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1、Introduction to ASP.NET DevelopmentTo overcome the performance and scalability problems that CGI brings, Microsoft developed a new way for developers to build scalable applications. This high performance alternative is called the Internet Server Application Programming Interface(ISAPI). Instead of h
2、ousing functionality in executable files, ISAPI uses DLLs. Using DLLs instead of executable programs has some definite performance and scalability advantagesThe ISAPI extension could also be called with arguments that will allow a single ISAPI extension to perform multiple tasks. Just as in the CGI
3、example, the directory must have execute permissions enabled, or the DLL will be downloaded to the client rather than run on the server. ISAPI extensions are typically used to process client requests and output a response as HTML, which is very similar to the way CGI programs are used.ISAPI filters
4、perform a function that cant be directly duplicated with CGI applications. ISAPI filters are never explicitly called; instead, they are called by IIS in response to certain events in the life of a request. The developer can request that an ISAPI filter be called whenever any of the following events
5、occur:1When the server has preprocessed the client headers2When the server authenticates the client3When the server is mapping a logical URL to a physical URL4Before raw data is sent from the client to the server5After raw data is sent from the client to the server but before the server processes it
6、 6When the server logs information7When the session is endingAs with any filter, ISAPI filters should request only the notifications it requires and process them as quickly as possible. One of the more common uses of ISAPI filters is to provide custom authentication. Another use is to modify the HTM
7、L that will be sent to the client. For example, an ISAPI filter could be used to change the background color of each page. Because ISAPI filters arent nearly as common as ISAPI extensions, I wont cover them any further in this book. If you want to learn more about ISAPI extensions, you can check out
8、 my book Inside Server-Based Applications (Microsoft Press, 1999).ISAPI specifies several entry-point functions that must be exported from the DLL. Using these entry points, IIS can load the DLL; call the functions that it implements, passing in parameters as required; and receive the data to write
9、back to the browser. ISAPI requires only two entry-point functions to be implemented these entry points, IIS can load the DLL; call the functions that it implements, passing in parameters as required; and receive the data to write back to the browser. ISAPI requires only two entry-point functions to
10、 be implementedA Better Solution: Active Server PagesIf youre wondering why weve dwelt on the alternatives to ASP.NET in a book about programming ASP.NET, the answer lies in the details of the implementation of ASP.NET and its predecessor, Active Server Pages (ASP). Understanding ISAPI is required f
11、or adept understanding of ASP and thus ASP.NET.During the beta of IIS 2.0, which became part of Windows NT 4.0, Microsoft introduced a new technology initially codenamed “Denali.” This was during Microsofts “Active” period and so the technology was eventually named Active Server Pages, or ASP. Sever
12、al versions of ASP.NET have been released, most notably the versions included with Windows NT 4.0 Option Pack (ASP 2.0 and IIS 4.0) and Windows 2000 (ASP 3.0 and IIS 5.0). For the purposes of this discussion, Ill consider ASP as a whole, without referring t o version differencesASP.NET became an ins
13、tant hit, in large part because it made something that was difficult(create dynamic Web content) relatively easy. Creating CGI applications and ISAPI applications wasnt terribly difficult, but using ASP was much simpler By default, ASP uses VBScript. Literally millions of developers are at least som
14、ewhat familiar with Visual Basic, Visual Basic for Applications (VBA), or VBScript. For these developers, ASP was the way to enter the Internet age. Certainly the developers could have learned a new programming language, but they didnt have to with ASP. Partly because of its use of VBScript, ASP bec
15、ame a viable way to build Web applications.Just as important was the relatively easy access to databases allowed through Microsoft ActiveX Data Objects (ADO). When you need to generate dynamic content, that dynamic content obviously needs to come from somewhere, and ADO made it easy to get at that d
16、ata.Finally, and perhaps most important, the ASP.NET development model allowed developers to essentially write code and run it. There was no need to perform compilation or elaborate installation steps. the ASP.NET architects were careful to capture this same development model, even though whats going on under the covers is quite a bit different.A New Solution: ASP.NETWhen version 3.0 of ASP.NET was released along with Windows 2000, it became c
