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10 Don't build your own lock Most locks are built out of simple principles at the core There's a state variable, a few interlocked instructions (exposed to managed code through the I nt e r l o c ked class), and some form of spinning and possibly waiting on an event when contention is detected Given this, it may look straightforward to build your own This is deceivingly difficult CLR locks have to coordinate with hosts so that they can perform deadlock detection and sophisticated user-mode scheduling for hosted customer authored code Some of NET's locks (Mo n itor) make higher reliability guarantees so that they can be safely used during AppDomain teardown Real locks are tuned to use an ideal mixture of spinning and waiting across many OS SKUs, CPU archi tectures, and cache hierarchy arrangements Such spinning must be written to work correctly with Intel HyperThreading and to avoid priority induced starvation Locks must mark critical regions of code so that would-be thread aborts will be performed correctly while sensitive shared state manipulation is under way And the C# and VB languages offer the " lo c k " and " Sy n c Loc k " keywords (as high lighted earlier) whose code generation pattern ensures that code won' t orphan locks in the face of asynchronous thread aborts To get all of this right requires a lot of hard work, time, and testing With that said, NET may not currently have every lock you could ever want Spin locks are a popular request that can help with performance scalability of highly concurrent and leaf-level code, as demonstrated in 1 4, Performance and Scalability It's best to make do with what is available out-of-the-box and to look for third party locks only if necessary 1 1 Don't call Monitor Enter on AppDomain agile obj ects (Types and Strings) Instances of some Type objects are shared across AppDomains The most notable are Ty p e s for domain neutral assemblies (such as mscorlibdll) and cross assembly interned St r i n g s While it may look innocuous, locks taken on these things are visible across all
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AppDomains i n the process A s a n example, two AppDomains executing this same code will interfere with each other
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This can cause severe reliability problems should a lock get orphaned in an add-in or hosted scenario, possibly causing cross AppDomain deadlocks stemming {seemingly inexplicably} from deep within your library The resulting code also leads to false con tention between code running in different domains and, therefore, can impact scalability in a way that is very difficult for customers {and library authors} to reason about 12 Don't use a machine- or process-wide synchronization primitive when AppDomain-wide would suffice The Mutex and Sema p h o r e types in the NET Framework should only be used for legacy, interoperability, cross AppDomain, and cross process reasons They are heavier weight-several orders of magni tude slower than a CLR Mo n it o r, as mentioned in 6, Data and Control Synchronization-and they introduce reliability and affinity problems They can be orphaned, out of process denial of service attacks can be mounted, and they can introduce scalability bottlenecks Moreover, they are associated with the as thread and, therefore, impose thread affinity 1 3 A race condition or deadlock in library code is always a bug This seems like it should be obvious But it's not always cut and dried Race conditions and deadlocks can be very difficult to fix Sometimes fixing one requires refactoring a lot of mostly working code to make some {seemingly} corner case and obscure sequence of events work correctly It's tempting to rearrange things to narrow the window of the race or reduce the likelihood of a deadlock But never lose sight of the fact that, no matter how narrow the likelihood, a race or deadlock is a severe correctness problem Sometimes fixing a bug like this requires making breaking changes Sometimes you may not have enough time to fix the bug in time to ship your product In either case, this is something that
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