是 C # 中的 bool 读/写原子

在 C # 中访问 布尔字段是原子的吗:

class Foo
{
private bool _bar;


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


//... same for a read
if (_bar) { ... }
}
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最佳答案

Yes.

Reads and writes of the following data types are atomic: bool, char, byte, sbyte, short, ushort, uint, int, float, and reference types.

as found in C# Language Spec.

Edit: It's probably also worthwhile understanding the volatile keyword.

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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.

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As stated above, bool is atomic, but you still need to remember that it also depends on what you want to do with it.

if(b == false)
{
//do something
}

is not an atomic operation, meaning that the value of b could change before the current thread executes the code after the if statement.

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the approach I have used, and I think is correct, is

volatile bool b = false;


.. rarely signal an update with a large state change...


lock b_lock
{
b = true;
//other;
}


... another thread ...


if(b)
{
lock b_lock
{
if(b)
{
//other stuff
b = false;
}
}
}

the goal was basically to avoid having to repetively lock an object on every iteration just to check if we needed to lock it in order to provide a large amount of state change information which occurs rarely. I think this approach works. And if absolute consistancy is required, I think volatile would be appropriate on the b bool.


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