();
conn.Close();
code download DataReaderExample.cs
The preceding code includes many familiar aspects of C# already covered in this chapter. To compile the
example, issue the following command:
csc /t:exe /debug+ DataReaderExample.cs /r:System.Data.dll
The following code from the previous example creates a new OLE DB .NET database connection,
based on the source connection string:
OleDbConnection conn = new OleDbConnection(source);
conn.Open();
OleDbCommand cmd = new OleDbCommand(select, conn);
The third line creates a new OleDbCommand object, based on a particular SELECT statement, and the
database connection to be used when the command is executed. When you have a valid command, you need
to execute it, which returns an initialized OleDbDataReader :
OleDbDataReader aReader = cmd.ExecuteReader();
An OleDbDataReader is a forward - only “ connected ” reader. In other words, you can only traverse the
records returned in one direction, and the database connection used is kept open until the data reader has
been closed.
An OleDbDataReader keeps the database connection open until it is explicitly closed.
The OleDbDataReader class cannot be instantiated directly — it is always returned by a call to the
ExecuteReader() method of the OleDbCommand class. Once you have an open data reader, there are
various ways to access the data contained within the reader.
When the OleDbDataReader object is closed (via an explicit call to Close() , or the object being garbage
collected), the underlying connection may also be closed, depending on which of the ExecuteReader()
methods is called. If you call ExecuteReader() and pass CommandBehavior.CloseConnection , you can
force the connection to be closed when the reader is closed.
The OleDbDataReader class has an indexer that permits access (although not type - safe access) to any fi eld
using the familiar array style syntax:
object o = aReader[0];
or
object o = aReader["CategoryID"];
Assuming that the CategoryID fi eld was the fi rst in the SELECT statement used to populate the reader,
these two lines are functionally equivalent, although the second is slower than the fi rst; to verify this, a test
application was written that performed a million iterations of accessing the same column from an open data
reader, just to get some numbers that were big enough to read. You probably don ’ t read the same column a
million times in a tight loop, but every (micro) second counts, so you should write code that is as optimal
as possible.
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