C h a pter 8 : Asyn c h ro n o u s Prolra m m i n l M o d e l s in Visual Studio .NET

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C h a pter 8 : Asyn c h ro n o u s Prolra m m i n l M o d e l s
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(Later in this book, in 1 6, Graphical User Interfaces, we'll examine a useful abstraction with the name of B a c k g r o u n d Wo r k e r This is a component that is specifically meant for maintaining responsive UIs with progress indicators, cancellation, and so on) Similarly, we could use a timeout to put an actual upper bound on the time we're willing to wait for Foo Say we are willing to wait for only a max imum of 500 milliseconds for Foo to complete and, if this timeout expires, we will throw an exception of some sort:
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This approach has one big problem Even i f w e timed out, we really should handle calling E n d Foo so that exceptions from the call to Foo are han dled and the IAsyn c R e s u lt resources can be cleaned up It would be terri ble if Foo threw a TheMa c h i n e I sO n F i re E x c e p t i o n and the thread calling f and g caught and swallowed the Timeo u t E x c e p t i o n thrown by g, with out E n d F oo ever having been called One way of handling this is to queue
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This approach makes some assumptions and isn't universally appealing We're assuming that it's OK to run 54 at any arbitrary point in the future, including after the calls to f and g have returned It also is not semantically equivalent to the sequential program We're also blocking a thread pool thread If the timeout may have happened because of a deadlock, we may completely tie up the thread pool What we really want is a way to cancel the work after 500 milliseconds, and to go back to waiting on it (hoping that
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cancellation is responsive) We will explore cancellation a bit more in 1 3, Data and Task Parallelism To take this example further, say we wanted to run two APM-capable oper ations, Foo and B a r concurrently, and wanted to handle them in whatever order they complete This is another example where the Asyn cWa itH a n d l e offers an advantage because we can wait for either (or both) to complete with WaitHa n d l e ' 5 Wa itAny and Wa itAl l methods If this were the simple syn chronous version of the code we wanted to modify to be asynchronous:
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