DataReaders

DataReaders are a programming concept used to access data from a data source in a forward-only, read-only fashion. They provide a streaming view of query results, allowing applications to read one row at a time without buffering the entire result set in memory. This makes data readers efficient for large result sets or when low memory usage is important, in contrast to data structures that cache all results.

In many environments, the data reader interface exposes core capabilities such as advancing to the next row, inspecting the number of columns, and retrieving column values by ordinal index or by name. Typical operations include moving to the next row with a method like Read(), obtaining the value of a column, and handling nulls or database-specific nulls. Because data readers are typically connected to an open data source, they rely on an active connection during use and must be closed or disposed when reading is complete to release resources.

A common realization of this concept is the DataReader found in .NET data access libraries. The System.Data.IDataReader interface defines the shared contract, with concrete implementations such as SqlDataReader, OleDbDataReader, and providers for other databases. These readers support typed access methods, GetValue, GetInt32, GetString, and similar helpers, as well as GetOrdinal to resolve column positions efficiently. They may offer optional features such as sequential access for large columns and asynchronous reading variants like ExecuteReaderAsync.

Beyond .NET, many database drivers and data processing libraries provide analogous streaming readers for CSV, JSON, or other formats. While the exact APIs vary, the underlying pattern remains: read data sequentially from a source with minimal buffering, access values efficiently, and manage the lifecycle of the underlying connection or stream.

Limitations include the inability to navigate backward, the need for an open connection, and potential performance trade-offs when random access or data caching is required.