All data-related classes reside in the System.Data namespace, which can be found in the System.Data.dll assembly and its secondary namespaces:
Let’s look at the purpose and contents of the System.Data namespace and, after this, at the purpose and contents of its secondary namespaces.
The System.Data namespace consists of classes, interfaces, delegates, and enumerations. The central class of ADO.NET is the DataSet class. This is the disconnected data container that serves as an in-memory cache of data. The DataSet, together with managed providers, supports reading from and writing modified data back to a data source.