When does a dictionary throw an IndexOutOfRangeException on Add or ContainsKey?

Question

On a busy ASP .NET website, I have a Dictionary, which acts as a cache, basically storing key/value pairs for later retrieval.

On high load, the Dictionary some times get into a state, where it always throws an IndexOutOfRangeException whenever i call the ContainsKey or Add method. The exception happens inside the private FindEntry method.

I am suspecting that this might be due to a synchronization issue, but I am not sure.

Can anyone tell me under which circumstances this can happen ? My goal is to gather enough information so that I can reproduce the issue in the dev environment.

Answer 1

The documentation for Dictionary states:

A Dictionary<TKey,TValue> can support multiple readers concurrently, as long as the collection is not modified. ... To allow the collection to be accessed by multiple threads for reading and writing, you must implement your own synchronization.

If you are not synchronizing access to the Dictionary, then you might get issues like those you describe (presumably because the internal state is no longer valid). If you want to try and reproduce this in your development environment, then try creating a program that uses multiple threads to continuously read and write from a Dictionary without synchronization.

Answer 2

I agree that this is almost certainly a synchronisation issue.

I'm not aware of any documentation that describes exactly when and how this can happen - the behaviour is undefined if you use a dictionary in a non-threadsafe manner.

For testing in your dev environment, I'd suggest running some parallel threads that randomly insert, remove, update etc in a continuous loop (basically a "concentrated" version of what's happening in your production environment).