Programming ASP.NET 3.5, 4th Edition

Tracking the Data Source with Events

Some programmers get nervous when a control does so much work invisibly. After all, when all goes well, it is great not to have to sweat the details, but if something does go wrong, how can you tell whether your connection failed, no records were updated, an exception was thrown, or exactly what happened? Related to that, what if you want to modify the behavior of the control in some way?

The ASP.NET controls in general, and the data controls in particular, overcome these concerns by providing numerous events that you can handle. For example, the SqlDataSource control described in this chapter has nine events you can handle that are not inherited from its parent control. There is an event you can handle when the DataSource is about to run a SELECT statement (Selecting) and one that you can handle immediately after the SELECT statement has finished executing (Selected). Indeed, there are three other pairs of events for each of the other SQL statements that a DataSource works with, as shown in Table 7-4. The ninth event, Filtering, can be used to validate or alter the values being used in WHERE clauses before they are used with a statement.

Table 7-4. SQL statements and their respective DataSource events

SQL statement	Event occurring before statement is run	Event occurring after statement has run
SELECT	Selecting	Selected
INSERT	Inserting	Inserted
UPDATE	Updating	Updated
DELETE	Deleting	Deleted
WHERE clause	Filtering	N/A

To see this at work, let’s use the Selected ...