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System.Linq.Data is a namespace in the .NET Framework that provides classes and methods for querying data using LINQ (Language Integrated Query). It is part of the System.Linq namespace and is specifically designed for working with data sources that implement the IQueryable interface, such as Entity Framework and LINQ to SQL.

The primary class in System.Linq.Data is the DataLoadOptions class, which is used to specify related data that should be loaded from the data source when a query is executed. This can help to reduce the number of database round-trips and improve performance. DataLoadOptions can be used to specify which related entities should be loaded eagerly, rather than lazily, when a query is executed.

Another important class in System.Linq.Data is the DataContext class, which represents a session that is used to query and update a database. The DataContext class provides methods for executing queries, inserting, updating, and deleting data, as well as for managing the lifecycle of objects in the context.

System.Linq.Data also provides extension methods for working with data sources, such as the ToList and ToArray methods, which can be used to execute a query and return the results as a list or array. Additionally, System.Linq.Data provides methods for working with related data, such as the LoadWith method, which can be used to specify that related data should be loaded when a query is executed.

Overall, System.Linq.Data provides a powerful and flexible way to query and manipulate data using LINQ, and is an important part of the .NET Framework for developers working with data. However, it is important to note that System.Linq.Data is specific to data sources that implement the IQueryable interface, and may not be suitable for all types of data sources.