What is Data Reader?

You can use the ADO.NET DataReader to retrieve a read-only, forward-only stream of data from a database. Using the DataReader can increase application performance and reduce system overhead because only one row at a time is ever in memory.
After creating an instance of the Command object, you create a DataReader by calling Command. ExecuteReader to retrieve rows from a data source, as shown in the following example.

SqlDataReader myReader = myCommand.ExecuteReader();

You use the Read method of the DataReader object to obtain a row from the results of the query.

while (myReader.Read())
Console.WriteLine("\t{0}\t{1}", myReader.GetInt32(0), myReader.GetString(1));