Using the DataReader

The following are some tips for best performance using a DataReader, as well as, answers to common questions regarding the use of the DataReader.

The DataReader must be closed before accessing any output parameters for the associated Command.
Always close the DataReader when you are finished reading the data. If the Connection you are using is only used to return the DataReader, close it immediately after closing the DataReader.
An alternative to explicitly closing the Connection is to pass CommandBehavior.CloseConnection to the ExecuteReader method to ensure that the associated connection is closed when the DataReader is closed. This is especially useful if you are returning a DataReader from a method and do not have control over the closing of the DataReader or associated connection.

The DataReader cannot be remoted between tiers. The DataReader is designed for connected data access.
When accessing column data use the typed accessors like GetString, GetInt32, and so on. This saves you the processing required to cast the Object returned from GetValue as a particular type.
Only one DataReader can be open at a time, off of a single connection. In ADO, if you opened a single connection and requested two recordsets that used a forward-only, read-only cursor, ADO implicitly opens a second, unpooled connection to the data store for the life of that cursor, and then implicitly closes it. With ADO.NET, little is done for you "under-the-covers". If you want two DataReaders open at the same time, off the same data store, you have to explicitly create two connections, one for each DataReader. This is one way that ADO.NET gives you more control over the use of pooled connections.
By default, the DataReader loads an entire row into memory with each Read. This allows for random access of columns within the current row. If this random access is not necessary, for increased performance, pass CommandBehavior.SequentialAccess to the call to ExecuteReader. This changes the default behavior of the DataReader to only load data into memory when it is requested. Note that, CommandBehavior.SequentialAccess requires you to access returned columns in order. That is, once you have read past a returned column, you can no longer read its value.
If you are finished reading the data from a DataReader, but still have a large number of unread results pending, call Cancel on the Command prior to calling Close on the DataReader. Calling Close on the DataReader causes it to retrieve pending results and empty the stream prior to closing the cursor. Calling Cancel on the Command discards results on the server so that the DataReader does not have to read though them when it is closed. If you are returning output parameters from your Command, calling Cancel discards them as well. If you need to read any output parameters, do not call Cancel on the Command; just call Close on the DataReader.

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