Under what conditions can a thread enter a lock (Monitor) region more than once concurrently?

Question

(question revised): So far, the answers all include a single thread re-entering the lock region linearly, through things like recursion, where you can trace the steps of a s

Answer 1

Re-Entrance is possible if you have a structure like so:

Object lockObject = new Object(); 

void Foo(bool recurse) 
{
  lock(lockObject)
   { 
       Console.WriteLine("In Lock"); 
       if (recurse)  { foo(false); }
   }
}

While this is a pretty simplistic example, it's possible in many scenarios where you have interdependent or recursive behaviour.

For example:

• ComponentA.Add(): locks a common 'ComponentA' object, adds new item to ComponentB.
• ComponentB.OnNewItem(): new item triggers data-validation on each item in list.
• ComponentA.ValidateItem(): locks a common 'ComponentA' object to validate the item.

Same-thread re-entry on the same lock is needed to ensure you don't get deadlocks occurring with your own code.