1.2 Connected and Disconnected Data
As mentioned
earlier, ADO.NET supports two different programming environments:
connected and disconnected.
The connected environment provides
forward-only, read-only access to data in the data source and the
ability to execute commands against the data source. The connected
classes provide a common way to work with connected data regardless
of the underlying data source. They include
Connection, Command,
DataReader, Transaction,
ParameterCollection, and
Parameter classes.
The disconnected environment allows data
retrieved from the data source to be manipulated and later reconciled
with the data source. The disconnected classes provide a common way
to work with disconnected data regardless of the underlying data
source. They include the DataSet,
DataTable, DataColumn,
DataRow, Constraint,
DataRelationship, and DataView
classes.
Finally, ADO.NET introduces the connected
DataAdapter class to bridge the data source and
disconnected classes by way of the connected classes. The
DataAdapter is an abstraction of the connected
classes that simplifies filling the disconnected
DataSet or DataTable classes
with data from the data source and updating the data source to
reflect any changes made to the disconnected data. Figure 1-1 shows the relationship between the connected
and disconnected classes in ADO.NET.
1.2.1 Connected Classes
The following classes are used by ADO.NET to communicate directly
with the data source:
- Connection
-
Maintains information required to connect to the data source through
a connection string. The connection string contains information such
as the name of the data source and its location, and authorization
credentials and settings. The Connection class has
methods to open and close the connection, for transactions to be
initiated on the connection, as well as control other properties of
the connection.
- Command
-
Executes SQL statements or stored procedures against the data source.
The command class has a ParameterCollection object
containing Parameter objects that allow
parameterized SQL statements and stored procedures to be used against
the data source.
- DataReader
-
Provides connected forward-only, read-only access to the data source.
It is optimized for speed. The DataReader is
instantiated through a Command object.
- Parameter
-
Allows parameters for both parameterized queries and stored
procedures to be defined and set to appropriate values. The
Parameter class is accessed through the
ParametersCollection object within a
Command object. It supports input and output
parameters as well as return values from stored procedures.
- Transaction
-
Allows transactions to be created on a connection so that multiple
changes to data in a data source are treated as a single unit of work
and either all committed or cancelled.
- DataAdapter
-
Bridges the data source and the disconnected
DataSet or DataTable classes.
The DataAdapter wraps the connected classes to
provide this functionality. It provides a method to retrieve data
into a disconnected object and a method to reconcile modified data in
the disconnected object with the data source. The
CommandBuilder class can generate the logic to
reconcile changes in simple situations; custom logic can be supplied
to deal with complex situations and optimize performance.
1.2.2 Disconnected Classes
The following ADO.NET classes allow data to be retrieved from the
data set, examined and modified offline, and reconciled with the data
source through the DataAdapter:
- DataSet
-
Provides a consistent way to deal with disconnected data completely
independently of the data source. The DataSet is
essentially an in-memory relational database, serving as a container
for the DataTable, DataColumn,
DataRow, Constraint, and
DataRelation objects.
The XML format serializes and transports a
DataSet. A DataSet can be
accessed and manipulated either as XML or through the methods and
properties of the DataSet interchangeably; the
XmlDataDocument class represents and synchronizes
the relational data within a DataSet object with
the XML Document Object Model (DOM).
- DataTable
-
Allows disconnected data to be examined and modified through a
collection of DataColumn and
DataRow classes. The DataTable
allows constraints such as foreign keys and unique constraints to be
defined using the Constraint class.
- DataColumn
-
Corresponds to a column in a table. The DataColumn
class stores metadata about the structure of the column that,
together with constraints, defines the schema of the table. The
DataColumn can also create expression columns
based on other columns in the table.
- DataRow
-
Corresponds to a row in a table and can examine and update data in
the DataTable. The DataTable
exposes DataRow objects through the
DataRowCollection object it contains. The
DataRow caches changes made to data contained in
its columns, storing both original and current values. This allows
changes to be cancelled or to be later reconciled with the data
source.
- Constraint
-
Allows constraints to be placed on data stored within a
DataTable. Unique and foreign key constraints can
be created to maintain data integrity.
- DataRelation
-
Provides a way to indicate a relationship between different
DataTable objects within a
DataSet. The DataRelation
relates columns in the parent and child tables allowing navigation
between the parent and child tables and referential integrity to be
enforced through cascading updates and deletes.
- DataView
-
Allows data, once retrieved into a DataSet or
DataTable, to be viewed in different ways. It
allows data to be sorted based on column values and for a subset of
the data to be filtered so that only rows matching specified criteria
are displayed.
Chapter 2 examines .NET data providers in more
detail and describes the way in which they provide connected
functionality and serve as a bridge to disconnected
functionality.
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