when locking a variable, must i lock it everywhere?

Question

I was wondering if when accessing from multiple threads a variable, must i lock it for every access ? For example, i have a list variable and would like to not be able to access that list while i remove an item from it. Must i lock it even in the threads that just access the data or it is sufficient to lock it in the "remove item" thread. My concern is that if a thread accesses the list stops in the middle of the process , goes to another thread that removes from the list and locks the variable, an error will appear .

Answer 1

Your bathroom door does not have a lock on it. Instead, it has a double-sided sign that says "occupied" on one side, and "unoccupied" on the other. When someone approaches the bathroom, if it is marked occupied, they wait until it is marked unoccupied. If it is marked unoccupied, they set the sign to occupied and go in. When they leave, they set the sign back to unoccupied.

(Some negotiation mechanism also must exist to handle the situation where there are two or more people waiting - who gets in first? And also, what happens when two people approach an unoccupied bathroom at the same time -- who wins? But we'll ignore those wrinkles; they are not germane to the analogy.)

This scheme works just fine. Your question is "if someone ignores the sign, or forgets to change it, can I ever be in a situation where there are two people in the bathroom at the same time?"

Yes, obviously you can. If you don't believe me, then I encourage you to try it with your bathroom some time and see what happens when someone ignores the protocol. A resource access protocol only protects access to that resource if everyone honours the protocol!

Answer 2

If you wish to keep a collection in a consistent state (or any other object you are protecting with a lock), you must lock on reads and writes.

There are some 'exceptions', like using a ReaderWriterSlimLock, you still need to acquire a lock for writes but you can perform multi-threaded reads efficiently:

Use ReaderWriterLockSlim to protect a resource that is read by multiple threads and written to by one thread at a time. ReaderWriterLockSlim allows multiple threads to be in read mode, allows one thread to be in write mode with exclusive ownership of the lock, and allows one thread that has read access to be in upgradeable read mode, from which the thread can upgrade to write mode without having to relinquish its read access to the resource.