c# - Does Interlocked provide visibility in all threads?

Question

Suppose I have a variable "counter", and there are several threads accessing and setting the value of "counter" by using Interlocked, i.e.:

int value = Interlocked.Increment(ref counter);

and

int value = Interlocked.Decrement(ref counter);

Can I assume that, the change made by Interlocked will be visible in all threads?

If not, what should I do to make all threads synchronize the variable?

EDIT: someone suggested me to use volatile. But when I set the "counter" as volatile, there is compiler warning "reference to volatile field will not be treated as volatile".

When I read online help, it said, "A volatile field should not normally be passed using a ref or out parameter".

Answer 1

Can I assume that, the change made by Interlocked will be visible in all threads?

This depends on how you read the value. If you "just" read it, then no, this won't always be visible in other threads unless you mark it as volatile. That causes an annoying warning though.

As an alternative (and much preferred IMO), read it using another Interlocked instruction. This will always see the updated value on all threads:

int readvalue = Interlocked.CompareExchange(ref counter, 0, 0);

which returns the value read, and if it was 0 swaps it with 0.

Motivation: the warning hints that something isn't right; combining the two techniques (volatile & interlocked) wasn't the intended way to do this.

Update: it seems that another approach to reliable 32-bit reads without using "volatile" is by using Thread.VolatileRead as suggested in this answer. There is also some evidence that I am completely wrong about using Interlocked for 32-bit reads, for example this Connect issue, though I wonder if the distinction is a bit pedantic in nature.

What I really mean is: don't use this answer as your only source; I'm having my doubts about this.