问题
I have a Dictionary that tracks objects (ClientObject). Both the dictionary and ClientObject's are accessed by multiple threads. When I modify or read any object in this dictionary, I obtain a read or write lock on the dictionary using ReaderWriterLockSlim (rwl_clients) then obtain an exclusive lock on the actual object.
I just wanted to know if I am using these .net threading facilities correctly
Example:
rwl_clients.EnterReadLock();
ClientObject clobj;
if(!m_clients.TryGetValue(key, out clobj))
return;
rwl_clients.ExitReadLock();
SomeMethod(clobj);
SomeMethod(ClientObject clobj) would do something like:
lock(clobj) {
/// Read / Write operations on clobj
}
Does getting and locking a value (ClientObject) from the dictionary in one thread mean that other threads will respect that lock? In other words, does .net see a value in the dictionary as a single resource (and not a copy) and will therefore respect a lock on that resource in all threads?
One more question, when removing a resource from the dictionary - should I lock it before performing Remove()
Example:
rwl_clients.EnterWriteLock();
ClientObject clobj;
if(m_clients.TryGetValue(key, out clobj)) {
lock(clobj) {
m_clients.Remove(key);
}
}
rwl_clients.ExitWriteLock();
I have learned so much from this site and appreciate any responses! Thanks.
回答1:
Does getting and locking a value (ClientObject) from the dictionary in one thread mean that other threads will respect that lock? In other words, does .net see a value in the dictionary as a single resource (and not a copy) and will therefore respect a lock on that resource in all threads?
It depends on the type - if a reference type then yes, if a value type no. This is also why you should never, ever lock on a value type since the value type will be boxed and any subsequent attempts to lock on that value will actually acquire a lock on a different object.
One more question, when removing a resource from the dictionary - should I lock it before performing Remove()
Yes, you should lock before any operation that mutates the state of the object.
As a side note - are you sure that this setup is the best possible solution to your problem? Mutable objects shared across threads tend to create more problems then they solve.
回答2:
If you are adding or removing items from the dictionary, lock the dictionary.
When you put an object in the dictionary, you are putting a REFERENCE to that object in the dictionary. To prevent that object from being changed by a second thread while the first thread is in the process of changing it, lock the object, not the dictionary.
来源：https://stackoverflow.com/questions/1482783/c-sharp-locking-a-resource-when-obtained-from-dictionary