Correct way to implement a resource pool

Question

I\'m trying to implement something that manages a pool of resources such that the calling code can request an object and will be given one from the pool if it\'s available,

Answer 1

There are a few problems with what you're doing, but your specific race condition is likely caused by a situation like the following. Imagine you have a capacity of one.

1) There is one unused item in the pool.

2) Thread #1 grabs it and signals the event.

3) Thread #2 finds no available event and gets inside the capacity block. It does not add the item yet.

4) Thread #1 returns the item to the pool and signals the event.

5) Repeat steps 1, 2, and 3 using two other threads (e.g. #3, #4).

6) Thread #2 adds an item to the pool.

7) Thread #4 adds an item to the pool.

There are now two items in a pool with a capacity of one.

Your implementation has other potential issues, however.

• Depending on how your Pool.Count and Add() are synchronized, you might not see an up-to-date value.
• You could potentially have multiple threads grab the same unused item.
• Controlling access with an AutoResetEvent opens yourself up to difficult to find issues (like this one) because you are trying to use a lockless solution instead of just taking a lock and using Monitor.Wait() and Monitor.Pulse() for this purpose.