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Understanding Code-Behind in .NET
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Page_PreRender: This method executes after all control events for the page have been executed, but before any HTML has been generated for the page Page_Unload: This event is typically where you put any cleanup tasks required by the application, such as closing files or terminating database connections Auto-event wireup is controlled (no surprise) by the AutoEventWireup attribute on the Page directive that appears at the top of every aspx file For example:
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<%@ Page Language= C# AutoEventWireup= true CodeFile= Defaultaspxcs Inherits= _Default %>
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Understanding Postback, Events, and Code-Behind
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When you use Visual Basic, AutoEventWireup is set to False Then the methods that handle page-level events must specify a Handles clause, as in this example:
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Protected Sub Page_Load(ByVal sender As Object, _ ByVal e As SystemEventArgs) Handles MeLoad End Sub
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Code-behind is one of the most important concepts (and techniques) in ASPNET programming It uses a separate file to contain the VBNET or C# code that s executed in response to events and only that code rather than intermingling the VBNET or C# code with the aspx code that defines the Web page The main benefit of code-behind is to separate the code that defines the appearance of a Web page from the code that defines the behavior of the page Result: code that is easier to write, debug, and maintain And it lets you direct your focus while you re developing your applications You can begin by creating a basic page that has the controls you need, without worrying too much about how the page looks Then you can focus on getting the code to work so the page does its job correctly After you have the code working, you can return to the aspx file to refine the appearance of the page In the real world, there isn t always a clean separation between the appearance of a page and its behavior Often, as a result, you end up alternating between tweaking the aspx file and tinkering with the code-behind file; changes in one file inevitably affect code in the other file Even so, separating the appearancemaking code from the behavioral code is a big step forward
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To understand how code-behind works, you need to understand two things: partial classes and code generation A partial class is a VBNET or C# class defined by two or more files Partial classes are integral to code-behind because a Web page must be defined by a single class that represents the controls on the page (that is, the appearance of the page) as well as the event handlers for the page (that is, the behavior of the page) The code-behind file is a partial class that supplies the methods that handle events The other part of the class is what the aspx file provides Code generation (that is, the automatic creation of code) is a required step because the aspx file is not written in either C# or Visual Basic Instead, it consists of a mixture of HTML and ASPNET markup tags When ASPNET processes a page, it reads this markup and then generates C# or VBNET code which represents the page, its HTML, and all its controls This code is a partial class that matches the partial class represented by the code-behind file Thus the complete processing for an ASPNET page consists of the following events:
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1 The aspx file is processed by ASPNET, which generates a source file
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that defines a partial class that represents the HTML and controls for the page This source file is generated in C# or VBNET, depending on the language specified for the page
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2 The source file generated in Step 1 is compiled by the C# or VBNET
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compiler to create a partial class
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3 The code-behind file is compiled by the C# or VBNET compiler to
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create a partial class
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4 The partial classes created in Steps 2 and 3 are combined to create
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the complete class that represents the page
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5 The resulting class is loaded into memory and executed
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Code-behind isn t the only code model supported by ASPNET If you prefer, you can also mix code directly in the aspx file, eliminating the need for a code-behind file To do so, you simply uncheck the Place code in separate file option when you create a new Web page Then, Visual Studio inserts special <script> tags that contain the code that would otherwise be placed in the code-behind file