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Is a bool read/write atomic in C#

Asked 2008-09-12T16:19:28.657
104

Is accessing a bool field atomic in C#? In particular, do I need to put a lock around:

class Foo
{
   private bool _bar;

   //... in some function on any thread (or many threads)
   _bar = true;

   //... same for a read
   if (_bar) { ... }
}
c# .net concurrency locking boolean
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1 Answer

33

bool accesses are indeed atomic, but that isn't the whole story.

You don't have to worry about reading a value that is 'incompletely written' - it isn't clear what that could possibly mean for a bool in any case - but you do have to worry about processor caches, at least if details of timing are an issue. If thread #1 running on core A has your _bar in cache, and _bar gets updated by thread #2 running on another core, thread #1 will not see the change immediately unless you add locking, declare _bar as volatile, or explicitly insert calls to Thread.MemoryBarrier() to invalidate the cached value.

answered 2008-09-12T16:53:24.033

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