Extend
the
form to add a label that will display the selected book. Set the
autopostback
attribute of the drop-down control to
true
, so that when a selection is made, the page
is posted back to the server and the label can be filled with the
selected title.
<asp:DropDownList ID="ddlBooks" autopostback="True"
The problem you’ll encounter is that when the page
is reloaded the data will be reloaded as well, and your selection
will be lost. You must protect against that by checking the
page’s
IsPostBack property, which is set to
true
when the page is posted back, as shown in
Example 9-1 (VB.NET) and Example 9-2 (C#).
It is a recurring theme throughout this book that the differences between C# and VB.NET are syntactic sugar, and not terribly important. Even more interesting, if you know one language, the other is quite understandable. This example shows that quite clearly.
Example 9-1. Checking IsPostBack in VB.NET
Private Sub Form_Load(ByVal sender As object, _ ByVal e As System.EventArgs) Handles MyBase.LoadIf Not IsPostBack( ) Then
' create the array list
Dim bookList As New ArrayList( )
' add all the books
bookList.Add("Programming ASP.NET")
bookList.Add("Programming C#")
bookList.Add("Teach Yourself C++ In 21 Days")
bookList.Add("Teach Yourself C++ In 24 Hours")
bookList.Add("TY C++ In 10 Minutes")
bookList.Add("TY More C++ In 21 Days")
bookList.Add("C++ Unleashed")
bookList.Add("C++ From Scratch")
bookList.Add("XML From Scratch")
bookList.Add("Web Classes FS")
bookList.Add("Beg. OO Analysis & Design")
bookList.Add("Clouds To Code")
bookList.Add("CIG Career Computer Programming")
' set the data source
ddlBooks( ).DataSource = bookList
' bind to the data
ddlBooks().DataBind( )
Else
lblMsg().Text = "Selected book: " & ddlBooks( ).SelectedItem.Text
End If
End Sub
Example 9-2. Checking IsPostBack in C#
private void Page_Load(object sender, System.EventArgs e) {if (! Page.IsPostBack)
{
// create the array list
ArrayList bookList = new ArrayList( );
// add all the books
bookList.Add("Programming ASP.NET");
bookList.Add("Programming C#");
bookList.Add("Teach Yourself C++ In 21 Days");
bookList.Add("Teach Yourself C++ In 24 Hours");
bookList.Add("TY C++ In 10 Minutes");
bookList.Add("TY More C++ In 21 Days");
bookList.Add("C++ Unleashed");
bookList.Add("C++ From Scratch");
bookList.Add("XML From Scratch");
bookList.Add("Web Classes FS");
bookList.Add("Beg. OO Analysis & Design");
bookList.Add("Clouds To Code");
bookList.Add("CIG Career Computer Programming");
// set the data source
ddlBooks.DataSource=bookList;
// bind to the data
ddlBooks.DataBind( );
}
else
{
lblMsg.Text = "Selected book: " + ddlBooks.SelectedItem.Text;
}
}
In Example 9-1 and Example 9-2, the IsPostBack property is
tested. If it returns true
, then the page has been
posted back to itself by a user taking an action, and the selected
item is displayed. If the IsPostBack property returns
false
, then the book list is populated with data
from the
ArrayList.
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