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CHAPTER 3 s WEB FORMS WITH ASP.NET
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Listing 3-10. Web UI Containing a List Box
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<%@ Page Language="C#" AutoEventWireup="true" Inherits="_Default" %> CodeFile="Default.aspx.cs"
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" > <head runat="server"> <title>Untitled Page</title> </head> <body> <form id="form1" runat="server"> <div> <asp:ListBox ID="ListBox1" runat="server" OnSelectedIndexChanged="ListBox1_SelectedIndexChanged"> <asp:ListItem Value="Test1"></asp:ListItem> <asp:ListItem Value="Test2"></asp:ListItem> <asp:ListItem Value="Test3"></asp:ListItem> </asp:ListBox> </div> </form> </body> </html>
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Visual Studio will create a code-behind page for this, which includes a Page_Load event handler. Add a handler for the ListBox1_SelectedIndexChanged event that looks like this:
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protected void ListBox1_SelectedIndexChanged(object sender, EventArgs e) { string strTest = ListBox1.SelectedItem.Text; }
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Put a breakpoint in this code (by selecting the line and pressing F9) and run the application. Select an item on the list. Nothing happens the event doesn t fire because there is no postback typically associated with a list box. Only form submit buttons cause postbacks, at least when you re not using ASP.NET. Now, there are a couple of things you can do here. First of all, you can put a button on the form, and when you press the button, a postback will be triggered. The postback contains information for all raised events, not just the action that caused the postback (the
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CHAPTER 3 s WEB FORMS WITH ASP.NET
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submit button). To see the postback and how it contains information for all events, add an ASP .NET button to the form, run it, select a list item, and then click the button. You ll see that your breakpoint on the ListBox s SelectedIndexChanged handler still trips, even though your action was to click the form s submit button. (Note that if you provided a handler for the submit button, that handler code would also be executed.) This demonstrates that ASP.NET passed the event information for everything on the page when you triggered the postback. The information for the list selection changed got passed along, too, and when the server received the postback, it triggered the event. Alternatively, you can use the automatic postback feature that the ASP.NET ListBox provides you with by setting the ListBox.AutoPostBack property to true. You can do this with the Property Editor or simply by adding the AutoPostBack='True' attribute to the <asp:ListBox> declaration. Remove the button from the page and try running the application again with the AutoPostBack='True' attribute present in the list box s markup. Now whenever you select an item on the list box, the page triggers a postback, and the SelectedIndexChanged event fires. Right now, this just loads the text of the selection into a dummy string. In your application, you could use this to give feedback to the user, or you could determine its index to for example enter selection data into a database. Thus, when using ASP .NET, you have a number of options for how and when events are handled by the framework, and you can design your UI accordingly.
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View State
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I ve mentioned view state a couple of times in this chapter, but what is it And what does it do for you Typically, web applications are stateless. Normally when we think of a stateless web application, we re thinking of an application that doesn t correlate invocations of the various pages in the application. Conversely, if you visit an online store and place an item in your shopping cart, you expect that item to be available in your shopping cart for checkout. This is known as session state. But there is another type of stateless behavior going on here the server will lose the page s control information between postbacks unless we do something about it. From HTML s inception, servers (for performance reasons) throw control values away when pages are posted back. To do otherwise typically consumes resources on the server and dramatically affects the scalability of the application. And as we saw earlier, even in the simplest of applications adding two numbers statelessness drastically affects the usability of the application because of this. A number of methods, such as cookies, could be used to maintain state on a page, but they can lead to security problems and privacy issues. Additionally, cookie use often requires a lot of development overhead, particularly for sophisticated applications. .NET has introduced the concept of view state to overcome these problems while ASP .NET page, you also minimizing the impact on the developer. When you define an ASP
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