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Disconnected Object

The Disconnected Objects

Disconnected applications typically connect as late as possible and disconnect as early as they can. While they are working in a disconnected fashion, ADO.NET pools the actual physical connection between various requests.

The various objects in consideration under the disconnected model of ADO.NET are as follows:

DataSet: The DataSet is at the central core of the disconnected mode of ADO.NET data access. The best way to think of a DataSet is like having your own very mini relational database management system (RDBMS) completely represented in memory. While it isn’t quite an RDBMS and should never be thought to replace an RDBMS, it helps to understand a DataSet if its various components are connected on a one-to-one basis with most major RDBMS objects. Also, it is important to realize that DataSets are available at System.Data.DataSet, i.e., above any .NET provider, thus making them .NET data provider–independent (more about .NET data providers in the next section). A DataSet can also be thought of as a logical collection of DataTables and DataRelations.

DataTable: A DataTable is most similar to a table in a database. It consists of DataColumns, DataRows, and various constraints set upon them. It stores data in a row/column format. Starting with ADO.NET 2.0, a DataTable is fully convertible to XML and can be serialized just like a DataSet. For data access needs where your DataSet might contain only one DataTable, it may make more sense to use a DataTable instead. As you’ll see in future chapters, this is not only more convenient, but it’s also better performing.

DataRow: One of the properties of DataTable is Rows of DataRowCollection type, which represents an enumerable collection of DataRow objects. As data is filled into a DataTable, the DataRowCollection gets new DataRow objects added to itself. The best logical equivalent of a DataRow in a database is a row in a table.

 DataColumn: A DataTable also contains a Columns property of DataColumnCollection type. Essentially, this represents the structure of a DataTable. The best logical equivalent of a DataColumn object in a database is an individual column in a given table in a database.

 DataView: A DataView is most similar to a view in a database. A DataView allows you to create a “view” on a DataTable and view a subset of the data based on a preset condition specified in its Filter property. You could also use the Sort property to sort the filtered subset of the DataTable’s data. One DataTable can have multiple views defined on it.

Constraint: A DataTable contains yet another property called Constraints of ConstraintsCollection type. This lets you create ForeignKeyConstraint or UniqueConstraint objects and associate various columns to certain conditions based on which data in the DataTable must pass for it to exist in the DataTable. The most logical equivalent of a ForeignKeyConstraint is a foreign key in a database, and UniqueConstraint specifies a Unique condition on a given column in a database.

DataRelation: A DataSet, like a database, might contain various interrelated tables. A DataRelation object lets you specify relations between various tables that allow you to both validate data across tables and browse parent and child rows in various DataTables. Its most logical equivalent is a foreign key specified between two tables in a database. The difference between a ForeignKeyConstraint and a DataRelation is that a DataRelation, in addition to validating data, gives you a convenient mechanism to browse parent and child rows in a DataSet.

Figure below shows where the various connected and disconnected objects fit into the bigger picture.  

When compared with ADO, the data holder object, which used to be Recordset, is now a DataSet. However, there is a critical difference. While a Recordset was also responsible for communicating with the database, a DataSet is not responsible for communicating with the database. Instead, it uses the gateway object between connected and disconnected modes—the DataAdapter.
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