What exactly is transmitted or used in the locking mechanism in multithreaded programming?

In the locking (lock) mechanism in multithreaded programming, an object or synchronization primitive is used to provide exclusive access to a shared resource. When a thread acquires the lock, other threads attempting to acquire the same lock will wait until it is released.

That is, "lock" involves either passing or using:

• Lock state (occupied/free).
• Waiting mechanism (e.g., a queue of threads waiting for release).

In C#, the lock keyword works with an object that serves as a lock marker. Example:

private readonly object _lockObj = new object();

void CriticalSection()
{
    lock(_lockObj)
    {
        // Code that should only be executed by one thread at a time
    }
}

Here, a reference to the _lockObj object is passed to lock, which is used to control access. Internally, the CLR uses this object to monitor the lock state.