ASP.NET学生网上选课系统论文+开题报告+英文文献+任务书 第4页
a problem. Developers are forced to test their websites with differ-ent operating systems and browsers, and they might even need to distribute browser updates to their clients. In other words, the client-side model sacrifices some of the most important benefits of web development.or that reason, ASP.NET is designed as a server-side technology. All ASP.NET code executes on the server. When the code is finished executing, the user receives an ordinary HTML page, which can be viewed in any browser. Figure 1-3 shows the difference between the server-side and client-side model.These are some other reasons for avoiding client-side programming:Isolation: Client-side code can’t access server-side resources. For example, a client-side application has no easy way to read a file or interact with a database on the server (at least not without running into problems with security and browser compatibility).Security: End users can view client-side code. And once malicious users understand how an application works, they can often tamper with it.ome cases, ASP.NET allows you to combine the best of client-side programming with server-side programming. For example, the best ASP.NET controls can intelligently detect the features of the client browser. If the browser supports JavaScript, these controls will return a web page that incorporates JavaScript for a richer, more responsive user interface. However, no matter what the capabilities of the browser, your code is always executed on the server.State limitations: To ensure optimum performance, the Web is built on stateless protocols, which means as soon as a page is sent to a user, the connection is closed and any user-specific information is discarded. ASP includes a session state feature that allows programmers to work around this problem. Using session state, a web application can retain temporary information about each client in server memory. However, session state is useless in scenarios where a website is hosted by several separate web servers. In this scenario, a client might access server B while its session information is trapped on server A and essentially abandoned. ASP.NET corrects this problem by allowing state to be stored in a central repository, such as a separate process or a database that all servers can access.ASP.NET deals with these problems (and many more) by introducing a completely new model for web pages. This model is based on a remarkable piece of technology called the NET Framework.You should understand that the .NET Framework is really a cluster of several technologies:The .NET languages: These include C# and VB .NET (Visual Basic .NET), the object-oriented and modernized successor to Visual Basic 6.0; these languages also include JScript .NET (a server-side version of JavaScript), J# (a Java clone), and C++ with Managed Extensions.he CLR (Common Language Runtime): The CLR is the engine that executes all .NET programs and provides automatic services for these applications, such as security checking, memory management, and optimization.The .NET Framework class library: The class library collects thousands of pieces of prebuilt functionality that you can “snap in” to your applications. These features are sometimes organized into technology sets, such as ADO.NET (the technology for creating database applications) and Windows Forms (the technology for creating desktop user interfaces).ASP.NET: This is the engine that hosts web applications and web services, with almost any feature from the .NET class library. ASP.NET also includes a set of web-specific services.sual Studio: This optional development tool contains a rich set of productivity and debugging features. The Visual Studio setup CDs (or DVD) include the complete .NET Framework, so you won’t need to download it separately.Interestingly, C# and VB .NET are actually far more similar than Java and C# or than VB6 and VB .NET. Though the syntax is different, both C# and VB .NET use the .NET classlibrary and are supported by the CLR. In fact, almost any block of C# code can be translated, line by line, into an equivalent block of VB .NET code. An occasional language difference pops up (for example, C# supports a language feature called anonymous methods, while VB .NET doesn’t), but for the most part, a developer who has learned one .NETlanguage can move quickly and efficiently to another.n short, both C# and VB .NET are elegant, modern languages that are ideal for creating the next generation of web applications..NET 1.0 introduced completely new languages. However, the changes in the .NET 2.0 languages are much more subtle. Both C# 2005 and VB 2005 add a few new features, but most parts of these languages remain unchanged. As a result, any code written according to version 1.0 of the C# language will work identically with version 2.0. In Chapters 2 and 3, you’ll sort through the syntax of C# and learn the basics of object-oriented programming. By learning the fundamentals before you start creating simple web pages, you’ll face less confusion and move more rapidly to advanced topics such as database access and web services.he CLR runs only IL code, which means it has no idea which .NET language you orig-inally used. Notice, however, that the CLR actually performs another compilation step—it takes the IL code and transforms it to native machine language code that’s appropriate for the current platform. This step occurs when the application is launched, just before the code is actually executed. In an ASP.NET application, these machine-specific files are cached while the web application is running so that they can be reused, ensuring optimum performance.You might wonder why .NET compilers don’t compile straight to machine code. The reason is that the machine code depends on several factors, including the CPU. For example, if you create machine code for a computer with an Intel processor, the compiler may be able to use Hyper-Threading to produce enhanced code. This machine-specific version isn’t suitable for deployment to other computers, because no guarantee exists that they’re using the same processor.Another welcome change in Visual Studio 2005 is the support for different coding models. While Visual Studio .NET 2003 locked developers into one approach, Visual Studio 2005 supports a range of different coding models, making it a flexible, all-purpose design tool. That means you can choose to put your HTML tags and event handling code in the same file or in separate files without compromising your ability to use Visual Studio and benefit from helpful features such as IntelliSense.
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