List with non-null elements ends up containing null. A synchronization issue?
- by Alix
Hi. First of all, sorry about the title -- I couldn't figure out one that was short and clear enough.
Here's the issue: I have a list List<MyClass> list to which I always add newly-created instances of MyClass, like this: list.Add(new MyClass()). I don't add elements any other way.
However, then I iterate over the list with foreach and find that there are some null entries. That is, the following code:
foreach (MyClass entry in list)
if (entry == null)
throw new Exception("null entry!");
will sometimes throw an exception.
I should point out that the list.Add(new MyClass()) are performed from different threads running concurrently. The only thing I can think of to account for the null entries is the concurrent accesses. List<> isn't thread-safe, after all. Though I still find it strange that it ends up containing null entries, instead of just not offering any guarantees on ordering.
Can you think of any other reason?
Also, I don't care in which order the items are added, and I don't want the calling threads to block waiting to add their items. If synchronization is truly the issue, can you recommend a simple way to call the Add method asynchronously, i.e., create a delegate that takes care of that while my thread keeps running its code? I know I can create a delegate for Add and call BeginInvoke on it. Does that seem appropriate?
Thanks.
