ADO.NET is a module of .Net Framework used to connect an application and the data sources.
DataReader and DataSet are two common components of ADO.NET that are used to fetch and store data in a C# application.
DataReader is a connection-oriented service; so, whenever you want data, you must be connected to the database. It allows you to retrieve read-only and forward-only data from a database. In other words, a DataReader is a stream of data that is returned from a database query. It reads only one row at a time from the database and can only move forward. This helps application performance since only one row of data is stored at a time. However, the DataReader cannot edit data while traversing over it.
DataSet is an in-memory representation of data. Unlike DataReader, it has a disconnected nature, meaning you fill data in DataSet once and then get data from DataSet instead of connecting with the database. A great plus about DataSet is that it can be filled using multiple data sources.
The DataSet represents a complete set of data among the tables that include related tables, constraints, and relationships. However, this greater functionality comes with performance overhead; therefore, DataSet is slower than DataReader.
DataReader provides faster performance, but has read-only and forward-only access. DataSet, on the other hand, is high resource-consuming, but offers more control and a disconnected nature.
As a rule of thumb, if you want forward-only access to query results, and do not need to edit data, then DataReader is the better option due to its speed.
If you want random access and do not need to worry about having a constant connection with the database, go with DataSet.
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