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Creating Quick Response Code in C#.NET Handling Events and Managing State

ChAPTER 3
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Handling Events and Managing State
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FigURE 3-5 Setting AutoPostBack to true.
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1 . Which event would you handle to set the value of a Label control 2. What happens in the Init event of a page
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1 . Usually, you would handle the Page.Load event. Because other parts of a
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form probably do not rely on the value of a Label, you could also handle the Page.PreInit, Page.Init, or Page.Render event.
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2. Each child control of the page is initialized to its design-time value.
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Exploring the Webpage Life Cycle Events
In this practice, you explore the webpage life cycle to gain an understanding of the events and when they are triggered.
oN the CoMpaNIoN MedIa
If you encounter a problem completing an exercise, you can find the completed projects in the samples installed from this book s companion CD. For more information about the project files and other content on the CD, see Using the Companion Media in this book s Introduction.
Lesson 1: Understanding the ASP.NET Life Cycle and Handling Events
ChAPTER 3
E xErcIsE 1 Configuring Webpage Event Handlers
In this exercise, you configure event handlers for some of the webpage and server control events. You then run the webpage to display the order in which these events are fired by ASP.NET.
Open Visual Studio and create a new website called LifeCycleEvents by using your preferred programming language. This practice assumes that you are using the codebehind model (and not the single-page model). Open the code-behind file for the Default.aspx page. Add a Page.Load event handler to the page (the handler is there by default in C#). Recall that the Visual Basic process for this is different than the one for C# (see Creating Event Handlers, earlier in this lesson). In the Page.Load event handler, add code that will write to the Output window in Visual Studio through the System.Diagnostics.Debug class. The following code provides an example.
Sample of Visual Basic Code Protected Sub Page_Load(ByVal sender As Object, _ ByVal e As System.EventArgs) Handles Me.Load System.Diagnostics.Debug.WriteLine("Page_Load") End Sub Sample of C# Code protected void Page_Load(object sender, EventArgs e) { System.Diagnostics.Debug.WriteLine("Page_Load"); }
2. 3.
Add event handlers for the PreInit, Init, PreRender, and Unload events. In each, place a call to Debug.Write. Each call should write out the respective event name associated with the handler. These additional handlers should look like the following.
Sample of Visual Basic Code Protected Sub Page_PreInit(ByVal sender As Object, _ ByVal e As System.EventArgs) Handles Me.PreInit System.Diagnostics.Debug.WriteLine("Page_PreInit") End Sub Protected Sub Page_Init(ByVal sender As Object, _ ByVal e As System.EventArgs) Handles Me.Init System.Diagnostics.Debug.WriteLine("Page_Init") End Sub
ChAPTER 3
Handling Events and Managing State
Protected Sub Page_PreRender(ByVal sender As Object, _ ByVal e As System.EventArgs) Handles Me.PreRender System.Diagnostics.Debug.WriteLine("Page_PreRender") End Sub Protected Sub Page_Unload(ByVal sender As Object, _ ByVal e As System.EventArgs) Handles Me.Unload System.Diagnostics.Debug.WriteLine("Page_Unload") End Sub Sample of C# Code protected void Page_PreInit(object sender, EventArgs e) { System.Diagnostics.Debug.WriteLine("Page_PreInit"); } protected void Page_Init(object sender, EventArgs e) { System.Diagnostics.Debug.WriteLine("Page_Init"); } protected void Page_PreRender(object sender, EventArgs e) { System.Diagnostics.Debug.WriteLine("Page_PreRender"); } protected void Page_Unload(object sender, EventArgs e) { System.Diagnostics.Debug.WriteLine("Page_Unload"); }
Run the web application in Debug mode (click the Start Debugging button on the standard toolbar). You might receive a prompt stating that the website cannot be debugged until you enable debugging in the Web.config file. Allow Visual Studio to enable debugging, and click OK to continue. The Default.aspx page should be displayed in a browser window. In Visual Studio, locate the Output window (View Output). You should see the results of the Debug.WriteLine calls at the bottom of the Output window, as shown in Figure 3-6. Notice the order in which the events fired.
Lesson 1: Understanding the ASP.NET Life Cycle and Handling Events
ChAPTER 3
FigURE 3-6 The Output window in Visual Studio showing the order in which the page events fired.
Reload the webpage in your browser. Notice that all the same events are processed again.
E xErcIsE 2 Using Postback
In this exercise, you configure a web form that allows a user to either log on with a user name and password or to access the site as a guest by using an email address. If the user chooses to log on as a registered user, he or she should be prompted for both a user name and a password. If the user chooses to log on as a guest, he or she should be prompted for an email address only.
Open Visual Studio and create a new website by using the ASP.NET Web Site template and your preferred programming language. This practice assumes that you are using the code-behind model (and not the single-page model). Create a second content page named home.aspx. Remove any existing controls from your Default.aspx page. Then add the following controls:
2. 3.
A DropDownList named UserTypeDropDownList. Add two items to the DropDownList with the text Registered User and guest. A Label named UsernameLabel. A TextBox named UsernameTextbox. A Label named PasswordLabel. Set PasswordLabel.Text to Password:. A TextBox named PasswordTextbox. A Button named Logonbutton.
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