Question

As per my understanding a lock is not released until the runtime completes the code block of the lock(obj) ( because when the block completes it calls Monitor.Exit(obj).

With this understanding i am not able to understand the reason behind the behaviour of the following code :

private static string obj = "";
        private static void RecurseSome(int number)
        {
            Console.WriteLine(number);
            lock (obj)
            {
                RecurseSome(++number);
            }
        }

//Call: RecurseSome(0)

//Output: 0 1 2 3...... stack overflow exception

There must be some concept that i am missing. Please help.

Answer 1

A lock knows which thread locked it. If the same thread comes again it just increments a counter and does not block.

So, in the recursion, the second call also goes in – and the lock internally increases the lock counter – because it is the same thread (which already holds the lock).

ms-help://MS.VSCC.v90/MS.MSDNQTR.v90.en/dv_csref/html/656da1a4-707e-4ef6-9c6e-6d13b646af42.htm

Or MSDN: http://msdn.microsoft.com/en-us/library/c5kehkcz.aspx

states:

The lock keyword ensures that one thread does not enter a critical section of code while another thread is in the critical section. If another thread tries to enter a locked code, it will wait, block, until the object is released.

Note the thread references and the emphasis on “ANOTHER” thread.