Basically, usage of the volatile keyword can sometimes be misleading. Its purpose is to allow that the latest value (or actually, an eventually fresh enough value)1 of the respective member is returned when accessed by any thread.
In fact, this is true to value types only2. Reference type members are represented in memory as the pointers to a location in the heap where the object is actually stored. So, when used on a reference type, volatile ensures you only get the fresh value of the reference (the pointer) to the object, not the object itself.
If you have a volatile List<String> myVolatileList which is modified by multiple threads by having elements added or removed, and if you expect it to be safely accessing the latest modification of the list, you are actually wrong. In fact, you are prone to the same issues as if the volatile keyword was not there -- race conditions and/or having the object instance corrupted -- it does not assist you in this case, neither it provides you with any thread safety.
If, however, the list itself is not modified by the different threads, but rather, each thread would only assign a different instance to the field (meaning the list is behaving like an immutable object), then you are fine. Here is an example:
public class HasVolatileReferenceType
{
public volatile List<int> MyVolatileMember;
}
The following usage is correct with respect to multi-threading, as each thread would replace the MyVolatileMember pointer. Here, volatile ensures that the other threads will see the latest list instance stored in the MyVolatileMember field.
HasVolatileReferenceTypeexample = new HasVolatileReferenceType();
// instead of modifying `example.MyVolatileMember`
// we are replacing it with a new list. This is OK with volatile.
example.MyVolatileMember = example.MyVolatileMember
.Where(x => x > 42).ToList();
In contrast, the below code is error prone, because it directly modifies the list. If this code is executed simultaneously with multiple threads, the list may become corrupted, or behave in an inconsistent manner.
example.MyVolatileMember.RemoveAll(x => x <= 42);
Let us return to value types for a while. In .NET all value types are actually reassigned when they are modified, they are safe to be used with the volatile keyword - see the code:
public class HasVolatileValueType
{
public volatile int MyVolatileMember;
}
// usage
HasVolatileValueType example = new HasVolatileValueType();
example.MyVolatileMember = 42;
1The notion of lates value here is a little misleading, as noted by Eric Lippert in the comments section. In fact latest here means that the .NET runtime will attempt (no guarantees here) to prevent writes to volatile members to happen in-between read operations whenever it deems it is possible. This would contribute to different threads reading a fresh value of the volatile member, as their read operations would probably be ordered after a write operation to the member. But there is more to count on probability here.
2In general, volatile is OK to be used on any immutable object, since modifications always imply reassignment of the field with a different value. The following code is also a correct example of the use of the volatile keyword:
public class HasVolatileImmutableType
{
public volatile string MyVolatileMember;
}
// usage
HasVolatileImmutableType example = new HasVolatileImmutableType();
example.MyVolatileMember = "immutable";
// string is reference type, but is *immutable*,
// so we need to reasign the modification result it in order
// to work with the new value later
example.MyVolatileMember = example.MyVolatileMember.SubString(2);
I'd recommend you to take a look at this article. It thoroughly explains the usage of the volatile keyword, the way it actually works and the possible consequences to using it.
Typically, the common language runtime controls the physical layout of the data fields of a class or structure in managed memory.(msdn.microsoft.com/en-us/library/…) so you can't be sure of the layout of a struct unless you use theStructLayoutAttribute. – Jamimanative (platform-specific) size integer#1148677 – Jamima