System.Data is the namespace in the .NET Framework that provides all of .NET’s data access functionality. Some functionality is contained directly in System.Data, and much more exists in its subnamespaces.

The Entity Framework adds Exception classes directly to this namespace, as well as EntityKey, which provides a durable reference to an entity.

System.Data.Common provides base classes that are common to all of the data providers written for .NET. For example, DbDataReader is the base of SqlDataReader, OleDbDataReader, OracleDataReader, and more. The Entity Framework adds a few high-level DbProvider members into this namespace along with DataRecordInfo to expose query results in the form of a DbDataRecord and EntityRecordInfo, which provides access to the metadata of an entity.

The provider information that allows the ADO.NET Entity Framework to communicate with Microsoft SQL Server is added into the System.Data.SqlClient class through additional classes added into the System.Data.Entity assembly.

System.Linq.Expressions adds LINQ to Entities query functionality to the System.Linq.Expressions namespace.

System.Data.Common.CommandTrees adds logic for building Entity Framework command trees from LINQ to Entities and Entity SQL expressions. Each provider that is written to work with the Entity Framework will have the ability to turn these command trees into store queries.

System.Data.Entity does not contain any classes or methods; it is the base for a hierarchy of other namespaces—namely, System.Data.Entity.Design and System.Data.Entity.Design.ASP.NET (discussed shortly).

System.Data.EntityClient is a standard ADO.NET managed provider supporting access to the data described in the EDM. This namespace is comparable to System.Data.SqlClient or System.Data.OracleClient and provides classes such as EntityConnection, EntityCommand, and EntityDataReader.

System.Data.Mapping provides logic for performing view generation from query expressions.

System.Data.Metadata.Edm contains the types that are represented in the conceptual, mapping, and store schemas that define and support the EDM. Using these types directly, it is possible to programmatically work with the metadata of the model.

System.Data.Objects is the most important namespace in the Entity Framework. It provides the classes for querying, change tracking, relationship management, and updating the data store through the EDM. The functionality provided by System.Data.Object and its child namespace, DataClasses, is referred to as “Object Services.”

System.Data.Objects.DataClasses contains the classes and interfaces that allow types described in the EDM to be instantiated as objects. With these classes you can programmatically interact with the data that is provided as a result of querying the EDM.

System.Linq.Expressions adds the necessary expressions for performing LINQ to Entities queries.

System.Data.Entity.Design provides functionality for generating an EDM as well as performing the code generation to create classes from the EDM.

System.Data.Entity.Design.ASP.NET provides the build providers used in the build environment for ASP.NET.

System.Data.Query.InternalTrees provides the tools for converting query expressions to be converted to command trees that are executed against the Entity Framework. This is very low-level and all of the members in this namespace are sealed.

System.Data.Query.PlanCompiler is another namespace filled with low-level functionality for processing queries.

System.Data.QueryResultAssembly, the third subnamespace in System.Data.Query, is also low-level and sealed. When data is returned from the data store, it needs to be transformed into objects. This namespace contains the tools that are used internally to perform this transformation.