create and print barcode c# On-Demand Compilation in Visual Studio .NET

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On-Demand Compilation
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Visual Studio .NET builds the compiled DLL before running the browser on the .aspx page, but ASP.NET works well even if no DLL has been created for the code-behind por tion of the form. You can deploy only the .aspx and the .vb (or .cs) source file, in which case the latter file is automatically compiled the first time a browser posts a request for the companion .aspx file. ASP.NET looks at the file s extension to decide which com piler Visual Basic, C#, or any other valid .NET language must be used to compile the source file, so it s crucial that you use the .vb extension for your Visual Basic .NET files.
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24:
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Web Forms and Controls
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The association between the .aspx file and its code-behind source module is held in the Src attribute of the @Page directive:
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<%@ Page Src="WebForm1.aspx.vb Inherits="FirstWebForm.WebForm1"%>
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The on-demand compilation model has no advantage over the Visual Studio .NET approach, except the ability to create Web Forms pages using a tool as simple as Notepad, and it has a couple of drawbacks. First, the compilation step adds a short delay the first time a page is requested. Second, and more important, you must deploy the source code file to have it compiled on the fly, so you might not be able to protect your intellectual property if you aren t in full control of the server computer. ASP.NET also supports a third code model, in which the .aspx file contains both the HTML text and the server-side code. In this model, server-side code is enclosed in <script> blocks with a runat="server" attribute, exactly as you find in classic ASP:
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<%@ Page Language="VB %> <SCRIPT RUNAT="server"> Private Sub btnAdd_Click(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles btnAdd.Click Dim res As Double = CDbl(txtOne.Text) + CDbl(txtTwo.Text) txtResult.Text = res.ToString End Sub </SCRIPT>
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This code model is often adopted in MSDN documentation and in many ASP.NET books and articles because it shows the HTML and the language code in a single place. But because this book is about using Visual Studio .NET, all my code samples use sep arate listings for the HTML and Visual Basic portions.
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The Page Life Cycle
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The great innovation of Web Forms is that they let you adopt the same event-driven programming model that made Visual Basic the most popular programming language under Windows. By and large, Web Forms controls expose the same events as their Windows Forms counterparts: Button controls expose a Click event, TextBox controls expose a TextChanged event, ListBox controls expose the SelectedIndexChanged event, and so on. The Page class is the ASP.NET equivalent of the form and exposes events such as Load and Unload. Even if the event-driven model is similar, you ll find an important difference between the Windows Forms and the Web Forms worlds. Events in Windows Forms applications fire as soon as end users operate on the user interface element that is, when they click on buttons, type something in a TextBox, or select another element in a ListBox control. On the other hand, Web Forms events fire on the server, not inside the browser, so their han dlers run only when the form is posted back to the server. This usually occurs when the user clicks on the submit button, even though other controls can fire a postback.
Part VI:
Internet Applications
This is what happens when a form is posted back to the server: 1. ASP.NET loads the code-behind class and fires the Page_Init event. At this time, your code can t determine whether this is the first time the form has been requested, nor can it retrieve the values the user typed in the form s controls. 2. After the page object and all its controls have been initialized, the Page_Load event fires. At this time, your code can determine whether the page is executing because of a postback operation (by checking the IsPostBack property) and can access current values in controls. Typically, you use this event to initialize controls and bind them to data sources. In other words, you use it for the kind of opera tions you perform in a Windows Forms Load event. 3. If this is a postback, all the events related to controls fire at this time. For example, TextChanged events for TextBox controls that have been modified by the end user fire now, as well as CheckedChanged events for CheckBox and RadioButton con trols that have been clicked. 4. The last control event that fires is the one that caused the postback action, usually the Click event of a Button or ImageButton control. 5. Finally, the Page object fires an Unload event when your code is expected to release all resources, close files and database connections, and so on. After this event all class-level variables are destroyed and are gone forever, unless you save their values somewhere (for example, in a session variable). The order in which control events fire isn t necessarily the order in which the end user operated on the corresponding controls. The Web Forms object fires these events by analyzing the current state of all the controls on the form posted back to the server and comparing it with the control s state at the time the form was sent to the browser. (This information is stored in the hidden __VIEWSTATE field.) For example, a TextBox s TextChanged event fires if the control contains a string other than its original value. Of course, you receive a single TextChanged event even if the end user modified the con trol s value several times before posting the form back to the server. Web Forms controls fire fewer events than their Windows Forms counterparts. For example, Web Forms controls don t expose events such as MouseEnter, MouseExit, GotFocus, and LostFocus because these events are typically used to provide instanta neous feedback to the end user. It makes little sense to cache them and fire them when the form is posted back to the server.
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