Operation is not valid due to the current state of the object

Question

Sometimes, when running a Mono .NET application (it seems to not depend on the input data, as it does not change), I run into following situation: when trying to Dequeue an element from a (non-empty, I check its Count beforehand) Queue, an exception is thrown:

Unhandled Exception: System.InvalidOperationException: Operation is not valid due to the current state of the object at System.Collections.Generic.Queue1[DBWorkItem].Peek ()[0x00000] at System.Collections.Generic.Queue1[DBWorkItem].Dequeue () [0x00000] at DBProcessor.process (System.Object q) [0x0006b] in <...>

Though I can certainly catch this exception, the element is dequeued and lost. Has anyone else encountered this behavior, too?

Answer 1

Do you have multiple threads using this queue? It may be non-empty when you check the count, but then empty when you try to peek the item.

If you have a short but complete program demonstrating the problem, we could tell for sure. Of course, it's possible that it's a Mono bug, although my experience is that system frameworks are usually more robust than the code calling into them.