Question

I have implemented a generic ObjectPool class but have experienced that it sometime deadlocks (happens at Monitor.Wait(poolLock))

Can anyone spot the error?

public class ObjectPool<T> where T : new()
{
    private readonly object poolLock = new object();
    Stack<T> stack = null;

    public ObjectPool(int count)
    {
        stack = new Stack<T>(count);
        for (int i=0; i<count; i++)
            stack.Push(new T());
    }

    public T Get()
    {
        lock (poolLock)
        {
            //if no more left wait for one to get Pushed
            while (stack.Count < 1)
                Monitor.Wait(poolLock); 
            return stack.Pop();
        }
    }

    public void Put(T item)
    {
        lock (poolLock)
        {
            stack.Push(item);
            //If adding first send signal
            if (stack.Count == 1)
                Monitor.Pulse(poolLock); 
        }
    }

usage

        try
        {
            service = myPool.Get();
        }
        finally
        {
            if (service != null)
                myPool.Put(service);
        }

Answer 1

The deadlock is probably happening with a stack.Count > 0 . That means you have a Wait/Pulse problem. It is not a bad idea to always call Pulse after a Push(). Or at least when Count < 5 or so. Remember that the Wait/Pulse mechanism does not have a memory.

A scenario:

Thread A tries to Get from an empty
Pool, and does a Wait()
Thread B tries
to Get from an empty Pool, and does a
Wait()

Thread C Puts into the Pool, Does a Pulse()
Thread D Puts back into the Pool and does not Pulse (Count == 2)

Thread A is activated and Gets its Item.
Thread B is left Waiting. With little hope fro recovery.