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ADO.NET in Disconnected Mode
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Figure 22-4 When the end user deletes a department in the upper grid, the DeptId fields of the child rows in the Employees table become null (lower grid).
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The DataAdapter Class
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In the first portion of this chapter, I showed you how to create a DataSet object, load it with data produced by your application (or read from a text file), create constraints and relationships, and define calculated fields. In other words, I showed you how to use the DataSet as a sort of scaled-down client-side database that your code defines and fills with data. While this functionality can be useful in many scenarios, the major ity of .NET applications have to process data coming from a real database, such as Access, SQL Server, or Oracle. The key to using the DataSet in this way is the DataAdapter object, which works as a connector between the DataSet and the actual data source. The DataAdapter is in charge of filling one or more DataTable objects with data taken from the database so that the application can then close the connection and work in a completely discon nected mode. After the end user has performed all his or her editing chores, the appli cation can reopen the connection and reuse the same DataAdapter object to send changes to the database. Admittedly, the disconnected nature of the DataSet complicates matters for developers, but it greatly improves its versatility. You can now fill a DataTable with data taken from any data source whether it s SQL Server, a text file, or a mainframe and process it with the same routines, regardless of where the data comes from. The decoupled archi tecture based on the DataSet and the DataAdapter makes it possible to read data from one source and send updates to another source when necessary. You have a lot more freedom working with ADO.NET, but also many more responsibilities.
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Note The code samples that follow assume a proper connection string was defined previ ously and stored in a global variable, as described in Setting the ConnectionString Property section in 21.
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Part V:
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Introducing the DataAdapter
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The first thing you need to know about the DataAdapter is that there s actually one DataAdapter class for each .NET data provider, so you have an OleDbDataAdapter, an SqlDataAdapter, an OdbcDataAdapter, and an OracleDataAdapter class. All these classes expose the same set of properties and methods because they inherit from the DbDataAdapter abstract type. Except for their names and a few other details such as how they deal with parameters you use these classes in exactly the same way.
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The DataAdapter s constructor is overloaded to take zero, one, or two arguments. In its most complete form, you pass to it a SQL SELECT statement (or an ADO.NET Com mand object containing a SQL SELECT statement) and a Connection object, as in this code snippet:
Dim cn As New OleDbConnection(BiblioConnString) cn.Open() Create a DataAdapter that reads and writes the Publishers table.
Dim sql As String = SELECT * FROM Publishers"
Dim da As New OleDbDataAdapter(sql, cn)
Or you can create a DataAdapter and then assign an ADO.NET Command object to its SelectCommand property:
da = New OleDbDataAdapter()
da.SelectCommand = New OleDbCommand(sql, cn)
Filling a DataTable
Once you ve created a DataAdapter object and defined its SELECT command, you can use the object to fill an existing or new DataTable of a DataSet with the Fill method, which takes the name of the target DataTable in its second argument:
Read the Publishers database table into a local DataTable.
Dim ds As New DataSet()
da.Fill(ds, Publishers )
cn.Close()
Because of the disconnected nature of the DataSet, the action of opening a connection only for the short time necessary to read data from a single database table is so fre quent that Microsoft engineers provided the DataAdapter object with the ability to open the connection automatically and close it immediately at the completion of the Fill method. For this reason, the preceding code can be written in a more concise way, as you see here:
Define the connection; no need to open it. Dim cn As New OleDbConnection(BiblioConnString)
22:
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