create barcode bitmap c# HTTP Handlers, Modules, and Routing in C#

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4 HTTP Handlers, Modules, and Routing
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The context argument is the current instance of the HttpContext class and represents the context of the request. A reference to the HTTP context is also passed as the custom data sent to the handler to process the request. The extraData parameter in the BeginProcessRequest signature is used to represent the status of the asynchronous operation. The BeginProcessRequest method returns an object of type HttpAsyncResult a class that implements the IAsyncResult interface. The IAsyncResult interface contains a property named AsyncState that is set with the extraData value in this case, the HTTP context. The OnCompletionCallback method is an internal method. It gets automatically triggered when the asynchronous processing of the request terminates. The following listing illustrates the pseudocode of the HttpRuntime private method:
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// The method must have the signature of an AsyncCallback delegate private void OnHandlerCompletion(IAsyncResult ar) { // The ar parameter is an instance of HttpAsyncResult HttpContext context = (HttpContext) ar.AsyncState; // Retrieves the instance of the asynchronous HTTP handler // and completes the request IHttpAsyncHandler asyncHandler = context.AsyncAppHandler; asyncHandler.EndProcessRequest(ar); // Finalizes the request as usual ... }
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The completion handler retrieves the HTTP context of the request through the AsyncState property of the IAsyncResult object it gets from the system. As mentioned, the actual object passed is an instance of the HttpAsyncResult class in any case, it is the return value of the BeginProcessRequest method. The completion routine extracts the reference to the asynchronous handler from the context and issues a call to the EndProcessRequest method:
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void EndProcessRequest(IAsyncResult result);
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The EndProcessRequest method takes the IAsyncResult object returned by the call to BeginProcessRequest. As implemented in the HttpApplication class, the EndProcessRequest method does nothing special and is limited to throwing an exception if an error occurred.
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Implementing Asynchronous Handlers
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Asynchronous handlers essentially serve one particular scenario a scenario in which the generation of the markup is subject to lengthy operations, such as time-consuming database stored procedures or calls to Web services. In these situations, the ASP.NET thread in charge of the request is stuck waiting for the operation to complete. Because threads are valuable resources, lengthy tasks that keep threads occupied for too long are potentially the perfect scalability killer. However, asynchronous handlers are here to help.
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Part I The ASP.NET Runtime Environment
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The idea is that the request begins on a thread-pool thread, but that thread is released as soon as the operation begins. In BeginProcessRequest, you typically create your own thread and start the lengthy operation. BeginProcessRequest doesn t wait for the operation to complete; therefore, the thread is returned to the pool immediately. There are a lot of tricky details that this bird s-eye description just omitted. In the first place, you should strive to avoid a proliferation of threads. Ideally, you should use a custom thread pool. Furthermore, you must figure out a way to signal when the lengthy operation has terminated. This typically entails creating a custom class that implements IAsyncResult and returning it from BeginProcessRequest. This class embeds a synchronization object typically a ManualResetEvent object that the custom thread carrying the work will signal upon completion. In the end, building asynchronous handlers is definitely tricky and not for novice developers. Very likely, you are more interested in having asynchronous pages than in generic asynchronous HTTP handlers. With asynchronous pages, the lengthy task is merely the ProcessRequest method of the Page class. (Obviously, you configure the page to execute asynchronously only if the page contains code that starts I/O-bound and potentially lengthy operations.) ASP.NET offers ad hoc support for building asynchronous pages more easily and more comfortably than through HTTP handlers. Caution I ve seen several ASP.NET developers use an .aspx page to serve markup other than HTML markup. This is not a good idea. An .aspx resource is served by quite a rich and sophisticated HTTP handler the System.Web.UI.Page class. The ProcessRequest method of this class entirely provides for the page life cycle as we know it Init, Load, and PreRender events, as well as rendering stage, view state, and postback management. Nothing of the kind is really required if you only need to retrieve and return, say, the bytes of an image. HTTP handlers are an excellent way to speed up particular requests. HTTP handlers are also a quick way to serve AJAX requests without writing (and spinning up) the whole machinery of Windows Communication Foundation (WCF) services. At the very end of the day, an HTTP handler is an endpoint and can be used to serve data to AJAX requests. In this regard, the difference between an HTTP handler and a WCF service is that the HTTP handler doesn t have a free serialization engine for input and output values.
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