Then the APM version using Wa i tAny would go as follows in .NET

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IAsyn c R e s u lt fooAs y n c R e s u lt IAsyn c R e s u l t ba rAsy n c R e s u lt WaitHandle [ ] handles { fooAsy n c R e s u l t AsyncWa itHa n d l e , b a rAsyn c Re s u l t AsyncWa itHandle }; int awoken if ( awok e n
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Of course things become more complicated if we need to handle the possibility of failure coming from E n d Foo or E n d B a r Would we block wait ing for the other to finish inside of a f i n a l ly block This is a difficult ques tion to answer, but without doing something like this we'd run the risk of losing exceptions The topic of cancellation once again comes up
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Mechanism #3: Polling the IsCompleted Flag
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The IAsyn c R e s u l t object offers an I sCompleted flag, of type boo l When the asynchronous work has finished, this gets set to t r u e 50 your rendezvous logic can guard the call to E n d Foo on this value, allowing you to avoid blocking and instead do other work while the asynchronous computation completes
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I n this example, w e introduced a new statement, 56, that does some thing useful while the concurrent operation is executing This is a little like the waiting with timeout example shown before (where we provided status to the user) with one distinction: checking I sCom p l et e d does not block the calling thread You must use this tactic with care: if 56 is something com putationally expensive, it may end up using CPU resources that could have otherwise been used to finish running Foo It would also be bad if 56 were an empty statement, because it amounts to a completely inappropriately written spin wait
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C h a pter 8: Asyn c h ro n o u s P rog ra m m i n g M o d e l s
Mechanism #,,: Callbacks
The callback rendezvous technique can be more complicated to deal with than the others It requires a style of programming referred to as continu
ation passing style (CPS), where the continuation of whatever you would have done after Foo completed (in a synchronous program) has to be rep resented with callback delegate instead It can be difficult to save enough information at the time of a Beg i n F oo call to be able to resume the entire log ical continuation of work asynchronously at some point in the future Moreover, the thread pool is meant only for short bursts of work, so you probably wouldn' t want to save the whole logical continuation (Le, the whole stack's worth), meaning this technique works best when the amount of work to do in response is fairly small (much like an event handler) The other mechanisms, by contrast, allow you to write your code similar to a synchronous program, with little regions carved out where the work hap pens asynchronously Attempting to use the callback rendezvous approach for this particular sample highlights these challenges Several callers in the current stack may depend on the output of calling F oo, because it is returned from both f and g We need to move the continuation statements 53, 54, 55, and 51 in the callback, requiring a lot of code refactoring to turn Foo into Begi n F oo And that alone is insufficient: since the caller of f also needs the output of F oo, we would need to make the things that happen after f returns part of the continuation too, possibly requiring callers to supply their own call backs as arguments Depending on the amount of code on the callstack you own, this may be possible, but this can get very complex very quickly For purposes of discussion, and to illustrate when a callback might be useful, pretend g looks like the following