Volatile and Cross-Thread Variable Changes

[TonyValenti]

What is the correct way to ensure that variables changed on one thread are observed on another?

Also, why is it that a long cannot be declared as volatile but you can Volatile.Read/Write longs? I expected that the API would be consistent with the keyword.

[CyrusNajmabadi]

Generally speaking, i view the correct way as using a normal 'lock'. If there are exceptional circumstances that do not allow for that, then the answer will depend on those circumstances.

[alexrp]

Also, why is it that a long cannot be declared as volatile but you can Volatile.Read/Write longs? I expected that the API would be consistent with the keyword.

Unfortunate historical reasons. The compiler does not use that API at all.

C#'s volatile gives you acquire/release semantics + atomicity. But at the CIL level, it is emitted as the volatile. prefix opcode (giving the acquire/release semantics only) + a regular load/store. In CIL, regular loads/stores are only atomic for a given type T if sizeof(T) <= sizeof(void*). That means that a C#-level volatile long cannot be guaranteed to be atomic on 32-bit machines. So the language forbids it to avoid the footgun.

AFAIK, there's no technical reason why the compiler has to continue to emit volatile in this way. For newer TFMs (i.e. anything .NET Standard 1.0+) it could theoretically be changed to emit calls to the Volatile class, and this restriction could be lifted. But I don't think there's much appetite for doing this, partly because of compatibility concerns, and partly because adding volatile to the language has been widely regarded as a mistake inherited from C/C++.

[TonyValenti]

Thanks so much for the answers here.

With my code, I want all threads to always be able to read the current version of the variables (ideally, without a lock).

When updating the variables, I do lock.

Will a volatile variable that has a Lock around all writes guarantee that all threads always see the current value?

[NN---]

This question is not new :)

https://stackoverflow.com/questions/4727068/why-not-volatile-on-system-double-and-system-long