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    <p>The reason has to do with memory barriers and guaranteeing thread safety.</p> <p>Shared variables (conditionals) that are used to determine whether a Pulse() is needed will be checked by all threads involved. Without a memory barrier, the changes might be kept in a register and be invisible from one thread to another. Reads and writes can also be re-ordered when viewed across threads.</p> <p>However, variables that are accessed from within a lock use a memory barrier, so they are accessible to all related threads. All operations within the lock appear to execute atomically from the perspective of other threads holding the same lock.</p> <p>Also, multiple context switches aren't required, as you postulated. Waiting threads are put in a (nominally FIFO) queue, and while they're triggered with Pulse(), they aren't fully runnable until the lock is relinquished (again, in part due to memory barriers).</p> <p>For a good discussion of the issues, see: <a href="http://www.albahari.com/threading/part4.aspx#_Wait_and_Pulse" rel="nofollow noreferrer">http://www.albahari.com/threading/part4.aspx#_Wait_and_Pulse</a></p>
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    1. POWhy does Monitor.Pulse need locked mutex? (.Net)
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    1. USRickNZ
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    1. COPutting a memory barrier in Pulse() wouldn't address the variables that are checked to see if Pulse() should be called. You know: `lock (someLock) { if (someVariable) Monitor.Pulse(); someVariable = false; }` It is actually possible to build a lightweight signaling mechanism with memory barriers alone... A memory barrier alone also wouldn't assure the (effective) atomicity of multiple operations inside the lock.
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    2. COThis link doesn't smell very good, for example: ` X86 and X64 processors always apply acquire-fences to reads and release-fences to writes` is wrong. he is confusing the MESI protocol and the flush of read writes operation from the instruction pipeline. c.f. intel documentation of XADD and other Locked prefix instructions. http://www.codemaestro.com/reviews/8
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    3. CO@RickNZ: If the code that does a pulse sets a flag which could be cleared by code in the wait loop, the code that calls pulse needs to acquire the lock before setting those flags. It would seem semantically useful, however, to have a version of pulse which did not need to be within the lock, but which--if the lock is busy--would set a flag so that if the current holder of the lock calls wait *before exiting the lock* it would act as though it was pulsed immediately. For example, suppose a wrapper object is supposed to encapsulate a thread. If the wrapper is abandoned, the thread...
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