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nously) In fact, the thread from which the exception was thrown will have been returned to the pool This means any thread local state, including local variables on the thread's stack, will not be available We always return f a l s e for the CompletedSyn c h ro n o u s ly property Returning t r u e is a relatively obscure situation that doesn't happen much It must return t r u e if the thread being used to execute the callback is the same thread that was used to invoke the B e g i n Foo operation in the first place Because our code always queues work to run in the thread pool, this isn't ever possible Some APM implementations are clever enough to run the callback on the current thread if it doesn't make sense to run the code asynchronously In these cases, your callback could end up using a lot of stack (unexpectedly) if it tries to continue to call Beg i n Foo over and over again from within the completion callbacks The F i leSt re a m class's Begi n R e a d and Beg i nW r i t e operations, for example, can result in this behavior because Windows asynchronous I / O may be able to finish the I / O opera tion so quickly that transferring the callback to another thread isn't neces sary We discuss this possibility more in 1 5, Input and Output Most programs can remain unaware of Comp letedSy n c h ro n o u s ly Once we have the S i m p l eAsy n c R e s u l t < T > class, we can wrap it with standard Begi n F oo and E n d Foo APM methods For example, Listing 82 demonstrates a simple APM variant of some synchronous Wo r k method that calls T h r e a d S l e e p and then returns a new random number:
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LI STI N G 82: A sim ple APM im plementation using S i m p leAsyncRes u lt<T> p u b l i c c l a s s Simp leAsyncOperation { p u b l i c int Work ( i nt s l e epyTime ) { Thread S leep ( s leepyTime ) ; ret u r n new Random ( ) Next ( ) ;
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p u b l i c IAsyn c R e s u lt BeginWo r k ( int s l e e pyTime , Asyn c C a l l ba c k c a l l ba c k , obj e c t state ) { ret u rn new S imp leAsyn c R e s u lt < i nt > ( delegate { ret u r n Wo rk ( s leepyTime ) ; } , c a l l ba c k , state
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C h a pte r 8: Asyn c h ro n o u s Prolra m m l n l M o d e l s
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p u b l i c int E ndWo r k ( IAsy n c R e s u lt a sy n c R e s u lt ) { Simp leAsyn c R e s u lt < i n t > s impleRe s u lt a s y n c R e s u l t a s SimpleAsy n c R e s u lt < i nt > ; i f ( s impleRe s u lt nUll ) t h row new Argument E x c eption ( " Bad a sync res u lt " ) ; ret u r n s i m p l e R e s u l t E nd ( ) ; } }
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A significantly more efficient approach to implementing the APM involves lazily allocating the Asy n c W a i t H a n d l e object only when it is requested (ie, a caller accesses Asyn cWa i t H a n d l e directly or calls E n d F oo before I sCompleted i s t r u e ) Though there are many more complicated examples of how to do this, it is very straightforward with the help of some additional lazy initialization abstractions that we will explore later in 1 0, Memory Models and Lock Freedom
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Where the APM Is Used i n the N ET Framework The APM is used in many places in the platform in various ways Here is a list of some of the most important APM-capable operations in the core assem blies that ship as part of the NET Framework 30 (ms c o r l i b d l l , Sy s
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t em d l l , System Core d l l , System Data d l l , System T r a n s a ction s d l l):
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All delegate types, by convention, offer a Beg i n I nvoke and E n d I nvoke method alongside the ordinary synchronous I n voke method While this is a nice programming model feature, you should stay away from them wherever possible The implementation uses remoting infrastructure that imposes a sizeable overhead to asynchronous invocation Queuing work to the thread pool directly is often a better approach, though that means you have to coordinate the ren dezvous logic yourself (or use the APM implementation we're about to examine)
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Asyn c h ro n o u s Prolra m m l n l M o d e l (A P M )
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