ReaderWriterLock vs lock{}

Please explain what are the main differences and when should I use what.
The focus on web multi-threaded applications.

lock allows only one thread to execute the code at the same time. ReaderWriterLock may allow multiple threads to read at the same time or have exclusive access for writing, so it might be more efficient. If you are using .NET 3.5 ReaderWriterLockSlim is even faster. So if your shared resource is being read more often than being written, use ReaderWriterLockSlim. A good example for using it is a file that you read very often (on each request) and you update the contents of the file rarely. So when you read from the file you enter a read lock so that many requests can open it for reading and when you decide to write you enter a write lock. Using a lock on the file will basically mean that you can serve one request at a time.

Consider using ReaderWriterLock if you have lots of threads that only need to read the data and these threads are getting blocked waiting for the lock and and you don’t often need to change the data.

However ReaderWriterLock may block a thread that is waiting to write for a long time.

Therefore only use ReaderWriterLock after you have confirmed you get high contention for the lock in “real life” and you have confirmed you can’t redesign your locking design to reduce how long the lock is held for.

Also consider if you can't rather store the shared data in a database and let it take care of all the locking, as this is a lot less likely to give you a hard time tracking down bugs, iff a database is fast enough for your application.

In some cases you may also be able to use the Aps.net cache to handle shared data, and just remove the item from the cache when the data changes. The next read can put a fresh copy in the cache.

Remember

"The best kind of locking is the locking you don't need (i.e. don't share data between threads)."

Monitor and the underlying "syncblock" that can be associated with any reference object—the underlying mechanism under C#'s lock—support exclusive execution. Only one thread can ever have the lock. This is simple and efficient.

ReaderWriterLock (or, in V3.5, the better ReaderWriterLockSlim) provide a more complex model. Avoid unless you know it will be more efficient (i.e. have performance measurements to support yourself).

The best kind of locking is the locking you don't need (i.e. don't share data between threads).

ReaderWriterLock allows you to have multiple threads hold the ReadLock at the same time... so that your shared data can be consumed by many threads at once. As soon as a WriteLock is requested no more ReadLocks are granted and the code waiting for the WriteLock is blocked until all the threads with ReadLocks have released them.

The WriteLock can only ever be held by one thread, allow your 'data updates' to appear atomic from the point of view of the consuming parts of your code.

The Lock on the other hand only allows one thread to enter at a time, with no allowance for threads that are simply trying to consume the shared data.

ReaderWriterLockSlim is a new more performant version of ReaderWriterLock with better support for recursion and the ability to have a thread move from a Lock that is essentially a ReadLock to the WriteLock smoothly (UpgradeableReadLock).