Is this Method Thread-Safe?

Can someone please tell me if the following method is thread safe. Also, please assume the call to _cache.GetOrCreate(...) is thread-safe. This method is the ONLY place in the application that creates or updates the region (dictionary). The class containing this method is a si开发者_开发百科ngleton, so multiple threads will access it.

    public IEnumerable<string> GetReportLookupItems<T>(string cacheKey, Func<IEnumerable<string>> factory)
    {
        Dictionary<string, IEnumerable<string>> region = _cache.GetOrCreate("Cache-Region:LookupItems", () => new Dictionary<string, IEnumerable<string>>());

        IEnumerable<string> items;

        if (!region.TryGetValue(cacheKey, out items))
        {
            region[cacheKey] = items = factory();
        }

        return items;
    }     

No. It's definitely not thread safe.

You're using a Dictionary<T,U> here, and changing its contents. Since Dictionary<T,U> is not thread-safe, the calls to TryGetValue and setting the dictionary by key are not thread safe.

If you were to change this to use a thread-safe dictionary (such as ConcurrentDictionary<T,U>), you could potentially have this be a thread-safe method.

I would say it depends on what _cache.GetOrCreate doess.

But from the sound of it, it does not sound thread safe since it is accessing the same Dictionary.

No, this code is not thread safe because calling the instance method TryGetValue on a shared dictionary (returned by _cache.GetOrCreate) is not thread safe. Multiple threads could potentially invoke this instance method on the same dictionary instance at the same time.

It is not thread safe.

Thread #1 executes all the way up to region[cacheKey] = items = factory();

Thread #1 is preempted by Thread #2

Thread #2 executes all the way up to region[cacheKey] = items = factory();

Thread #2 is preempted by Thread #1

Thread #1 executes factory()

Thread #1 is preempted by Thread #2

Thread #2 executes factory()

At this point in time you have two threads with two different instances of "factory()".