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copied!<p>On problem with <code>ConcurrentBag</code> is that it is unordered so you cannot pull items out by index the same way you are doing it currently. However, you can iterate it via <code>foreach</code> to get the same effect. This iteration will be thread-safe. It will not go bizerk if an item is added or removed while the iteration is happening.</p> <p>There is another problem with <code>ConcurrentBag</code> though. It actually copies the contents to a new <code>List</code> internally to make the enumerator work correctly. So even if you wanted to just pick off a single item via the enumerator it would still be a O(n) operation because of the way enumerator works. You can verify this by disassembling it.</p> <p>However, based on context clues from your update I assume that this collection is going to be small. It appears that there is only one entry per "game client" which means it is probably going to store a small number of items right? If that is correct then the performance of the <code>GetEnumerator</code> method will be mostly insignificant.</p> <p>You should also consider <code>ConcurrentDictionary</code> as well. I noticed that you are trying to match items from the collection based on <code>zoneId</code>. If you store the items in the <code>ConcurrentDictionary</code> keyed by <code>zoneId</code> then you would not need to iterate the collection at all. Of course, this assumes that there is only one entry per <code>zoneId</code> which may not be the case.</p> <p>You mentioned that you did not want to use "local lists/copies", but you never said why. I think you should reconsider this for the following reasons.</p> <ul> <li>Iterations could be lock-free.</li> <li>Adding and removing items appears to be infrequent based context clues from your code.</li> </ul> <p>There are a couple of patterns you can use to make the list copying strategy work really well. I talk about them in my answers <a href="https://stackoverflow.com/a/10557130/158779">here</a> and <a href="https://stackoverflow.com/a/10018399/158779">here</a>.</p>

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