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PART III
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NOTE The exception class is always called e; this has become the convention
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The exception processing proceeds from the top down through the catch statement If a base exception class is higher in the listing than a child exception class, the child exception will never be caught EXAM TIP The DataException is the base exception for all the ADONET exceptions You will see this structure in use in the rest of this chapter as we start to use data sources programmatically in the Web Forms
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Accessing Data Programmatically
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Even though we can quickly build a Web Form with very little coding, the end result will, in most cases, be an application that we still want to tune and control a bit more This can be done by using the ADONET objects directly in code In this section, we will look closer at how we can work with databases programmatically
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First, Table 16-2 gives a refresher of the objects from 10 and what they are used for The prefix xxx used in the table identifies the three object variations for the three managed data providers SQL, OLE DB, and ODBC The prefixes are sql, oleDb, and odbc EXAM TIP Remember that the SQL managed provider only works with Microsoft SQL Server 7 and above
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Connecting to a Data Source
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To connect to a data source, we need to select the appropriate managed provider For the rest of this chapter, we will assume (as does Microsoft) that we are using Microsoft SQL Server 2000 as the data source When you create the connection object, you need to supply a connection string that defines, among other things, the name of the database and how you authenticate A sample connection string looks like this:
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data source=KENSNABBEN; initial catalog=Address; user id=sren; password="123";
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The data source parameter defines the server name, the initial catalog is the name of the database you want to connect to, the user id is the name you want to connect as, and the password is the password for the user id The connection string can be built by hand, or you can use the Data Link Properties dialog box (shown earlier in Figure 16-2) to build it If you use the Data Link dialog box, the connection string will be saved by Visual Studio NET so you can retrieve the string easily in other projects The Data Link dialog box was described in the Using a DataGrid Server Control section, earlier in this chapter
Object
Description
xxxConnection xxxDataAdapter
xxxCommand DataSet xxxDataReader Table 16-2
Defines the way we find and authenticate with a data source Defines four commands that are used to SELECT, INSERT, UPDATE, and DELETE data from the data source The DataAdapter is commonly used to create a DataSet that is then used for data processing Creates an object that defines an SQL statement or a stored procedure that will be reused throughout the application Provides a disconnected representation of the data It contains multiple tables that can be edited, displayed, or used Provides a connected, read-only, forward-only way of manipulating the data
The NET Data Provider Objects
16: Consuming and Manipulating Data from Data Sources
The recommended procedure for creating a connection to a data source is as follows: Create a global string to hold the connection string, and use the Application object to store the string This way; if the string needs to be changed, you only need to perform the change once, and because all connections to a particular database use the same string, the Connection Manager can take advantage of connection pooling Create the connection as a local object, and explicitly destroy the connection when it is no longer needed If the procedure where the object was created goes out of scope, the connection should be destroyed by the garbage collector That will not happen directly, nor can it be predicted the connection will be active until the garbage collector runs The connections that are held open on the database server will effectively limit the performance of the server The following code segment illustrates how to connect to a database In this example, the connection is made to the Address database on a Microsoft SQL Server 2000 using the name KENSNABBEN
string strSQL = "user id=sa;password;initial catalog=Address;" + "data source=KENSNABBEN"; SqlConnection addressConnection = new SqlConnection(strSQL); try { addressConnectionOpen(); // perform the data processing } catch (SqlException e) { // we will get here if the connection failed to open // process the error by, for example, throwing an exception to the caller // to try another connection } finally { // close the connection and set the reference to null addressConnectionClose(); addressConnection = null; }
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