ASP.NET PDF Viewer using C#, VB/NET

The Main method waits until a key is pressed much like with work items in the thread pool, having active asynchronous requests will not keep the process alive, so the program would exit before finishing its work without that ReadKey. (A more robust approach for a real program that needed to wait for work to complete would be to use the CountdownEvent described earlier.) The Dns class will call the OnGetHostEntryComplete method once it has finished its lookup. Notice that the first thing we do is call the EndGetHostEntry method the other half of the APM. The End method always takes the IAsyncResult object corresponding to the call recall that this identifies the call in progress, so this is how EndGetHostEn try knows which particular lookup operation we want to get the results for.

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12.4 Summary

The APM says nothing about which thread your callback will be called on. In practice, it s often a thread pool thread, but not always. Some individual implementations might make guarantees about what sort of thread you ll be called on, but most don t. And since you don t usually know what thread the callback occurred on, you will need to take the same precautions you would when writing multithreaded code where you explicitly create new threads. For example, in a WPF or Windows Forms application, you d need to use the SynchronizationContext class or an equivalent mechanism to get back to a UI thread if you wanted to make updates to the UI when an asynchronous operation completes.

It involves understanding how your site will work and how users will add content My general rule of thumb is that if a content type is part of a section (that is, part of the forums, blogs, products, catalog items, and so on), requires a specific module, or has multiple nodes with multiple settings or fields, then you should probably create a content type Content types can also help solve problems For example, you can sit and stare at the screen for hours asking the question, How do I get a specific field to appear at this specific place Frequently, creating a content type will solve this problem..

The End method in the APM returns any data that comes out of the operation. In this case, there s a single return value of IPHostEntry, but some implementations may return more by having out or ref arguments. Example 16-16 then prints the results, and finally prints the AsyncState property of the IAsyncResult, which will be 42 this is where the value we passed as the final argument to BeginGetHostEntry pops out. This is not the only way to use the Asynchronous Programming Model you are allowed to pass null as the delegate argument. You have three other options, all revolving around the IAsyncResult object returned by the Begin call. You can poll the IsCompleted property to test for completion. You can call the End method at any time if the work is not finished this will block until it completes.* Or you can use the Asyn cWaitHandle property this returns an object that is a wrapper around a Win32 synchronization handle that will become signaled when the work is complete. (That last one is rarely used, and has some complications regarding ownership and lifetime of the handle, which are described in the MSDN documentation. We mention this technique only out of a pedantic sense of duty to completeness.)

* This isn t always supported. For example, if you attempt such an early call on an End method for a networking operation on the UI thread in a Silverlight application, you ll get an exception.

Ajax is an important technique to use with today s web applications. Using it effectively means that the majority of your users will see a quicker interaction with the web server, but it doesn t prevent users with JavaScript disabled from accessing the site. This is sometimes referred to as progressive enhancement. Unfortunately, with raw JavaScript the technique is cumbersome and error-prone. With good libraries such as jQuery and Microsoft Ajax, you can be much more productive.

You are required to call the End method at some point, no matter how you choose to wait for completion. Even if you don t care about the outcome of the operation you must still call the End method. If you don t, the operation might leak resources.