CHAPTER 21 ADO.NET PART I: THE CONNECTED LAYER
Figure 21-1. System.Data.dll is the core ADO.NET assembly
It turns out most Visual Studio project templates automatically reference this key data access
assembly. You should also understand that there are other ADO.NET-centric assemblies beyond
System.Data.dll, which you might need to reference manually in your current project using the Add
Reference dialog box.
The Three Faces of ADO.NET
You can use the ADO.NET libraries in three conceptually unique manners: connected, disconnected, or
through the Entity Framework. When you use the connected layer (the subject of this chapter), your code
base explicitly connects to and disconnects from the underlying data store. When you use ADO.NET in
this manner, you typically interact with the data store using connection objects, command objects, and
data reader objects.
The disconnected layer (you will learn more about this in Chapter 22) allows you to manipulate a set
of DataTable objects (contained within a DataSet) that functions as a client-side copy of the external
data. When you obtain a DataSet using a related data adapter object, the connection is automatically
opened and closed on your behalf. As you would guess, this approach helps free up connections for
other callers quickly and goes a long way toward increasing the scalability of your systems.
After a caller receives a DataSet, it is able to traverse and manipulate the contents without incurring
the cost of network traffic. Also, if the caller wants to submit the changes back to the data store, the data
adapter (in conjunction with a set of SQL statements) is used to update the data source; at this point the
connection is reopened for the database updates to occur, and then closed again immediately.
Finally in Chapter 23, you will be introduced to a data access API termed the Entity Framework (or
simply, EF). Using EF, you are able to interact with a relational database using client-side objects that
encapsulate a number of low-level database specifics from view. As well, the EF programming model
allows you to interact with relational databases using strongly typed LINQ queries, using the grammar of
LINQ to Entities.
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