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Comment by FFFXXX | Hacker News Reader

I think you are quoting from https://learn.microsoft.com/en-us/dotnet/api/system.threadin...

"In C#, using the volatile modifier on a field guarantees that every access to that field is a volatile memory operation"

This makes it sound like you are right and the volatile keyword has the same behaviour as the Volatile class which explicitly says it has acquire-release ordering.

But that seems to contradict "The volatile keyword doesn't provide atomicity for operations other than assignment, doesn't prevent race conditions, and doesn't provide ordering guarantees for other memory operations." from the volatile keyword documentation?

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I too interpretat those docs as contradictory, and I wonder if, like how Java 5 strengthened volatile semantics, this happened at some point in C# too and the docs weren't updated? Either way the specification, which the docs say is definitive, says it's acquire/release.

https://learn.microsoft.com/en-us/dotnet/csharp/language-ref...

"When a field_declaration includes a volatile modifier, the fields introduced by that declaration are volatile fields. [...] For volatile fields, such reordering optimizations are restricted:

    A read of a volatile field is called a volatile read. A volatile read has “acquire semantics”; that is, it is guaranteed to occur prior to any references to memory that occur after it in the instruction sequence.

    A write of a volatile field is called a volatile write. A volatile write has “release semantics”; that is, it is guaranteed to happen after any memory references prior to the write instruction in the instruction sequence."

Acquire-release ordering provides ordering guarantees for all memory operations. If an acquire observes a releases, the thread is also guaranteed to see all the previous writes done by the other thread - regardless of the atomicity of those writes. (There still can't be any other data races though.)

This volatile keyword appears to only consider that specific memory location whereas the Volatile class seem to implement acquire-release.

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