Why use ReaderWriterLockSlim.EnterReadLock?

Question

Based on Microsoft documentation EnterReadLock is used on locking in read mode. It also says multiple threads can read but only one thread can write at a time.

Why we should use EnterReadLock while multiple threads can access the same data? What happens if we don't lock on reading, but lock on writing?

Answer 1

Read lock prevents acquiring write lock (EnterWriteLock will block until there are no more read or write locks acquired by others), but allows acquiring other read locks. If you don't use EnterReadLock on reads - the effect is other thread can acquire write lock and write data at the same time other threads read it, essentially defeating the usage of ReaderWriterLock completely.