What is "datareader"

In ADO.NET, a DataReader is a broad category of objects used to sequentially read data from a data source. DataReaders provide a very efficient way to access data, and can be thought of as a Firehose cursor from ASP Classic, except that no server-side cursor is used. A DataReader parses a Tabular Data Stream from Microsoft SQL Server, and other methods of retrieving data from other sources.

A DataReader is usually accompanied by a Command object that contains the query, optionally any parameters, and the connection object to run the query on.