C# bool is atomic, why is volatile valid
In C#, we know that a bool is atomic - then why is it valid to mark it as volatile? what is the difference and what is a good (or even practical) use-case for one versus the other?
The supposition of your question is that you believe that volatile makes an access atomic. But volatility and atomicity are completely different things, so stop conflating them.
Volatility is the property that the compiler and runtime are restricted from making certain optimizations involving moving reads and writes of variables forwards and backwards in time with respect to each other, and more generally, with respect to other important events such as starting and stopping threads, running constructors, and so on. Consult the C# specification for a detailed list of how operations may or may not be re-ordered with respect to visible side effects.
Atomicity is the property that a particular operation can only be observed as not started or totally completed, and never "halfway done".
As you can see from the definitions, those two things have nothing whatsoever to do with each other.
In C#, all accesses to references, bools, and integer types of size 4 and smaller are guaranteed to be atomic.
Now, in C# there is some slight non-orthogonality between atomicity and volatility, in that only fields of atomic types may be marked as volatile. You may not make a volatile double, for example. It would be really weird and dangerous to say "we're going to restrict how reads and writes may be optimized but still allow tearing". Since volatility does not cause atomicity, you don't want to put users in a position of thinking that an operation is atomic just because it is also volatile.
You should read my series of articles that explains in far more detail what the differences between these things are, and what volatile actually does, and why you do not understand nearly enough to be using it safely.
https://ericlippert.com/2011/05/26/atomicity-volatility-and-immutability-are-different-part-one/
https://ericlippert.com/2011/05/31/atomicity-volatility-and-immutability-are-different-part-two/
https://ericlippert.com/2011/06/16/atomicity-volatility-and-immutability-are-different-part-three/
https://web.archive.org/web/20160323025740/http://blog.coverity.com/2014/03/12/can-skip-lock-reading-integer/
If you think you understand volatility after reading all that, I invite you to try to solve the puzzle I pose here:
https://web.archive.org/web/20160729162225/http://blog.coverity.com/2014/03/26/reordering-optimizations/
If there are updates to variables in the preceding or subsequent code and the order in which the updates occurs is critical, then marking the field as volatile will ensure that an update to that field will happen after any previous updates and before any subsequent updates.
In other words, if _isPending is volatile, then the compiler will not cause these instructions to execute in a different order:
_someVariable = 10;
_isPending = true;
_someOtherVariable = 5;
Whether multi-threaded or not, if we've written code that breaks depending on whether these updates in adjacent lines occur in the specified order then something is wrong. We should ask why that sequence matters. (If there is a scenario where that matters, imagine trying to explain it in a comment so that no one makes a breaking change to the code.)
To nearly anyone reading the code above it would appear that the order of those operations doesn't matter at all. If they do matter that means that someone else who reads our code can't possibly understand what's going on. They could do some refactoring, reorder those lines of code, and break everything without knowing it. It might even work when they test it and then fail unpredictably and inconsistently when it's deployed.
I agree with Eric Lippert's comment in the answer you linked:
Frankly, I discourage you from ever making a volatile field. Volatile fields are a sign that you are doing something downright crazy: you're attempting to read and write the same value on two different threads without putting a lock in place.
I suppose I failed to directly answer the direction. volatile is valid for a type (including bool) because it's possible to perform an atomic operation on that type. volatile protects from compiler optimizations. According to the documentation for volatile,
This ensures that the most up-to-date value is present in the field at all times.
But if the field can't be represented in 32 bits or less then preventing compiler optimizations can't guarantee that anyway.