On which data structure is the locking mechanism implemented?

In C#, the locking mechanism (lock) is implemented based on an object that serves as a mutex. Usually, an object of type object is used, which acts as a monitor. When you write lock(someObject), the CLR uses an internal monitor mechanism to synchronize access to the critical section of code.

Example:

private readonly object _lockObj = new object();

public void ThreadSafeMethod()
{
    lock (_lockObj)
    {
        // Critical section
    }
}

Thus, the data structure is simply an object on which the monitor, implementing the lock, is based.