http://ayende.comAyende @ RahienCopyright (C) Ayende Rahien 2004 - 2021 (c) 202660No, it is never safe without proper locking in place. On a single processor, you have simply not been able to reliably reproduce the issue, but it certainly exists. http://ayende.com/2621/adding-to-list-from-multiple-threads#comment10http://ayende.com/2621/adding-to-list-from-multiple-threads#comment10Thu, 12 Jul 2007 22:23:38 GMTAyende, maybe I'm missing the point, so adding to a list is safe as long as you're on a single processor? http://ayende.com/2621/adding-to-list-from-multiple-threads#comment9http://ayende.com/2621/adding-to-list-from-multiple-threads#comment9Thu, 12 Jul 2007 21:22:13 GMTHowever it looks strange to me... I should be experimenting at least some issues since so many threads are trying to add items to the list concurrently, but instead I have no troubles doing it, as if the list was thread safe... http://ayende.com/2621/adding-to-list-from-multiple-threads#comment8http://ayende.com/2621/adding-to-list-from-multiple-threads#comment8Wed, 11 Jul 2007 12:29:50 GMTSimone, That can certainly be the reason http://ayende.com/2621/adding-to-list-from-multiple-threads#comment7http://ayende.com/2621/adding-to-list-from-multiple-threads#comment7Tue, 10 Jul 2007 18:19:07 GMTAyende, I'm having no troubles running your code, it runs fine with no errors in the additions... Can it be because your machine is a dual core? http://ayende.com/2621/adding-to-list-from-multiple-threads#comment6http://ayende.com/2621/adding-to-list-from-multiple-threads#comment6Tue, 10 Jul 2007 16:49:59 GMTJust throwing in my 2 cents supporting Oren's example above. I always wonder why developers insist on "clever" algorithms, especially involving multi-threading. They use gut-feel to tell them whether a lock is required or whether finer-grained locking is appropriate. If you think multi-threading is easy, you haven't written enough multi-threaded code. Implement the simplest locking scheme possible (with unit tests to prove it actually works). Then when you measure a perf bottleneck in the locking code, implement a more performant algorithm and ensure your tests still pass. Premature optimization is the root of all evil. Premature optimization of locking code is evil incarnate (and just plain stupid). http://ayende.com/2621/adding-to-list-from-multiple-threads#comment5http://ayende.com/2621/adding-to-list-from-multiple-threads#comment5Fri, 06 Jul 2007 16:55:48 GMTnow, why didn't you use the threadtester I wrote? it's just for these types of cases.. http://ayende.com/2621/adding-to-list-from-multiple-threads#comment4http://ayende.com/2621/adding-to-list-from-multiple-threads#comment4Fri, 06 Jul 2007 16:06:43 GMTthanks, fixed. http://ayende.com/2621/adding-to-list-from-multiple-threads#comment3http://ayende.com/2621/adding-to-list-from-multiple-threads#comment3Fri, 06 Jul 2007 08:46:00 GMTI think you mean "multiple threads" not "multiply". One means many the other is a mathematical operation. [)amien http://ayende.com/2621/adding-to-list-from-multiple-threads#comment2http://ayende.com/2621/adding-to-list-from-multiple-threads#comment2Fri, 06 Jul 2007 06:39:10 GMTMSDN documentation will tell you if a class is thread-safe or not: http://msdn2.microsoft.com/en-us/library/6sh2ey19.aspx Down at the bottom in the "Thread Safety" section, it says that List<T> is not thread-safe for instance methods. I usually start at MSDN documentation to determine if I need to lock a resource or not. http://ayende.com/2621/adding-to-list-from-multiple-threads#comment1http://ayende.com/2621/adding-to-list-from-multiple-threads#comment1Thu, 05 Jul 2007 22:30:24 GMT