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problems, all of which can lead to hangs when your API is used on the UI thread, reliability problems, and frustration for your cus tomer Locks don' t compose very well; ignoring this and attempting to compose parts of your components that use them in this way is fraught with peril 7 Avoid blocking while you hold a lock This is self explanatory Admittedly, it is sometimes unavoidable Trying to acquire a lock is an operation that can block under con tention, so by definition, if you need to hold more than one lock at once, you will be violating this advice But what's more important, blocking on high or variable latency operations such as I / O will effectively serialize any other thread trying to acquire that lock behind your I / O request If that other thread trying to acquire the lock is on the UI thread, you may have just indirectly caused a user visible hang The developer may not understand the cause of this hang if the lock is buried inside of your library, and it may be tricky and error prone to work around At the very least, extending lock hold times like this can cause convoys Aside from having scalability impacts, blocking while a lock is held can lead to deadlocks and invariants being broken Any time you block on an STA thread, the CLR uses it as a chance to run the message loop When run on pre-Windows 2000 that means running custom MsgWa i t F o rMu l t i p l eOb j e c t s pumping code, and OLE's CoWa i t F o rMu l t i p l e H a n d l e s post-Windows 2000 While this style of pumping processes only a tiny subset of CUI messages, it can dis patch arbitrary COM to CLR interop calls These calls include cross thread / apartment S e n d Me s s a ge calls, such as an MTA to STA call through a proxy If this happens while a lock is held, that newly dis patched work also runs under the protection of the lock If the same object is accessed, this can lead to surprising bugs where invariants are still broken inside the lock Try to minimize the time you hold a lock and move all blocking and communication across apartments, threads, processes outside the edges of those lock acquisition/releases All libraries should strive to only acquire locks at the leaves of callgraphs to the extent that it is possible
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8 Assert lock ownership Races often result when some leaf-level code assumes a lock has been taken at a higher level on the call stack, but the caller has for gotten to acquire it Or maybe the owner of that code recently refac tored it and didn't realize the implicit pre-condition that was broken in the process This may go undetected in test suites unless the race actually occurs in the world All new locks in the NET Framework provide APIs to test if the lock is held Mo n i t o r currently lacks an I sHeld API, so if you want to heed this advice with Mon it o r you'll need to maintain the extra state yourself I S H e ld-like functionality should never be used to dynami cally influence lock acquisition and release at runtime, for example avoiding recursion and taking or releasing based on its value It is meant as a debugging aid only 9 Avoid lock recursion in your design Use a non recursive lock where possible Recursion is one of the problems highlighted in 1 1 , Concurrency Hazards, that can lead t o reliability and reentrancy problems Lock recursion is typically an indication of an oversim plified synchronization policy For instance, many designs use lock recursion as a way to avoid splitting functions into those that take locks (nonrecursively) and those that assert that locks are already taken This can lead to a reduction in code size, but usually results in a more brittle design in the end For this reason, most new NET locks are nonrecursive by default and only offer it as an opt in setting Recursive lock acquires are redundant and add unnecessary per formance overhead But worse, depending on recursion can make it more difficult to understand the synchronization behavior of your program, in particular at what boundaries invariants are supposed to hold Usually we'd like to say that the first line after a lock acqui sition represents an invariant "safe point" for an object, but as soon as recursion is introduced this statement can no longer be made con fidently This in turn makes it more difficult to ensure correct and reliable behavior when dynamically composed
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