Visual Basic.NET How to Program (PDF)

Chapter 20 ASP .NET, Web Forms and Web Controls 1011
Fig. 20.39 Code-behind file for the login page for authors application (part 5 of 5).
In Fig. 20.39, the Page_Init event handler is defined in lines 42–77. If the page is
being loaded for the first time, lines 50–76 execute. The database code is contained within
a Try/Catch/Finally block (lines 53–74) to handle any database connectivity exceptions
and to ensure that the database is closed. Lines 58–59 execute the SQL query that
Visual Studio generates at design time—this query simply retrieves all the rows from the
Authors table of the Books database. Lines 63–65 iterate through the rows, placing the
item in the first column of each row (the author name) into nameList.
The reader might notice that we use an OleDbDataReader, an object that reads data
from a database. We did not use an object of this type before, because the OleDbDataReader
is not as flexible as other readers we discussed in Chapter 19. The object can read
data, but cannot update it. However, we use OleDbDataReader in this example because
we need only read the authors’ names; this object provides a fast and simple way to do so.
In this example, we use a CustomValidator to validate the user’s password. We
define a handler (lines 80–124) for the ServerValidate event of the CustomValidator,
which executes every time the user clicks Submit. This event handler contains
a ServerValidateEventArgs parameter called args. The object referenced by
args has two important properties: Value, which contains the value of the control that
the CustomValidator is validating, and IsValid, which contains a Boolean representing
the validation result. Once the event handler completes, if IsValid is True,
the HTML form is submitted to the Web server; if IsValid is False, the Custom-
Validator’s ErrorMessage is displayed, and the HTML form is not submitted to
the Web server.
To create and attach an event handler for the ServerValidate event, double-click
CustomValidator. The definition for this event handler (lines 80–124) tests the
selected user name against the password provided by the user. If they match, the user is