The behavior is undefined. When we develop a class that is not thread-safe, it means that we don't even test to see if what happens if multiple threads party on it at the same time. When you do that all bets are off.
From: ozdotnet-boun...@ozdotnet.com [mailto:ozdotnet-boun...@ozdotnet.com] On Behalf Of Wallace Turner Sent: Thursday, June 03, 2010 4:39 AM To: 'ozDotNet' Subject: Dictionary thread-safe-ness Hi, In short, could you encounter a deadlock whilst accessing a Dictionary object in a thread-unsafe manner ? Consider a class containing a Dictionary and a method to add to the Dictionary. Assume 2 threads call Add( ) public class SomeClass { private Dictionary<int, List<ClassA>> _dictionary = new Dictionary<int, List<ClassA>>(); private void Add(int id, ClassA someObj) { List<ClassA> list = null; if (!_dictionary.TryGetValue(id, out list))//thread1 hung here { list = new List<ClassA>(); list.Add(someObj);//thread2 hung here _dictionary.Add(id, list); } else { list.Add(someObj); } } } I encountered a deadlock recently. When I paused the debugger the threads were hung at the indicated position. That is, one was trying to Get and the other trying to Add. The docs regarding Dictionary and Thread Safety state: A Dictionary<TKey, TValue> can support multiple readers concurrently, as long as the collection is not modified. Even so, enumerating through a collection is intrinsically not a thread-safe procedure. In the rare case where an enumeration contends with write accesses, the collection must be locked during the entire enumeration. To allow the collection to be accessed by multiple threads for reading and writing, you must implement your own synchronization. It says 'must be locked' without explaining the consequences. I am aware of the non-deadlocking consequences but I didn't expect it to deadlock! (perhaps it did not and it was something else, I had lots of threads and did not investigate this at the time) Regards Wal
