Question

We can use Wait and Pulse to simulate a ManualResetEvent as follows:

code #1

readonly object _locker = new object();
bool _signal;
void WaitOne()
{
  lock (_locker)
   {
     while (!_signal) Monitor.Wait (_locker);
   }
}

void Set()
{ 
   lock (_locker) 
      { _signal = true; 
        Monitor.PulseAll (_locker); 
      }
}

void Reset() 
   {
    lock (_locker) _signal = false; 
   }

fine

now lets talk about enhancing it to be AutoREsetEvent :

an AutoResetEvent is simply a matter of replacing the code in WaitOne with this:

code #2

lock (_locker)
{
    while (!_signal) Monitor.Wait (_locker);
    _signal = false;//<---------------
}

and replacing PulseAll with Pulse in the Set method:

code #3

lock (_locker) 
   { _signal = true; 
     Monitor.Pulse (_locker); 
   }

and here is the question :
at code #2 , the line _signal = false;.

why is it necessary ? Pulse will only reach 1 wait handle and I know that AutoREsetEvent is letting only one blocked item to go through and automatically close the gate.

so why writing _signal = false is necessary ? .

Answer 1

If the _signal variable stays true, then if another thread calls WaitOne after the Set call, it wouldn’t wait. You don’t just have to consider how existing waiting threads are handled – you have to consider the overall state of the object.