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properties This information i s flowed across logical async points as required, and, therefore, using them doesn't imply any sort of hard OS thread affinity
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If you mark a P ! Invoke signature with [ Dl l lmpo rtAtt r i b u t e ( J Set L a s t E rro r=t r u e ) ] , then the CLR will store the Win32 last error on the logical CLR thread This ensures that, even if a cooperative scheduling switch happens after the P / Invoke but before you can check its value, your last error will be preserved The Win32 APIs Get L a st E r ro r and S et L a st E r ror, on the other hand, store this infor mation in the TEB If you are P / Invoking to get at the last error information, you are apt to be surprised if you are running in an environment that permits thread migration because the error may change before you get a chance to access it You can avoid this by always using the safe Ma r s h a l Get L a stWi n 3 2 E r ro r function 26 Avoid P/lnvoking to other Win32 APls that access data in the Thread Environment Block (TEB) Security and locale information is something Win32 stores in the TEB that NET offers APIs to access safely That is easy But many Win32 APIs access data from the TEB without necessarily saying so, or will look for and possible lazily create some thread affine data structure (eg, a window message queue in USER32), leading to silent thread affinity While there is no good list of which APIs acquire or depend on thread affinity, it's good to be aware of this issue
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27 Consider using a reader/writer lock for read-only synchronization Concurrent access to shared state often consists of a high read-to write ratio Given this, using exclusive synchronization (such as CLR monitors) can hurt scalability in situations with a large numbers of concurrent readers While starting off with a reader / writer lock could be a premature optimization, many situations warrant using one, particularly very hot read regions of code There has been a lot of negative press about NET's ReaderWrit e r Loc k I n particular, the performance i s a t about 6 times that o f success ful Mon ito r E n t e r calls Unfortunately, this has (in the past)
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prevented many library developers from using reader/writer locks altogether This is the primary motivation that the ReaderWrit e r Loc k S l im type was added i n NET 35 28 Avoid lock free code for all but the most critical performance needs Compilers and processors reorder reads and writes to get better per formance, but in doing so make it harder to write concurrent code without locks The CLR memory model gives a base level of guaran tees that we preserve across all hardware platforms 1 0, Memory Models and Lock Freedom, went into detail about when and how to exploit the memory model When in doubt, however, avoid it if at all possible The reason Lock free code is extraordinarily complicated to write, maintain, and debug for most developers, even those who have been doing it for years This is the type of code whose proliferation will lead to poor robustness in the face of adding more and more proces sors Use of volat i l e fields and calls to Thread Memo ryBa r r i e r should be viewed with great suspicion, as it probably means some body is trying to be cleverer than is required 29 Avoid hand-coded spin waits If you must do it, do it right Sometimes it is tempting to put a busy wait in very tightly synchro nized regions of code Unless written properly, however, this tech nique won't work well It's often simpler to use locks or events (such as Mo n it o r Wa it / P u l s e / P u l s eAl l ) for this type of cross thread com munication These internally employ some reasonable amount of spinning versus waiting automatically for you If you think spin waiting is appropriate for your situation, please consult 1 4, Performance and Scalability, where an overview is provided along with details of proper spin wait algorithms 30 When yielding the current thread's time slice, use
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