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create a watchdog thread that monitors the shutdown process Although changeable by CLR hosts, the CLR will by default allow the AppDoma i n P r o c e s s E x i t and all relevant finalizers to run for 2 seconds before becom ing impatient If this period of time is exceeded, the shutdown thread is sus pended, and the CLR shutdown process continues without running any more managed code This can be illustrated by the following code example
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u s ing System j u s ing System Th read i n g j c l a s s Program { p rivate stat i c o b j e c t s l o c k p u b l i c s t a t i c vo id Ma i n ( ) { II C reate la new f i n a l i z a b l e obj e ct s Program [ ] p new Program [ la ] j
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I I Obt a i n t h e l o c k a n d t h e n force a p roc e s s exit l o c k ( s_loc k ) { E n v i ronment E x it ( - l ) j } II E n s u re t h e obj e c t s don ' t b e c ome u n r e a c h a b l e before exit i n g GC K e e pAlive ( p ) j } - P rogram ( ) { Console Wr i t e L i n e ( " a c q u i ring s_loc k " ) j I I T h i s l o c k a c q u i s ition w i l l a lways hang loc k ( s_loc k ) { C o n s o l e Writ e L i n e ( " Got it ! } } Nope " ) j
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When this program runs, only one finalizer will run, and it will freeze for about 2 seconds after the shutdown is initiated by the call to E n v i
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ronment E x i t This happens because the attempt to acquire s_loc k from Program ' s finalizer deadlocks, and the watchdog eventually kills the thread,
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skipping the remaining 9 finalizers in the queue The code in Ma i n that initiated the shutdown will have orphaned s_loc k by calling Ex it while it was held The same would have occurred if we attached an event handler to AppDoma i n C u r rent Proce s s E x i t that tried to acquire s_loc k, for example This same policy applies to any synchronization objects including man aged reader/ writer locks, events and condition variables, and any other type of interthread communication You might expect that mutexes would behave in managed code as they do in Win32 during process exit, given that Mutex is a thin wrapper over the OS mutex APIs In other words, you'd expect a call to Mutex Wa it O n e on an orphaned mutex to throw a Mutex Ab a n do n e d E x c e p t i o n If that happened, the unhandled exception would probably crash the finalizer thread and, hence, the entire process during shutdown That's not what happens Because shutdown-oriented managed code runs before E x i t P roc e s s is called, threads that own abandoned mutexes are just suspended (not killed); thus, the mutexes aren't aban doned, and attempts to acquire them will hang The manifestation of these sorts of hangs is often not horrible Many finalizers are meant to clean up intra process state anyway, and because HAND L E lifetime is tied to the process lifetime, Windows will close them automatically during process exit But a hang means that additional library and application logic won' t run, like flushing F i l e S t r e a m write buffers And for any cross-process state, you should always have a fail safe plan in place, such as detecting corrupt machine-wide state and repairing it upon the next program restart This is similar to what must be done with native code, given that the process will terminate if you try to acquire an orphaned lock Finally, a 2 second pause doesn't seem like much, but it's long enough that most users will notice it Avoid ing cross-thread coordination during shutdown is considered a best practice, and it can help to (statistically) improve the user experience for shutdowns
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